Talk:Absorption spectroscopy

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Not all absorption techniques excite electrons; IR, for instance, only changes a molecule's vibrational energy level. It sounds like the author is also trying to include X-ray crystallography within absorption spectroscopy as well; that is also incorrect. Only IR spectra contain the "fingerprint" region mentioned in this article; UV spectra really don't provide such unique information. The analogy between calibration curves and Hooke's law is tenuous at best, and would be very confusing to someone not familiar with both topics. This page is listed under "pages needing expert attention," so my little list of inaccuracies might be a good place to start. Mihovil 19:03, 30 August 2006 (UTC)[reply]

I think that all absorption techniques affect the electrons. Unless one is trying to involve the nucleus, which would require quite high energy radiation, most of the techniques of general interest affect the electrons. The energy levels between which the electron switches are determined by the various energy modes available to the molecule (these include the rotational, vibrational and electronic energy level). The way the EM radiation interacts with the molecule is by affecting the electron cloud distribution around the nucleus (which according to

Born-Oppenheimer approximation

are fixed).

As far as "fingerprint" is concerned. Well the "author" hasn't made it specific. He is refering to the spectrum on the whole of which IR spectrum is a part. So I guess there is no problem in that. Also UV spectra is usually determined by the electronic configuration of the molecule under investigation. Since the configuration can change, how can you say that the UV spectra does not provide any unique information.

I agree with the fact that the analogy is bit confusing and can be put it in simple terms. Thank you for your suggestions. 69.180.9.141 00:37, 31 August 2006 (UTC)[reply]

Of course all spectroscopies "affect" the electron cloud; I never disputed that. But IR radiation absolutely does not promote electrons to higher energy levels, UV does. When a photon of infrared radiation is absorbed by the absorbing species, said species is promoted from its (usually) ground vibrational state to a higher vibrational state. Yes, the electron cloud will be polarized, but that is a matter for the Raman spectroscopist, and Raman in not an absorption technique. UV spectra of most organic molecules are basically broad Gaussian blobs, and it is very common for different molecules to have UV absorption spectra which are centered around the same wavelength. This means that it will be difficult or impossible to distinguish them on the basis of their UV spectra alone. For this reason, UV is not considered to give any "fingerprint" information. Mihovil 02:05, 31 August 2006 (UTC)[reply]

Well even in case of IR, it's only the electron cloud that is affected. Only difference is that in this case we are not concerned with the atomic orbital, but the molecular orbitals. Also IR does not have enough energy to affect the nucleus of most of the atoms, so one cannot say that the nucleus is absorbing the energy from the radiation. One can also think of it as absorption of radiation in IR range affects the vibration mode of the molecule. When the molecule absorbs the IR radiation, it starts vibrating at higher amplitudes. This is because the electrons are displayed to higher energy orbitals, so there is more repulsion between the nucleus and hence the amplitude is higher.

When we talk of UV exciting the electrons to higher energy levels, it is usually the transfer between the atomic orbitals, where the energy difference between the levels is more compared to molecular orbitals.

Also when we talk of broad absortion septrum, it is because we are not resolving it enough. Given ehough resolution power, we can distinguish between the different lines, only limitation will be due to the line broadining mechanisms. So even rotational spectrum can distinguish between species, but the problem with that is multiple species can have overlapping rotational spectrum or some of them have no rotational spectrum at all, so one needs the vibrational spectrum or the vibrotational spectrum to make sure. I agree that just using one kind of spectra may not be sufficient to given enough information about the molecule. 69.180.9.141 03:56, 2 September 2006 (UTC)[reply]

At this point, I'm really not sure if you are actually reponding to any of my comments, or just trying to demonstrate your knowledge of spectroscopy. Mihovil 00:24, 4 September 2006 (UTC)[reply]

I am just trying to understand the process of absorption. Most of the books do not mention what happens during an absorption. All they state is that the energy is absorbed by the molecule and it gets transfered to a different energy level. Some of them do state that it's the electrons that gets transfered. So I was confused when you said that the electron are not affected. I just wanted to make "sure" of what happens during an absorption process, so that I do not make unnecessary changes/reverts in the article. I am sorry if I have offended you. 69.180.9.141 21:34, 4 September 2006 (UTC)[reply]

There are a lot of misconceptions in this discussion. In order to prevent other readers from being confused I will point out that Mihovil's comments are sound. The problem user 69.180.9.141 has is the notion that energy is absorbed by a single particle instead of by the atom/molecule as a whole.

The books mentioned above that refer to electrons being transferred to a different level are invoking an approximation akin to the Bohr model. A better description would state that an electronic degree of freedom is excited. In a molecule, there are also vibrational and rotational degrees of freedom that can be excited. In the end, however, all these degrees of freedom interact to some extent. 128.146.32.223 (talk) 22:45, 7 September 2009 (UTC)[reply]

Of course you haven't! Sorry if I sounded cranky. Mihovil 23:50, 23 September 2006 (UTC)[reply]

Anyway, it depends on the type of spectroscopy. For example, in UV-visible spectroscopy, electrons absorb radiation, and are promoted to higher energy levels. In infrared spectroscopy, though, radiation is absorbed, but the electrons are essentially not affected. Mihovil 23:50, 23 September 2006 (UTC)[reply]

I have added an introductory paragraph and some referencesCinnamon colbert 03:54, 14 December 2006 (UTC)[reply]

I have changed the name back to absorbtion spectroscopy, as atomic absorbtion spectroscopy , at least in the USA, typically refers to techniques such as flame spectroscopy; I think most workers would consider this a different article.

I have added an introduction to try to reduce the problem of specificity. DJDahm 7, April, 2007

I'm not sure what absorption spectroscopy means when I'm reading this article. It deals primarily with UV visible spectroscopy when that topic has its own article. This doesn't make any sense. From the looks of it this article needs to get rid of the UV visible section and be clear on its purpose that distinguishes it from other spectroscopy techniques. LostLucidity (talk) 00:55, 27 January 2008 (UTC)[reply]

I agree with many previous commenters that the current article does not clarify what is unique about absorption spectroscopy as a topic and contains so many concepts that it is confusing. I think many of the topics touched on are already dealt with well in other articles. I don't, though, think this article should be done away with. I have spent some time working on substantial changes to this article, using the articles on

There is a line in the absorption spectrum section which I find extremely confusing:

Radiation is more likely to be absorbed at frequencies that match the energy difference between two quantum mechanical states of the molecules.

If you follow the link to quantum mechanical states it still makes little to no sense. Can we simplify this a little? Paskari (talk) 17:08, 24 April 2013 (UTC)[reply]

Not the slightest hint that any human discovered this process. What? One day an astronomer walked into the observatory, and there on the bench was a spectrometer, with an unsigned note explaining how to use it? Yep, I am being sarcastic. How about someone who knows writing a "History" section. I came here to get that information. Nick Beeson (talk) 00:36, 28 June 2014 (UTC)[reply]

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