Talk:Rotational spectroscopy

message: 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)
replies: text: I've noticed that the rotational energy levels in the figure (:Image:Vibrationrotationenergy.svg) become progressively closer with increasing J. It is my understanding, however, that the energy level gap between successive levels grows as J increases.--GregRM (talk) 23:54, 15 February 2008 (UTC)
replies: text: Whoops, that was a bit stupid. I've fixed it now. David-i98 (talk) 15:07, 11 March 2008 (UTC)
thread_title:  figures required 

message: 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)
replies: text: The statement that "rotational and microwave spectroscopy are not exactly synonymous" would be more helpful to the reader if we indicate the differences. For example 1) rotational transitions are not always at frequencies observed with microwave apparatus, 2) some microwave transitions involve other molecular changes such as inversion, and 3) (most important I think) even typical rotational transitions can be observed by microwave OR Raman spectroscopy, and some molecules can only be studied in Raman. Dirac66 (talk) 17:38, 21 May 2012 (UTC)

text: Raman has some application to pure rotational spectroscopy as well, and is of especial interest for non-dipolar molecules which have no microwave spectrum. However mentioning Raman to the article intro might well be confusing. Instead I will start a new section at the end to mention Raman, and mention only pure rotational transitions. Dirac66 (talk) 00:15, 23 May 2012 (UTC)
thread_title:  work in progress! 

message: 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)
thread_title:  Missing quantum number M 

message: 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)
thread_title:  Does C60 belong in this article? 

message: For the following reasons
#The lead is too long and too detailed
#Duplication of categories linear, spherical top etc.
#Selection rules (both IR and Raman) should be a separate section
#Early on there needs to be a clear statement of the rigid rotor and how it is modified by the effects of centrifugal distortion, Coriolic coupling, hyperfine splitting etc.
#There does not appear to be any mention of intensities (Boltzmann distribution)
#I would like to see some spectra, even vibration-rotation spectra would be preferable to none. I've see the vib-rot spectra of CO and CH4 in the version 13 March. they are not good because the resolution insufficient to show separate lines. HCl might be better as the speparation between lines is greater, making it easier to observe. Incidentally, the ir spectrum of the C-H stretching vibration of methane shows the effect of Coriolis coupling even though the resolution is poor.
How to proceed? Petergans (talk) 12:22, 8 November 2012 (UTC)
thread_title:  This article needs to be revised 

message: 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)
replies: text: I think Nnrw's point is that we have another article on ro-vib spectra, which is called Rovibrational coupling although a better title (I think) would be Rotational-vibrational spectroscopy. That article does need much improvement, but in principle all the ro-vib material could be included there. There is some merit in dividing the two subjects into two articles, since ro-vib uses IR which is a different experimental technique. It also gives slightly different information, such as the rotational constants of the vibrationally excited state. Good point. I've amended the bit on spherical tops accordingly. Petergans (talk) 13:32, 11 November 2012 (UTC)
replies: text:  I'm away all next week so I may not be working on this article during that time. Now I have to pack my bag and go to the airport. Petergans (talk) 13:32, 11 November 2012 (UTC)
thread_title:  Carbon dioxide 

message: 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)
replies: text: Do you mean v(J'-J") = 2B(J"+1)-4D(J"+1)3? This does follow from the previous equation F(J) = 2B(J"+1) - DJ"2(J"+1)2, since ΔJ2(J+1)2 = (J"+1)2(J"+2)2 - J"2(J"+1)2 = (J"+1)2[(J"+2)2-J"2] = (J"+1)2(4J"+4) = 4(J"+1)3. Also Atkins and de Paula agree - 8th edn, p.448.   Dirac66 (talk) 20:27, 15 November 2012 (UTC)
replies: text: Intuitively I see the effect of centrifugal distortion as a deviation from the rigid rotor, that is, the spacing between adjacent lines J and J+1 equal to 2B minus a correction. I'll check my reference books tomorrow after I return home. Petergans (talk) 10:07, 18 November 2012 (UTC)
replies: text: OK, I have now relabelled this formula which was incorrectly referred to as line spacing. Actually it is the correct formula for line position or line location, or at least the first-order approximation to same as Nnrw has pointed out above in the section Comments on above points. In the Linear molecule section of the article, only the line positions are given in the text, although the equal line spacings (without centrifugal distortion) are mentioned in the Figure caption. Dirac66 (talk) 12:45, 18 November 2012 (UTC)
thread_title:  centrifugal distortion 

message: The section on asymmetric tops seems the most confusing in the article. Some suggestions:
* The notation should conform to the rest of the article. Are A and B the rotational constants defined elsewhere as \tilde A and \tilde B? And since there is no \tilde C elsewhere in the article, we should specify that C is defined analogously for the third axis.
* Also the quantum number k should be K. Elsewhere in the article, k is the Boltzmann constant.
* Define what is meant by scaled rotational Hamiltonian and scaled rotational energy levels.
* The banded Hamiltonian is well described, but would still benefit from a diagram. 
* Finally, the most difficult sentence (and a half): The Hamiltonian can be formulated in six different settings, dependent on the mapping of the principal axes to lab axes and handedness. For the most asymmetric, right-handed representation, ... This is my preferred candidate for removal, as I have no idea what it means and I suspect it requires long explanations. Dirac66 (talk) 23:09, 18 November 2012 (UTC)
replies: text:  I agree. Hollas states briefly that an eigenvalue problem has to be solved for each J value. As far as I can see, the formulae shown refer to specific cases where the molecular structure is approximately that of a symmetric top. I would delete most of this, but give good citations for the theory, maybe in the microwave books? For illustration the rotational spectrum of water vapour (far infrared) has been studied in detail[ http://dx.doi.org/10.1063/1.1703330 http://dx.doi.org/10.1063/1.1703330]. Petergans (talk) 12:00, 2 December 2012 (UTC)
replies: text: I'm also in agreement, it doesn't belong here. I would not be in favour of deleting the section entirely- but of moving it to a sub-page and linking to it from the main page. I hope that someone in the future would give some attention to making sure everything on that sub-page is correct. Personally, I would also want to do that with the information for symmetric tops. As the page evolves, I would imagine this would mean we have the best of both worlds. Full mathematical derivations on the sub-pages but an interesting/engaging main page relatively free of maths.Nnrw (talk) 17:18, 2 December 2012 (UTC)
replies: text:  I've reduced the content to an absolute minimum, with a citation that gives the approximate formulae. BTW, if anyone has themselves obtained spectra, it is OK to upload them to WP as "own work" and then display them in the article. Petergans (talk) 12:01, 3 December 2012 (UTC)
replies: text: Much better now. It is at the same level as the rest, so readers who understand the article can understand this section. Dirac66 (talk) 19:41, 3 December 2012 (UTC)
replies: text: Comments on own work noted- I would be happy to cite the results I have from a ro-vib. spectrum, and to cite an IR spectrum I have as own work while providing full details, but not to display that same spectrum in the article because of introducing confusion with ro-vib. spectroscopy. WIll do this with the CO spectum when I have a moment.Nnrw (talk) 18:41, 4 December 2012 (UTC)
thread_title:  Asymmetric top section 

message: 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)
replies: text: Corrected. Petergans (talk) 12:01, 20 November 2012 (UTC)
thread_title: Incorrect page reference

message: 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)
thread_title:  Spherical top 

message: 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)
replies: text: There is an illustration at Rotational-vibrational spectroscopy#Polyatomic linear molecules which shows the intensity variation in C2H2 clearly. The separation between adjacent rotational lines is a little less than 1 cm-1, so the fine structure will not be fully resolved at the resolution quoted by Stoicheff. Certainly I think the effect was old hat by 1957. It's probably all in the old Gerhard Herzberg books, which used to be the standard reference source for this kind of stuff. Petergans (talk) 10:25, 8 February 2013 (UTC)
thread_title: Callomon-Stoicheff Raman paper now accessible

message: 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)
replies: text: We do also have a good article on C.V. Raman, probably because he did win a Nobel prize. And we have an article on Willis H. Flygare but it is too short and more is needed. Two names that come to mind with no Wikipedia articles now are W.(Walter?) Gordy, mentioned once in the history section and cited twice in the references. Also Takeshi Oka, a Japanese-Canadian in Herzberg's group in Ottawa. I believe both did quite a lot of microwave spectroscopy. And as mentioned above, Boris P. Stoicheff in Raman spectroscopy, but he has an article now. Dirac66 (talk) 01:30, 30 November 2012 (UTC)

text: Thanks for the suggestions. I agree that Gordy would be a valuable addition, and so I'll work on tracking down some information for his.ronningt (talk) 12:39, 1 December 2012 (UTC)
replies: text: Sorry- My memory failed with my comment above- Brebis Bleaney didn't do the first study in 1934, that was Cleeton and Williams. The first high resolution stuff using klystron/magnetron sources, right after the Second World War was done simultaneously by Townes and Bleaney (according to Townes in his memoirs, though Bleaney published first as I remember).Nnrw (talk) 17:27, 2 December 2012 (UTC)
thread_title:  Biographies 

message: 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)
:Your comments about intensities are interesting. A similar thing is observed with Carbon-13 NMR where the pulse length is often shorter than the relaxation time, affecting intensities. I see this as an experimental effect which causes intensities to differ from the "theoretical" values. The relative line intensities shown in the figure are the theoretical intensities, as usually observed in infrared spectra. Petergans (talk) 11:41, 2 December 2012 (UTC)
replies: text: I think focusing on the theoretical intensities alone is common in this sort of discussion and also appropriate here. It might be worth a note that each experimental technique requires calibration or reference measurements to establish the intensity, but even this might not be needed. I prefer the "rotational line intensities" label since it makes it clear why this section relates to rotational spectroscopy. The line intensities are strongly dependent on the population levels, so the content of the discussion feels appropriate to me. What seems to be missing is a comment linking the intensities to the transition dipole.ronningt (talk) 12:58, 2 December 2012 (UTC)
thread_title:  Rotational levels vs Rotational lines 

message: 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)
replies: text: That's mostly fine. I've changed the following (1) restored the sentence about molecular rotational motions being quenched in solids or liquids. This is essential because users may be interested to know why there's an FTIR in every teaching lab and an FT-NMR spectrometer in every university but not an FT-MW instrument. Good point, but does not belong in the lead. I've expanded and moved it into overview Petergans (talk) 09:17, 5 December 2012 (UTC)
replies: text: Another point- when saying that microwave spectra "can be measured in absorption or emission", I'm not sure it's wrong, but I'm not sure the statement is exactly right either, it might be slightly misleading (I wouldn't get away with it when writing a paper). Absorption is certainly correct. The problem is the emission I talked about in the instrumental section (which I will rephrase to remove jargon when I have a chance) is spontaneous coherent emission of the entire molecular ensemble, it's a decoherence phenomenon rather than a molecular emission of the sort Einstein describes. Someone who knows about NMR will know what I mean- it's the same principle there. You really can't do a straightforward "traditional emission" experiment in the lab-the reason is that the radiative lifetime of a rotationally-excited state is way too long, so the gas pulse involved in a Balle-Flygare FTMW experiment expires before your molecules emit. Fortunately the timescales involved in the decoherence that can be measured are only microseconds so you can do that experiment. As well, of course, what's picked up by radiotelescopes will be emission in the more traditional sense (ie the opposite of absorption). So like I say, it's not exactly wrong- but I'm not sure "absorption" and "emission" should be counterpoised in that sentence the way they are currently. I admit, unerstanding the whole "spontaneous coherent emission" idea is difficult- I still struggle with it, and the Bloch equations. Nnrw (talk) 18:50, 4 December 2012 (UTC)
replies: text: I had radioastronomy in mind when I put in emission. Petergans (talk) 19:05, 4 December 2012 (UTC)
thread_title:  Re-written lead 

message: 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)
replies: text: Perhaps you could ask User:Westeros91 who is listed as the author of this image. Dirac66 (talk) 22:20, 16 December 2012 (UTC)
thread_title: Zeeman Effect

message: The section Selection rules now gives the Raman rule for symmetric tops as
:ΔK = 0, ΔJ = 0, ±1, ±2
:ΔK ≠ 0, ΔJ = ±2
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)
replies: text: You are right. I mis-copied from Banwell&McCash. I hope the corrected text is OK. Petergans (talk) 10:31, 18 December 2012 (UTC)
thread_title: Raman selection rules

message: : (and other editors)
:The notation for rotational constants is inconsistent in this article. There are two obvious ways to harmonize the notation so I will consult before choosing one.
:In the sections Overview, Effect of rotation on vibrational spectra, and Rotational line intensities B is the rotational constant in cm-1, as in the books by Banwell and McCash and by Atkins and de Paula.
:However in the sections Linear molecules, Centrifugal distortion and Symmetric top, the rotational constant in cm-1 is B/hc or \bar B, as in the Article Rigid rotor.
:Clearly one article should have one notation for this quantity. My preference would be to follow the spectroscopy books and use B, with a note when we link the Rigid rotor article to say why we have divided by hc. But I will wait a week for other opinions before proceeding.
:And after this article is made consistent, we can look at related articles such as Rotational-vibrational spectroscopy and Rotational temperature.  Dirac66 (talk) 22:03, 5 April 2015 (UTC)
replies: text: OK, no response after 8 days so I have changed to B throughout. Dirac66 (talk) 23:54, 13 April 2015 (UTC)
thread_title:  Notational inconsistency for rotational constant