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For convenience molecules are divided into four classes: (1) Linear molecules (2) Symmetric Tops. (3) Asymmetric Tops. dealing with each in turn:
Clearly, only three classes are named (and only these three are discussed). However, a fourth class must also be considered, namely (4) Spherical Tops. A section on spherical tops should be added here.
For some reason, this page seems really lame. It really needs to be updated to offer a more structured description of the topic. Suprisingly bad.
I found some misinformation concerning the P branch and R branch (it has been corrected). I haven't had time to look through the rest of the article, though I think that the validity of the information should be checked-- consider the mixup that I found and other possible mixups in the article.
Need to add some figures, explaining the spectrum. Also a section for experimental determination and applications.
I've added a figure for a linear molecule, but some explanation of the Q-branch transitions that are sometimes observed is needed, and I'm not really qualified to do so. I have a scan of a spectrum recorded in my university labs on a FTIR machine of an HCl / DCl mixture. Does anyone know if there would be any problem with me putting this up? David-i98 ( talk) 10:52, 29 January 2008 (UTC)
I am a research-active academic who specialises in microwave spectroscopy. I agree with the general comments which say this page needs some work and wish to develop the page, acting on some of the criticisms. I will be providing some new figures in the coming days/weeks and suggesting other amendments also. I have started by tweaking the opening paragraphs. There were some factual inaccuracies (rotational and microwave spectroscopy are not exactly synonymous, for example). I have tried to retain the spirit of the previous draft.
I will be trying to engage the lively global community of researchers who use rotational spectroscopy so that we can get an excellent page in the coming weeks. — Preceding unsigned comment added by Nnrw ( talk • contribs) 16:01, 21 May 2012 (UTC)
I've removed the figure showing the ro-vibrational spectrum of CO and replaced it with a pure rotational spectrum of CF3I which I took at the University of Bristol. I've also removed the ro-vibrational spectrum of CH4 that was somewhat further down. I want to draw a distinction here between ro-vibrational spectroscopy (described elsewhere on wikipedia, linked to in the opening paragraph) and pure rotational spectroscopy which is somewhat different. — Preceding unsigned comment added by Nnrw ( talk • contribs) 14:40, 22 May 2012 (UTC)
Re: comment by Dirac. I agree with the need to change that sentence and have made a few changes. With regard to Dirac66 point 3, I think Raman is used most often for measurements of ro-vibrational or ro-vibronic transitions rather than rotational so have left that out. I'm not a Raman specialist though, my background is in pure-rotational spectroscopy. Further comments and edits welcomed. — Preceding unsigned comment added by Nnrw ( talk • contribs) 14:47, 22 May 2012 (UTC)
I've replaced the illustration that showed transitions in ro-vibrational specta with another that shows transitions in pure rotational spectra. All of the theory alongside the figure relates to pure rotational spectra so I think it's better this way. The same diagam also shows how these map onto the rotational transitions that can be observed experimentally. It is important to distinguish between the energies associated with rotational levels, and those associated with transitions and I try to do that with this plot. Nnrw ( talk) 16:12, 24 May 2012 (UTC)
I'm delighted someone's watching what I'm doing and thanks for the feedback. Please keep it coming. A few responses; In 2012, it's not true that absorption spectra are always measured. It's true that this was the usual way of doing things before 1975. Around that time, Fourier transform (FT) techniques were first applied in microwave spectroscopy and everything changed in a big way. A FT experiment involves (i) first cooling molecules through gas jet expansion (ii) irradiating molecules with a polarisation pulse (the molecules absorb) before finally (iii) detecting the molecular emission that accompanies the subsequent decoherence. FTMW is the electric dipole analogue of NMR and is governed by the Bloch equations in the same way. To cut a long story short, I can do no better on how FTMW works than the animation here: < http://www.chem.ualberta.ca/~jaeger/research/ftmw/ftmw.htm> which I will eventually link from the article, after having a chance to work on the experiment section. The great majority of research worldwide now uses FTMW spectrometers, absorption cells are very rarely used. Nnrw ( talk) 14:08, 25 May 2012 (UTC)
The splitting in each J-J transition is definitely nuclear quadrupole coupling. This has been studied a few times, I'm not sure if you have the fortunate privilege of a university library. If so, you might find a few papers by searching for "pure rotational CF3I trifluoroiodomethane" or something like that using a search engine. The nuclear quadrupole coupling constant of iodine in CF3I was last measured in 2010. Centrifugal distortion induces a small shift in the position of each J-J transition. In the frequency range covered by the diagram, this is a very, very small effect (so small it's invisible on the figure). Centrifugal distortion is a much bigger shift at mm wavelengths. The connections and distinctions between millimetre and microwave spectroscopies might usefully be covered in a future version of this page. I would like to overhaul the sections that go beyond the rigid rotor. The section on centrifugal distortion is currently a little misleading. It actually shouldn't really be referred to as the "non-rigid rotor", the phrase "semi-rigid rotor" is conventionally used because higher order distortion terms (ie there's others in addition to D) are neglected by the model presented on the page. Nnrw ( talk) 14:08, 25 May 2012 (UTC)
On the suggestion of moving the diagram down to the symmetric top section, I would really like to leave it where it is. I think the introductory preamble should be aimed at microwave spectroscopy very generally, not necessarily just for linear molecules. The spectrum of CF3I is a nice prototype because you can see the regular 2B spacing in the spectrum between the J-J transitions and the spectrum also illustrates hyperfine coupling very nicely (can be referred to in a future version of the page). Nnrw ( talk) 14:32, 25 May 2012 (UTC)
I agree this would be a good idea, if there's nothing like it already there. I need to concentrate on just one page at a time.
chirped-pulse FTMW involves exactly the same sequence of events ((i) cooling (ii) polarisation (absorption) (iii) detecting the emission) as does the older variant of FTMW. A single experiment (done in a fraction of a second) involves the molecules first absorbing the radiation and detecting the emission as they relax. Nnrw ( talk) 14:08, 25 May 2012 (UTC)
Perhaps the diagrams are all right (at least for astrophysics)but the Experimental section should explain when absorption is observed and when emission is observed.
I think the diagram is OK but welcome further opinions. I agree regarding the second point and we will get there but there is a huge amount more work needed before this can be easily summarised, given what I say above about how FTMW and CP-FTMW work. What is needed is (i) an explanation of how FTMW works, then (ii) why FTMW superceded absorption cells in research and the technological changes that made this possible then (iii) how a CP-FTMW works and (iv) the technology that now allows construction of a chirped-pulse FTMW spectrometer. Boiling this down to a reasonably basic level will require some good choices of words. Answering a different query, chirped-pulse FTMW spectrometers don't use chirped pulse lasers, they exploit high speed electronics (arbitrary waveform generators and oscilloscopes). Given that the page can be accessed by anyone at any time, I won't induce confusion by trying to explain all of this at once. I will get there in the end, I hope. Nnrw ( talk) 14:08, 25 May 2012 (UTC)
Also the mention of the Virginia work needs a hyperlink - have I found the most appropriate page? Dirac66 ( talk) 13:09, 25 May 2012 (UTC)
The link to chirped pulse amplification wasn't appropriate in this context. I've added a link to a page maintained by the research group that developed by CP-FTMW. This may be too complex for this audience. On the other hand, when a simple and more accessible description of CP-FTMW has been provided, it will be a useful complement to what we have on the main page. Nnrw ( talk) 14:39, 25 May 2012 (UTC)
OK, another concern today. In the section Structure of rotational spectra, subsection Linear molecules, the article now says "These molecules have two degenerate modes of rotation ... Since we cannot distinguish between the two modes, we need only one rotational quantum number (J) to describe the rotational motion of the molecule." I would say that the rotation is described by two degrees of freedom (not degenerate modes) which are the angles θ and φ. The Schrodinger equation is the one for a particle on the surface of a sphere, with spherical harmonics as eigenfunctions. The two quantum numbers are J and M. Of course the energy depends only on J in the absence of an external field, so only J is observable in ordinary microwave spectroscopy. But M does become observable in Stark-effect or Zeeman-effect experiments.
Similarly the section Structure of rotational spectra, subsection Symmetric top refers only to two quantum numbers J and K. Actually there are 3 degrees of rotational freedom = the two for a linear molecule plus a third rotation about the symmetry axis. This leads to 3 quantum numbers J, K and M. Again E is independent of M, unless there is an external field. Dirac66 ( talk) 17:45, 26 May 2012 (UTC)
This is a major omission and leads to several misleading statements which apply to infrared spectroscopy but not to Raman spectroscopy. I will attend to this eventually, but I can't give a time-scale at the moment. Anyone else interested? Petergans ( talk) 12:42, 30 October 2012 (UTC)
The Historical Achievements section contains a paragraph about buckminsterfullerene (C60) which could be interpreted to mean (although it does not actually say) that C60 was discovered by observation of its microwave spectrum. However, this is impossible because C60 has zero dipole moment by symmetry, and therefore has no microwave spectrum. It is true that microwave observations of astrophysical molecules such as HC5N and HC7N led to the research that resulted in the discovery of C60, the actual observation of C60 was made by other means - initially mass spectroscopy and later IR (vibrational) and NMR.
I therefore question whether C60 should even be mentioned in this article. If it is mentioned, then it should be made clear that although rotational spectra of other molecules stimulated the research, C60 itself does not have a rotational spectrum. Dirac66 ( talk) 03:14, 1 November 2012 (UTC)
For the following reasons
How to proceed? Petergans ( talk) 12:22, 8 November 2012 (UTC)
I put a lot of work into editing this back in August but have run out of time and don't expect to be able to do much soon. I welcome other contributors.
Nnrw ( talk) 10:42, 9 November 2012 (UTC)
I agree none of the underpinning theory should be deleted from this page unless there is something authoritative elsewhere. It would indeed be a big job to make it all available and understandable, not one I'm really ready for now. I understand that the centrifugal distortion expression is a first approximation but I think I still feel it is misleading, for the following reason; the relationship is reported as though it were an exact equality. If the accompanying text instead said something along the lines of...."after inclusion of a first-order correction to account for centrifugal distortion....", then I would be much happier. If an expression which is formally an approximation is not labelled as such, then it is incorrect, in my opinion. Nnrw ( talk) With the "Rotational Spectrum Example" file, I was aiming to make explicit the connection from transitions between rotational energy levels to the frequencies of transitions that are observed in laboratory spectra. I find that undergraduates don't always immediately understand that transition frequencies are observed in spectra (and not levels directly) and I wanted to help with that. I appreciate the intensities aren't explained. I could add a note to say that transition intensities are a function of temperature (this would be easy, can do if needed) but have the impression that this wasn't the only source of your dissatisfaction. Is there anything else wrong with the figure that you'd like fixed, or which would be a reason to delete it completely? Alternatively, do you think we should put anything else in it's place to make the connection between levels, transitions, and the rotational constant? I'll leave unchanged for now. Nnrw ( talk) 18:06, 16 November 2012 (UTC)
I note User:Nnrw's aversion to the inclusion of anything to do with ro-vibrational spectra. What about the case of CO2? This molecule is centrosymmetric so no pure rotation spectrum can be observed by microwave spectroscopy. However, the ro-vibrational spectrum of the asymmetric stretching vibration is easy to observe at about 2350 cm-1 in the infrared and can be used to determine rotational constants, gas temperature (e.g. in the atmosphere) etc. Worth inclusion? Methane? Petergans ( talk) 14:51, 10 November 2012 (UTC)
Dirac66 reports my thoughts about linking to the "ro-vibrational coupling" page exactly. I also agree with the proposal to re-name the "ro-vibrational coupling" page as "ro-vibrational spectroscopy" and will add my own comment to that page to that effect. The other page is even quieter than this one so I guess that change will probably happen. Nnrw ( talk) 18:06, 16 November 2012 (UTC)
Please check the formula for line spacing. I think it should be 2B + f(D), but I don't have access to reference books here in Italy. Petergans ( talk) 09:04, 15 November 2012 (UTC)
There is now an apparent inconsistency in the article as to why no spectrum is observed in molecules with no dipole. The intro says that a permanent (non-zero) electric dipole moment is required to have a spectrum, but the spherical top section says that spherical tops have no spectrum because there is no change in dipole moment with rotational quantum number. A reader may wonder why change in dipole is not mentioned in the intro.
I understand that this inconsistency is only apparent, because the statement in the intro refers to a molecular frame of reference in which a dipole is constant (at least in each quantum state), whereas the statement about spherical tops describes the rotation of the dipole vector with the molecule as a change in the lab frame of reference. But this is not mentioned in the article, and I think it would be unnecessarily complicated to explain properly. Instead I will change the language in the spherical top section to conform to the intro. Dirac66 ( talk) 23:56, 16 November 2012 (UTC)
The section on asymmetric tops seems the most confusing in the article. Some suggestions:
And one other loose end. In the section Rotational line intensities, the page in Banwell and McCash is given as p. 4P0 (sic). I don't have this book to check so could someone please correct this? (Perhaps try p.400 because the closest digit to P on a keyboard is 0.) Dirac66 ( talk) 23:09, 18 November 2012 (UTC)
Hollas, "Modern Spectrocopy",3rd. edition, p 103, section 5.2.5, describes how rotation about any 3-fold axis in SiH4 gives rise to centrifugal distortion and hence a small dipole moment is generated. In fact part of the far ir spectrum (that is, pure rotation spectrum) of SiH4 is illustrated, Fig. 5.10, from Rosenberg & Ozier, Can. J. Phys., 1974, 52, 575. Maybe this should go in the symmetric rotor section? I must also assume that a similar spectrum has been obseved with methane.
See also Arieh Rosenberg and Irving Ozier J. Chem. Phys. 65, 418 (1976); http://dx.doi.org/10.1063/1.432784 (7 pages)"Collision‐induced absorption of gaseous silane in the far infrared".
More goodies at http://dx.doi.org/10.1139/p75-250 (pdf at http://fermi.uchicago.edu/publications/PDF/oka061.pdf)
Petergans (
talk) 16:55, 20 November 2012 (UTC)
I've removed this section as there is nothing relevant to rotational spectra. Any objections? N.b. I've added nuclear spin statistics to the intensities section. Petergans ( talk) 16:25, 27 November 2012 (UTC)
I didn't really like that section, certainly no objection to its removal. On the other hand, we should put something in about hyperfine interactions in the future. Hyperfine interactions are relevant to rotational spectra- each J-J transition in the microwave spectrum of CF3I (see figure at the top of the page) is split by the effects of nuclear quadrupole coupling, for example (the nuclear electric quadrupole moment of a quadrupolar nucleus couples to the overall rotation of the nuclear framework). If we do, I suggest it should just be an explanation of the observed splittings rather than a full derivation. That would be consistent with the other formulae available above for energy levels and transition frequencies. Fine interactions (ie involving electron spin-rotation) are also important but molecules that have these have always been very hard to study so there's less in the literature. Doing so involves correcting for coupling of the unpaired electron spin with the magnetic field of the earth using Helmholtz coils. The described coupling otherwise leads to compicated splitting patterns and hence, very messy observations. Rotational spectroscopy of such radicals is a current research frontier because many radicals are important in the chemistry of the atmosphere. See, for example, Suma et al's paper in Science Vol. 311, pp. 1278 (2006). Nnrw ( talk) 11:46, 28 November 2012 (UTC) 128.240.229.68 ( talk) 11:44, 28 November 2012 (UTC)
{{
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: Unknown parameter |coauthors=
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help) for which the quadrupole moment was determined.
Petergans (
talk) 17:02, 28 November 2012 (UTC)No problem. I have the book widely regarded as the "bible" of microwave spectroscopy on my desk- "Microwave Molecular Spectra" by Walter L. Gordy and Robert L. Cook. This book has been out of print for a long time, unfortunately. It has a chapter (about 60 pages!) on nuclear hyperfine structure in rotational spectra which I hope, at some point, I can draw upon to put some basic information here. A note on the nuclear spin statistics section fo this page- it currently uses ethyne to illustrate nuclear spin statistics but the problem is, this molecule doesn't have a pure rotational spectrum (as stated further up the page). The description of the spin statistical weights is good as is but can we change this example? Some good alternatives suggested in Gordy and Cook would be NH3, CH3Cl, CH3CCH which are C3v so C3 rotation about the a axis interchanges hydrogens and transition intensities also reflect statistical weight distributions. Nnrw ( talk) —Preceding undated comment added 11:20, 29 November 2012 (UTC)
I found "Callomon, J.H. (1957). "HIGH RESOLUTION RAMAN SPECTROSCOPY OF GASES: VIII. ROTATIONAL SPECTRA OF ACETYLENE, DIACETYLENE, DIACETYLENE-d2, AND DIMETHYLACETYLENE". Canadian Journal of Physics. 35 (4): 373–382.
doi:
10.1139/p57-043. {{
cite journal}}
: Unknown parameter |coauthors=
ignored (|author=
suggested) (
help)". No details in the abstract. I can't see the contents unless I go in to Uni.
Petergans (
talk) 15:21, 29 November 2012 (UTC)
I tried again to access the 1957 paper by Callomon and Stoicheff on the Raman spectra of acetylenes which none of us could access in December, and this time I was able to download it. Perhaps the publisher had a technical problem which has been resolved, so that others with university subscriptions can now access this article also.
Anyway I have now read the paper. There is a plate after the second page with the spectra, and the intensity alternation is visible for acetylene and diacetylene, but not really for diacetylene-d2 and dimethylacetylene. However the text is concerned mostly with determination of bond lengths and does not mention the intensity alternation. Except for a sentence in the final paragraph which says that the resolution of 0.3 cm-1 corresponds to a certain moment of inertia for molecules having nuclei with spin ≠ 0 and twice this value for nuclei with zero spin. This rather cryptic mention suggests to me that by 1957 the effect of nuclear spin was well known and no longer required detailed explanation. However as a Wikipedia reference for the existence of intensity alternation, I think we need instead a reference which says explicitly that there is intensity alternation in some molecule, and at least briefly relates the alternation to nuclear spin statistics. Dirac66 ( talk) 00:31, 8 February 2013 (UTC)
We currently have both "references" and a "bibliography" at the bottom of this page. The references are pretty comprehensive, I think we can safely delete the entire bibliography for neatness. What do you think??? Nnrw ( talk) —Preceding undated comment added 16:29, 29 November 2012 (UTC)
I am surprised at how few biographical articles there are related to this field. Only Townes if I'm reading correctly. Any suggestions for key researchers who might deserve biographical articles? I would consider doing the necessary research to get articles started. ronningt ( talk) 00:02, 30 November 2012 (UTC)
The suggestion of Walter Gordy is a good one. The article on Flygare definitely needs expansion. There is a brief wikipedia article on E. Bright Wilson who was certainly a microwave pioneer and must be a leading candidate for a bio. The first measurements of microwave molecular spectra were reported by Brebis Bleaney in 1934. His results were obtained at lower resolution that that which followed and he didn't do as much thereafter as people like Gordy and Townes who were his contemporaries. With respect to microwave spectroscopy, William Klemperer has to be an important person to include from the last 30 years or so. He was amongst the first researchers to show that microwave spectroscopy could be used to study individual van der Waals and hydrogen bonds in isolation. I have now discovered he actually already has a good bio on wikipedia. We should link to it, when we have the right material to connect from. I don't know anything about rotational Raman spectroscopy and will stick to what I know! Nnrw ( talk)
I've added this section as a brief mention is essential. I think the present placings of this and the section on effect of vibration on rotation are wrong, so I'll probably include them with quadrupole splitting and Stark effect when I can think of a suitable sub-section title for the 4 topics. Petergans ( talk) 10:09, 1 December 2012 (UTC)
OK. I've added the following sentence; "Rotational constants can be reported either as wavenumber in units of cm-1 or frequency in units of MHz. The former is more appropriate when reporting data for small, light molecules and the latter is most often used by microwave spectroscopists working on larger species but there is no agreed standard." I don't know if this is the most appropriate place for it but some statement on units seems needed. The discussion on instruments is all in MHz/GHz now and that's what I use when reporting data. Most microwave spectroscopists operate between 6 and 18 GHz for practical reasons. If we worked in cm-1, all our rotational constants would be expressed as multiplied by 10-7 or some similar factor and it's just nicer to work with round numbers. What do you think? Nnrw ( talk) I've also added a note to the sentence comparing the rotational constants obtained from IR and microwave to point out that rotational constants obtained from microwave spectroscopy are generally obtained to higher precision. Nnrw ( talk)
I've now identified a few problems with the sentence; "For example, carbon monoxide, 12CO, has a vibration frequency of 2143 cm-1 and a rotational constant B of 1.925 cm-1, which compares well with the value of 1.92118 cm-1 obtained by microwave spectroscopy." It is properly and helpfully cited as Banwell and McCash so I looked up the source. The rotational constant is given in Banwell as "B" rather than Be (the equlibrium constant), or either of the vibrational state-specific B0 or B1. These labels are important because this section of the wiki page starts by explaining how rotational constants are affected by vibration and explicitly defines these three parameters. We therefore should use the labels within the article. The data from the microwave experiment isn't the most accurate available figure, it's from a Gordy paper in 1950 (instruments were not as accurate as they are now). A better current figure for a microwave spectroscopy Be is available through the NIST microwave spectral database (primary source from 1990 is cited from this link within the database); < http://physics.nist.gov/PhysRefData/MolSpec/Diatomic/Html/Tables/CO.html> and is 1.93160(2) cm-1. The number in brackets is one standard deviation in units of the last sig. fig. (ie a measure of the experimental precision) Banwell's citation to its own source for the IR data is "Miss J. Cook at York University" and not to a publication. Banwell does say though, that the rotational constant is obtained by simply dividing the interval between adjacent lines by 2. That's very crude, and it's neither B0, B1 or Be. The purpose of the sentence on the wiki page is to compare the results of data obtained through infrared spectroscopy with microwave and the problem is that it's giving a very false impression- you can do much better than implied by Banwell. It's possible to extract the ground state B0, an excited state B1, calculate Be and measure a centrifugal distortion constant from an easily-obtained IR spectrum, I regularly mark this exercise for our teaching lab and I have some data to work with. I get a Be of 1.9316(4) cm-1 from a typical data set after a more rigorous spectral assignment. This is much closer to the microwave result. The precision of modern spectroscopy is certainly much better than implied by the comparison in Banwell and McCash. Can we either delete the sentence highlighted above or do a comparison from less shaky foundations? Nnrw ( talk) —Preceding undated comment added 22:35, 1 December 2012 (UTC)
I've added a section on "Instruments and Methods" in place of the original "Experimental Determination of the Spectrum". All the information in the original section is subsumed into the new one. I'm not sure exactly what the right title is for the section. I know this will not be a comprehensive list of all experimental methods covered by this page. For example, I don't know the details of how a rotational Raman experiment is conducted. I also havn't said anything specific about mm-wave spectroscopy (components and instruments are somewhat different above 24 GHz in frequency) or about teraherz spectroscopy. I've selected a format that should make it easy for people to add new sections on particular methods as appropriate. Nnrw ( talk) Nnrw ( talk)
Something occurred to me whilst writing about the instruments. The section labelled "rotational line intensities" was really a discussion of rotational level populations. Temperature is a very important factor behind experimentally-observed spectral line intensities but there are also others. For example, in a FTMW experiment, it's important to ensure the pulse length and power level are optimised for the dipole of the molecule that you wish to observe. In a Balle-Flygare instrument there's also a major effect that comes from the response/performance of the cavity which changes as a function of frequency. In principal, the instrumental response function in CP-FTMW instruments is linear with frequency but it's not always achieved in practice. I have therefore relabelled the section on "rotational line intensities" as "rotational level populations" and made the same alteration in the text a few lines down, and also changing the first sentence in that section. I'm still thinking about whether the reference to line intensities before the part on nuclear spin statistics is correct (can't remember whether the spin statistics affect the level populations or the transition dipole moment and thus line intensities, need to check books). Nnrw ( talk)
I felt the first 3 sections of the page needed some rationalising. The "historical overview" section had outlived its purpose- I relocated and reworded almost all the historical material into either "Applications" or the "Absorption Spectroscopy and Stark Modulation" section. Very little material has actually been removed- just the comment about Charles Townes's recollections of RADAR attenuation by water vapour in his memoirs "How the Laser Happened". I'll hope to reinstate the reference at some point because that's a great book. Nnrw ( talk)
This is now more of a summary of the article contents. Anything significant that I may have removed should now go into the appropriate place in the body of the article. Petergans ( talk) 15:19, 4 December 2012 (UTC)
There's nothing here about the Zeeman effect yet. This is the effect of a magnetic field on transition frequencies and would be worth including because it's the reason that molecules with unpaired electrons are hard to study by microwave spectroscopy in the lab. Somewhat analogous to the Stark effect. There's already a more detailed page on wikipedia about the Zeeman effect to link to. I imagine we'll get around to this at some point in the future. Nnrw ( talk) 18:35, 4 December 2012 (UTC)
212.159.115.44 ( talk) 08:51, 13 February 2014 (UTC)== Oxygen?? ==
I found this spectrum in microwave transmission; I've added O2 to the section on linear molecues. However, this does not explain the spectrum shown here completely since O2 has magnetic-dipole allowed doublets between ca,. 54 and 66 Ghz and 120 Ghz. Where do the other spikes come from? Petergans ( talk) 10:38, 6 December 2012 (UTC)
The explanation of the spectrum as it appears on the page, "Pure rotation spectrum of atmospheric water vapour measured at Mauna Kea (33 cm-1 to 100 cm-1)", differs from that shown to the right. I know nothing about this subject area but the explanation on the page suggests that the spectrum is for water, rather than a spectrum of the mixture of gases present in air which is what I assume it is.
The section Selection rules now gives the Raman rule for symmetric tops as
The reference given is Banwell and McCash, which I do not have. However both Hollas (p.116) and Atkins and de Paula (8th ed, p.449) give only the case ΔK = 0, so I ask that someone recheck Banwell and McCash concerning ΔK ≠ 0. It is my understanding that a ΔK ≠ 0 Raman transition for a symmetric top is forbidden because rotation around the symmetry axis does not change the polarizability. Dirac66 ( talk) 22:20, 16 December 2012 (UTC)
For linear molecules the Selection rules section correctly gives the Raman rule as ΔJ = 0, ±2. However the Rotational Raman spectroscopy claims that the 15N2 spectrum is resolved into P, Q, R, and S branches, referenced to Hollas p.113. But in Hollas the mention of O, P, Q, R and S branches at equation (5.51) refers to a general molecule, not to a linear molecule. For 1he 15N2 spectrum in Figure 5.17, all labelled lines are S: S(0), S(10) and S(20) for both Stokes and anti-Stokes series, implying that the intermediate lines are also S - S(1), S(2), S(3), etc. The Q branch for pure rotational spectra is just the Rayleigh line, and there is no mention of P or R branches which would violate the selection rule. So I propose to delete the P and R branches from the discussion of 15N2, and just mention the (Stokes and anti-Stokes) S-branches and the Rayleigh line.
As for the intensity alternation, we could point out here that an analogous phenomenon is observed in the vib-rot spectrum of acetylene, with a link to the vib-rot article after the point has been included in that article. Dirac66 ( talk) 22:59, 16 December 2012 (UTC)
Details are fine, but put in the basic info too. I looked this up to show someone some spectroscopy and laughed very hard. 70.162.46.129 ( talk) 06:00, 2 May 2014 (UTC)
I do not know who wrote that section and what exactly they tried to convey, but it is mostly wrong. The energy of rotation is not "added to, or subtracted from" the energy of vibration, but is always added. Moreover, two different vibrational states have different rotational constants ( and ), so their energies are
and hence the vibration-rotation wavenumbers of transitions are
with for the R branch and for the P branch. The Q branch has , but since , the rotational energy is still changed, and thus this does not correspond to a "pure vibration". — Mikhail Ryazanov ( talk) 05:35, 24 June 2016 (UTC)
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For convenience molecules are divided into four classes: (1) Linear molecules (2) Symmetric Tops. (3) Asymmetric Tops. dealing with each in turn:
Clearly, only three classes are named (and only these three are discussed). However, a fourth class must also be considered, namely (4) Spherical Tops. A section on spherical tops should be added here.
For some reason, this page seems really lame. It really needs to be updated to offer a more structured description of the topic. Suprisingly bad.
I found some misinformation concerning the P branch and R branch (it has been corrected). I haven't had time to look through the rest of the article, though I think that the validity of the information should be checked-- consider the mixup that I found and other possible mixups in the article.
Need to add some figures, explaining the spectrum. Also a section for experimental determination and applications.
I've added a figure for a linear molecule, but some explanation of the Q-branch transitions that are sometimes observed is needed, and I'm not really qualified to do so. I have a scan of a spectrum recorded in my university labs on a FTIR machine of an HCl / DCl mixture. Does anyone know if there would be any problem with me putting this up? David-i98 ( talk) 10:52, 29 January 2008 (UTC)
I am a research-active academic who specialises in microwave spectroscopy. I agree with the general comments which say this page needs some work and wish to develop the page, acting on some of the criticisms. I will be providing some new figures in the coming days/weeks and suggesting other amendments also. I have started by tweaking the opening paragraphs. There were some factual inaccuracies (rotational and microwave spectroscopy are not exactly synonymous, for example). I have tried to retain the spirit of the previous draft.
I will be trying to engage the lively global community of researchers who use rotational spectroscopy so that we can get an excellent page in the coming weeks. — Preceding unsigned comment added by Nnrw ( talk • contribs) 16:01, 21 May 2012 (UTC)
I've removed the figure showing the ro-vibrational spectrum of CO and replaced it with a pure rotational spectrum of CF3I which I took at the University of Bristol. I've also removed the ro-vibrational spectrum of CH4 that was somewhat further down. I want to draw a distinction here between ro-vibrational spectroscopy (described elsewhere on wikipedia, linked to in the opening paragraph) and pure rotational spectroscopy which is somewhat different. — Preceding unsigned comment added by Nnrw ( talk • contribs) 14:40, 22 May 2012 (UTC)
Re: comment by Dirac. I agree with the need to change that sentence and have made a few changes. With regard to Dirac66 point 3, I think Raman is used most often for measurements of ro-vibrational or ro-vibronic transitions rather than rotational so have left that out. I'm not a Raman specialist though, my background is in pure-rotational spectroscopy. Further comments and edits welcomed. — Preceding unsigned comment added by Nnrw ( talk • contribs) 14:47, 22 May 2012 (UTC)
I've replaced the illustration that showed transitions in ro-vibrational specta with another that shows transitions in pure rotational spectra. All of the theory alongside the figure relates to pure rotational spectra so I think it's better this way. The same diagam also shows how these map onto the rotational transitions that can be observed experimentally. It is important to distinguish between the energies associated with rotational levels, and those associated with transitions and I try to do that with this plot. Nnrw ( talk) 16:12, 24 May 2012 (UTC)
I'm delighted someone's watching what I'm doing and thanks for the feedback. Please keep it coming. A few responses; In 2012, it's not true that absorption spectra are always measured. It's true that this was the usual way of doing things before 1975. Around that time, Fourier transform (FT) techniques were first applied in microwave spectroscopy and everything changed in a big way. A FT experiment involves (i) first cooling molecules through gas jet expansion (ii) irradiating molecules with a polarisation pulse (the molecules absorb) before finally (iii) detecting the molecular emission that accompanies the subsequent decoherence. FTMW is the electric dipole analogue of NMR and is governed by the Bloch equations in the same way. To cut a long story short, I can do no better on how FTMW works than the animation here: < http://www.chem.ualberta.ca/~jaeger/research/ftmw/ftmw.htm> which I will eventually link from the article, after having a chance to work on the experiment section. The great majority of research worldwide now uses FTMW spectrometers, absorption cells are very rarely used. Nnrw ( talk) 14:08, 25 May 2012 (UTC)
The splitting in each J-J transition is definitely nuclear quadrupole coupling. This has been studied a few times, I'm not sure if you have the fortunate privilege of a university library. If so, you might find a few papers by searching for "pure rotational CF3I trifluoroiodomethane" or something like that using a search engine. The nuclear quadrupole coupling constant of iodine in CF3I was last measured in 2010. Centrifugal distortion induces a small shift in the position of each J-J transition. In the frequency range covered by the diagram, this is a very, very small effect (so small it's invisible on the figure). Centrifugal distortion is a much bigger shift at mm wavelengths. The connections and distinctions between millimetre and microwave spectroscopies might usefully be covered in a future version of this page. I would like to overhaul the sections that go beyond the rigid rotor. The section on centrifugal distortion is currently a little misleading. It actually shouldn't really be referred to as the "non-rigid rotor", the phrase "semi-rigid rotor" is conventionally used because higher order distortion terms (ie there's others in addition to D) are neglected by the model presented on the page. Nnrw ( talk) 14:08, 25 May 2012 (UTC)
On the suggestion of moving the diagram down to the symmetric top section, I would really like to leave it where it is. I think the introductory preamble should be aimed at microwave spectroscopy very generally, not necessarily just for linear molecules. The spectrum of CF3I is a nice prototype because you can see the regular 2B spacing in the spectrum between the J-J transitions and the spectrum also illustrates hyperfine coupling very nicely (can be referred to in a future version of the page). Nnrw ( talk) 14:32, 25 May 2012 (UTC)
I agree this would be a good idea, if there's nothing like it already there. I need to concentrate on just one page at a time.
chirped-pulse FTMW involves exactly the same sequence of events ((i) cooling (ii) polarisation (absorption) (iii) detecting the emission) as does the older variant of FTMW. A single experiment (done in a fraction of a second) involves the molecules first absorbing the radiation and detecting the emission as they relax. Nnrw ( talk) 14:08, 25 May 2012 (UTC)
Perhaps the diagrams are all right (at least for astrophysics)but the Experimental section should explain when absorption is observed and when emission is observed.
I think the diagram is OK but welcome further opinions. I agree regarding the second point and we will get there but there is a huge amount more work needed before this can be easily summarised, given what I say above about how FTMW and CP-FTMW work. What is needed is (i) an explanation of how FTMW works, then (ii) why FTMW superceded absorption cells in research and the technological changes that made this possible then (iii) how a CP-FTMW works and (iv) the technology that now allows construction of a chirped-pulse FTMW spectrometer. Boiling this down to a reasonably basic level will require some good choices of words. Answering a different query, chirped-pulse FTMW spectrometers don't use chirped pulse lasers, they exploit high speed electronics (arbitrary waveform generators and oscilloscopes). Given that the page can be accessed by anyone at any time, I won't induce confusion by trying to explain all of this at once. I will get there in the end, I hope. Nnrw ( talk) 14:08, 25 May 2012 (UTC)
Also the mention of the Virginia work needs a hyperlink - have I found the most appropriate page? Dirac66 ( talk) 13:09, 25 May 2012 (UTC)
The link to chirped pulse amplification wasn't appropriate in this context. I've added a link to a page maintained by the research group that developed by CP-FTMW. This may be too complex for this audience. On the other hand, when a simple and more accessible description of CP-FTMW has been provided, it will be a useful complement to what we have on the main page. Nnrw ( talk) 14:39, 25 May 2012 (UTC)
OK, another concern today. In the section Structure of rotational spectra, subsection Linear molecules, the article now says "These molecules have two degenerate modes of rotation ... Since we cannot distinguish between the two modes, we need only one rotational quantum number (J) to describe the rotational motion of the molecule." I would say that the rotation is described by two degrees of freedom (not degenerate modes) which are the angles θ and φ. The Schrodinger equation is the one for a particle on the surface of a sphere, with spherical harmonics as eigenfunctions. The two quantum numbers are J and M. Of course the energy depends only on J in the absence of an external field, so only J is observable in ordinary microwave spectroscopy. But M does become observable in Stark-effect or Zeeman-effect experiments.
Similarly the section Structure of rotational spectra, subsection Symmetric top refers only to two quantum numbers J and K. Actually there are 3 degrees of rotational freedom = the two for a linear molecule plus a third rotation about the symmetry axis. This leads to 3 quantum numbers J, K and M. Again E is independent of M, unless there is an external field. Dirac66 ( talk) 17:45, 26 May 2012 (UTC)
This is a major omission and leads to several misleading statements which apply to infrared spectroscopy but not to Raman spectroscopy. I will attend to this eventually, but I can't give a time-scale at the moment. Anyone else interested? Petergans ( talk) 12:42, 30 October 2012 (UTC)
The Historical Achievements section contains a paragraph about buckminsterfullerene (C60) which could be interpreted to mean (although it does not actually say) that C60 was discovered by observation of its microwave spectrum. However, this is impossible because C60 has zero dipole moment by symmetry, and therefore has no microwave spectrum. It is true that microwave observations of astrophysical molecules such as HC5N and HC7N led to the research that resulted in the discovery of C60, the actual observation of C60 was made by other means - initially mass spectroscopy and later IR (vibrational) and NMR.
I therefore question whether C60 should even be mentioned in this article. If it is mentioned, then it should be made clear that although rotational spectra of other molecules stimulated the research, C60 itself does not have a rotational spectrum. Dirac66 ( talk) 03:14, 1 November 2012 (UTC)
For the following reasons
How to proceed? Petergans ( talk) 12:22, 8 November 2012 (UTC)
I put a lot of work into editing this back in August but have run out of time and don't expect to be able to do much soon. I welcome other contributors.
Nnrw ( talk) 10:42, 9 November 2012 (UTC)
I agree none of the underpinning theory should be deleted from this page unless there is something authoritative elsewhere. It would indeed be a big job to make it all available and understandable, not one I'm really ready for now. I understand that the centrifugal distortion expression is a first approximation but I think I still feel it is misleading, for the following reason; the relationship is reported as though it were an exact equality. If the accompanying text instead said something along the lines of...."after inclusion of a first-order correction to account for centrifugal distortion....", then I would be much happier. If an expression which is formally an approximation is not labelled as such, then it is incorrect, in my opinion. Nnrw ( talk) With the "Rotational Spectrum Example" file, I was aiming to make explicit the connection from transitions between rotational energy levels to the frequencies of transitions that are observed in laboratory spectra. I find that undergraduates don't always immediately understand that transition frequencies are observed in spectra (and not levels directly) and I wanted to help with that. I appreciate the intensities aren't explained. I could add a note to say that transition intensities are a function of temperature (this would be easy, can do if needed) but have the impression that this wasn't the only source of your dissatisfaction. Is there anything else wrong with the figure that you'd like fixed, or which would be a reason to delete it completely? Alternatively, do you think we should put anything else in it's place to make the connection between levels, transitions, and the rotational constant? I'll leave unchanged for now. Nnrw ( talk) 18:06, 16 November 2012 (UTC)
I note User:Nnrw's aversion to the inclusion of anything to do with ro-vibrational spectra. What about the case of CO2? This molecule is centrosymmetric so no pure rotation spectrum can be observed by microwave spectroscopy. However, the ro-vibrational spectrum of the asymmetric stretching vibration is easy to observe at about 2350 cm-1 in the infrared and can be used to determine rotational constants, gas temperature (e.g. in the atmosphere) etc. Worth inclusion? Methane? Petergans ( talk) 14:51, 10 November 2012 (UTC)
Dirac66 reports my thoughts about linking to the "ro-vibrational coupling" page exactly. I also agree with the proposal to re-name the "ro-vibrational coupling" page as "ro-vibrational spectroscopy" and will add my own comment to that page to that effect. The other page is even quieter than this one so I guess that change will probably happen. Nnrw ( talk) 18:06, 16 November 2012 (UTC)
Please check the formula for line spacing. I think it should be 2B + f(D), but I don't have access to reference books here in Italy. Petergans ( talk) 09:04, 15 November 2012 (UTC)
There is now an apparent inconsistency in the article as to why no spectrum is observed in molecules with no dipole. The intro says that a permanent (non-zero) electric dipole moment is required to have a spectrum, but the spherical top section says that spherical tops have no spectrum because there is no change in dipole moment with rotational quantum number. A reader may wonder why change in dipole is not mentioned in the intro.
I understand that this inconsistency is only apparent, because the statement in the intro refers to a molecular frame of reference in which a dipole is constant (at least in each quantum state), whereas the statement about spherical tops describes the rotation of the dipole vector with the molecule as a change in the lab frame of reference. But this is not mentioned in the article, and I think it would be unnecessarily complicated to explain properly. Instead I will change the language in the spherical top section to conform to the intro. Dirac66 ( talk) 23:56, 16 November 2012 (UTC)
The section on asymmetric tops seems the most confusing in the article. Some suggestions:
And one other loose end. In the section Rotational line intensities, the page in Banwell and McCash is given as p. 4P0 (sic). I don't have this book to check so could someone please correct this? (Perhaps try p.400 because the closest digit to P on a keyboard is 0.) Dirac66 ( talk) 23:09, 18 November 2012 (UTC)
Hollas, "Modern Spectrocopy",3rd. edition, p 103, section 5.2.5, describes how rotation about any 3-fold axis in SiH4 gives rise to centrifugal distortion and hence a small dipole moment is generated. In fact part of the far ir spectrum (that is, pure rotation spectrum) of SiH4 is illustrated, Fig. 5.10, from Rosenberg & Ozier, Can. J. Phys., 1974, 52, 575. Maybe this should go in the symmetric rotor section? I must also assume that a similar spectrum has been obseved with methane.
See also Arieh Rosenberg and Irving Ozier J. Chem. Phys. 65, 418 (1976); http://dx.doi.org/10.1063/1.432784 (7 pages)"Collision‐induced absorption of gaseous silane in the far infrared".
More goodies at http://dx.doi.org/10.1139/p75-250 (pdf at http://fermi.uchicago.edu/publications/PDF/oka061.pdf)
Petergans (
talk) 16:55, 20 November 2012 (UTC)
I've removed this section as there is nothing relevant to rotational spectra. Any objections? N.b. I've added nuclear spin statistics to the intensities section. Petergans ( talk) 16:25, 27 November 2012 (UTC)
I didn't really like that section, certainly no objection to its removal. On the other hand, we should put something in about hyperfine interactions in the future. Hyperfine interactions are relevant to rotational spectra- each J-J transition in the microwave spectrum of CF3I (see figure at the top of the page) is split by the effects of nuclear quadrupole coupling, for example (the nuclear electric quadrupole moment of a quadrupolar nucleus couples to the overall rotation of the nuclear framework). If we do, I suggest it should just be an explanation of the observed splittings rather than a full derivation. That would be consistent with the other formulae available above for energy levels and transition frequencies. Fine interactions (ie involving electron spin-rotation) are also important but molecules that have these have always been very hard to study so there's less in the literature. Doing so involves correcting for coupling of the unpaired electron spin with the magnetic field of the earth using Helmholtz coils. The described coupling otherwise leads to compicated splitting patterns and hence, very messy observations. Rotational spectroscopy of such radicals is a current research frontier because many radicals are important in the chemistry of the atmosphere. See, for example, Suma et al's paper in Science Vol. 311, pp. 1278 (2006). Nnrw ( talk) 11:46, 28 November 2012 (UTC) 128.240.229.68 ( talk) 11:44, 28 November 2012 (UTC)
{{
cite journal}}
: Unknown parameter |coauthors=
ignored (|author=
suggested) (
help) for which the quadrupole moment was determined.
Petergans (
talk) 17:02, 28 November 2012 (UTC)No problem. I have the book widely regarded as the "bible" of microwave spectroscopy on my desk- "Microwave Molecular Spectra" by Walter L. Gordy and Robert L. Cook. This book has been out of print for a long time, unfortunately. It has a chapter (about 60 pages!) on nuclear hyperfine structure in rotational spectra which I hope, at some point, I can draw upon to put some basic information here. A note on the nuclear spin statistics section fo this page- it currently uses ethyne to illustrate nuclear spin statistics but the problem is, this molecule doesn't have a pure rotational spectrum (as stated further up the page). The description of the spin statistical weights is good as is but can we change this example? Some good alternatives suggested in Gordy and Cook would be NH3, CH3Cl, CH3CCH which are C3v so C3 rotation about the a axis interchanges hydrogens and transition intensities also reflect statistical weight distributions. Nnrw ( talk) —Preceding undated comment added 11:20, 29 November 2012 (UTC)
I found "Callomon, J.H. (1957). "HIGH RESOLUTION RAMAN SPECTROSCOPY OF GASES: VIII. ROTATIONAL SPECTRA OF ACETYLENE, DIACETYLENE, DIACETYLENE-d2, AND DIMETHYLACETYLENE". Canadian Journal of Physics. 35 (4): 373–382.
doi:
10.1139/p57-043. {{
cite journal}}
: Unknown parameter |coauthors=
ignored (|author=
suggested) (
help)". No details in the abstract. I can't see the contents unless I go in to Uni.
Petergans (
talk) 15:21, 29 November 2012 (UTC)
I tried again to access the 1957 paper by Callomon and Stoicheff on the Raman spectra of acetylenes which none of us could access in December, and this time I was able to download it. Perhaps the publisher had a technical problem which has been resolved, so that others with university subscriptions can now access this article also.
Anyway I have now read the paper. There is a plate after the second page with the spectra, and the intensity alternation is visible for acetylene and diacetylene, but not really for diacetylene-d2 and dimethylacetylene. However the text is concerned mostly with determination of bond lengths and does not mention the intensity alternation. Except for a sentence in the final paragraph which says that the resolution of 0.3 cm-1 corresponds to a certain moment of inertia for molecules having nuclei with spin ≠ 0 and twice this value for nuclei with zero spin. This rather cryptic mention suggests to me that by 1957 the effect of nuclear spin was well known and no longer required detailed explanation. However as a Wikipedia reference for the existence of intensity alternation, I think we need instead a reference which says explicitly that there is intensity alternation in some molecule, and at least briefly relates the alternation to nuclear spin statistics. Dirac66 ( talk) 00:31, 8 February 2013 (UTC)
We currently have both "references" and a "bibliography" at the bottom of this page. The references are pretty comprehensive, I think we can safely delete the entire bibliography for neatness. What do you think??? Nnrw ( talk) —Preceding undated comment added 16:29, 29 November 2012 (UTC)
I am surprised at how few biographical articles there are related to this field. Only Townes if I'm reading correctly. Any suggestions for key researchers who might deserve biographical articles? I would consider doing the necessary research to get articles started. ronningt ( talk) 00:02, 30 November 2012 (UTC)
The suggestion of Walter Gordy is a good one. The article on Flygare definitely needs expansion. There is a brief wikipedia article on E. Bright Wilson who was certainly a microwave pioneer and must be a leading candidate for a bio. The first measurements of microwave molecular spectra were reported by Brebis Bleaney in 1934. His results were obtained at lower resolution that that which followed and he didn't do as much thereafter as people like Gordy and Townes who were his contemporaries. With respect to microwave spectroscopy, William Klemperer has to be an important person to include from the last 30 years or so. He was amongst the first researchers to show that microwave spectroscopy could be used to study individual van der Waals and hydrogen bonds in isolation. I have now discovered he actually already has a good bio on wikipedia. We should link to it, when we have the right material to connect from. I don't know anything about rotational Raman spectroscopy and will stick to what I know! Nnrw ( talk)
I've added this section as a brief mention is essential. I think the present placings of this and the section on effect of vibration on rotation are wrong, so I'll probably include them with quadrupole splitting and Stark effect when I can think of a suitable sub-section title for the 4 topics. Petergans ( talk) 10:09, 1 December 2012 (UTC)
OK. I've added the following sentence; "Rotational constants can be reported either as wavenumber in units of cm-1 or frequency in units of MHz. The former is more appropriate when reporting data for small, light molecules and the latter is most often used by microwave spectroscopists working on larger species but there is no agreed standard." I don't know if this is the most appropriate place for it but some statement on units seems needed. The discussion on instruments is all in MHz/GHz now and that's what I use when reporting data. Most microwave spectroscopists operate between 6 and 18 GHz for practical reasons. If we worked in cm-1, all our rotational constants would be expressed as multiplied by 10-7 or some similar factor and it's just nicer to work with round numbers. What do you think? Nnrw ( talk) I've also added a note to the sentence comparing the rotational constants obtained from IR and microwave to point out that rotational constants obtained from microwave spectroscopy are generally obtained to higher precision. Nnrw ( talk)
I've now identified a few problems with the sentence; "For example, carbon monoxide, 12CO, has a vibration frequency of 2143 cm-1 and a rotational constant B of 1.925 cm-1, which compares well with the value of 1.92118 cm-1 obtained by microwave spectroscopy." It is properly and helpfully cited as Banwell and McCash so I looked up the source. The rotational constant is given in Banwell as "B" rather than Be (the equlibrium constant), or either of the vibrational state-specific B0 or B1. These labels are important because this section of the wiki page starts by explaining how rotational constants are affected by vibration and explicitly defines these three parameters. We therefore should use the labels within the article. The data from the microwave experiment isn't the most accurate available figure, it's from a Gordy paper in 1950 (instruments were not as accurate as they are now). A better current figure for a microwave spectroscopy Be is available through the NIST microwave spectral database (primary source from 1990 is cited from this link within the database); < http://physics.nist.gov/PhysRefData/MolSpec/Diatomic/Html/Tables/CO.html> and is 1.93160(2) cm-1. The number in brackets is one standard deviation in units of the last sig. fig. (ie a measure of the experimental precision) Banwell's citation to its own source for the IR data is "Miss J. Cook at York University" and not to a publication. Banwell does say though, that the rotational constant is obtained by simply dividing the interval between adjacent lines by 2. That's very crude, and it's neither B0, B1 or Be. The purpose of the sentence on the wiki page is to compare the results of data obtained through infrared spectroscopy with microwave and the problem is that it's giving a very false impression- you can do much better than implied by Banwell. It's possible to extract the ground state B0, an excited state B1, calculate Be and measure a centrifugal distortion constant from an easily-obtained IR spectrum, I regularly mark this exercise for our teaching lab and I have some data to work with. I get a Be of 1.9316(4) cm-1 from a typical data set after a more rigorous spectral assignment. This is much closer to the microwave result. The precision of modern spectroscopy is certainly much better than implied by the comparison in Banwell and McCash. Can we either delete the sentence highlighted above or do a comparison from less shaky foundations? Nnrw ( talk) —Preceding undated comment added 22:35, 1 December 2012 (UTC)
I've added a section on "Instruments and Methods" in place of the original "Experimental Determination of the Spectrum". All the information in the original section is subsumed into the new one. I'm not sure exactly what the right title is for the section. I know this will not be a comprehensive list of all experimental methods covered by this page. For example, I don't know the details of how a rotational Raman experiment is conducted. I also havn't said anything specific about mm-wave spectroscopy (components and instruments are somewhat different above 24 GHz in frequency) or about teraherz spectroscopy. I've selected a format that should make it easy for people to add new sections on particular methods as appropriate. Nnrw ( talk) Nnrw ( talk)
Something occurred to me whilst writing about the instruments. The section labelled "rotational line intensities" was really a discussion of rotational level populations. Temperature is a very important factor behind experimentally-observed spectral line intensities but there are also others. For example, in a FTMW experiment, it's important to ensure the pulse length and power level are optimised for the dipole of the molecule that you wish to observe. In a Balle-Flygare instrument there's also a major effect that comes from the response/performance of the cavity which changes as a function of frequency. In principal, the instrumental response function in CP-FTMW instruments is linear with frequency but it's not always achieved in practice. I have therefore relabelled the section on "rotational line intensities" as "rotational level populations" and made the same alteration in the text a few lines down, and also changing the first sentence in that section. I'm still thinking about whether the reference to line intensities before the part on nuclear spin statistics is correct (can't remember whether the spin statistics affect the level populations or the transition dipole moment and thus line intensities, need to check books). Nnrw ( talk)
I felt the first 3 sections of the page needed some rationalising. The "historical overview" section had outlived its purpose- I relocated and reworded almost all the historical material into either "Applications" or the "Absorption Spectroscopy and Stark Modulation" section. Very little material has actually been removed- just the comment about Charles Townes's recollections of RADAR attenuation by water vapour in his memoirs "How the Laser Happened". I'll hope to reinstate the reference at some point because that's a great book. Nnrw ( talk)
This is now more of a summary of the article contents. Anything significant that I may have removed should now go into the appropriate place in the body of the article. Petergans ( talk) 15:19, 4 December 2012 (UTC)
There's nothing here about the Zeeman effect yet. This is the effect of a magnetic field on transition frequencies and would be worth including because it's the reason that molecules with unpaired electrons are hard to study by microwave spectroscopy in the lab. Somewhat analogous to the Stark effect. There's already a more detailed page on wikipedia about the Zeeman effect to link to. I imagine we'll get around to this at some point in the future. Nnrw ( talk) 18:35, 4 December 2012 (UTC)
212.159.115.44 ( talk) 08:51, 13 February 2014 (UTC)== Oxygen?? ==
I found this spectrum in microwave transmission; I've added O2 to the section on linear molecues. However, this does not explain the spectrum shown here completely since O2 has magnetic-dipole allowed doublets between ca,. 54 and 66 Ghz and 120 Ghz. Where do the other spikes come from? Petergans ( talk) 10:38, 6 December 2012 (UTC)
The explanation of the spectrum as it appears on the page, "Pure rotation spectrum of atmospheric water vapour measured at Mauna Kea (33 cm-1 to 100 cm-1)", differs from that shown to the right. I know nothing about this subject area but the explanation on the page suggests that the spectrum is for water, rather than a spectrum of the mixture of gases present in air which is what I assume it is.
The section Selection rules now gives the Raman rule for symmetric tops as
The reference given is Banwell and McCash, which I do not have. However both Hollas (p.116) and Atkins and de Paula (8th ed, p.449) give only the case ΔK = 0, so I ask that someone recheck Banwell and McCash concerning ΔK ≠ 0. It is my understanding that a ΔK ≠ 0 Raman transition for a symmetric top is forbidden because rotation around the symmetry axis does not change the polarizability. Dirac66 ( talk) 22:20, 16 December 2012 (UTC)
For linear molecules the Selection rules section correctly gives the Raman rule as ΔJ = 0, ±2. However the Rotational Raman spectroscopy claims that the 15N2 spectrum is resolved into P, Q, R, and S branches, referenced to Hollas p.113. But in Hollas the mention of O, P, Q, R and S branches at equation (5.51) refers to a general molecule, not to a linear molecule. For 1he 15N2 spectrum in Figure 5.17, all labelled lines are S: S(0), S(10) and S(20) for both Stokes and anti-Stokes series, implying that the intermediate lines are also S - S(1), S(2), S(3), etc. The Q branch for pure rotational spectra is just the Rayleigh line, and there is no mention of P or R branches which would violate the selection rule. So I propose to delete the P and R branches from the discussion of 15N2, and just mention the (Stokes and anti-Stokes) S-branches and the Rayleigh line.
As for the intensity alternation, we could point out here that an analogous phenomenon is observed in the vib-rot spectrum of acetylene, with a link to the vib-rot article after the point has been included in that article. Dirac66 ( talk) 22:59, 16 December 2012 (UTC)
Details are fine, but put in the basic info too. I looked this up to show someone some spectroscopy and laughed very hard. 70.162.46.129 ( talk) 06:00, 2 May 2014 (UTC)
I do not know who wrote that section and what exactly they tried to convey, but it is mostly wrong. The energy of rotation is not "added to, or subtracted from" the energy of vibration, but is always added. Moreover, two different vibrational states have different rotational constants ( and ), so their energies are
and hence the vibration-rotation wavenumbers of transitions are
with for the R branch and for the P branch. The Q branch has , but since , the rotational energy is still changed, and thus this does not correspond to a "pure vibration". — Mikhail Ryazanov ( talk) 05:35, 24 June 2016 (UTC)