Talk:Two-photon absorption

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How does this compare to "Two-photon excitation"? Should that term be linked back to here? Should a redirect of that come here? Should this be renamed to that? Or am I just missing it? -RJFerret 05:15, 12 July 2006 (UTC)

Hi,

as far as i know, two photon excitation also includes effects such as sum frequency generation (SFG). Thus, TPA should be a part of two photon excitation (a page with links should be enough). --lagaffe from wikipedia.fr (144.204.16.1 13:56, 22 November 2006 (UTC))

Contents

[edit] This needs work.

I just made some edits in the article, but I'm pretty unsatisfied. It's a bit of a dog's dinner and could do with a rewrite all over. Yeah, I know, I'm whining when what I should really do is get down and do it myself...

In particular the "Measurements" part, which was much needed, could now do with some expansion (I also personally don't like the use of "β" for cross-section, but that's a whole other discussion).

I'll make an effort to come back and do something when I can find time...

• TheBendster (talk) 7 May 2007, 07:56 (UTC)

  • Its me again... I am not an expert at all in this field just trying to learn. I have spent considerable time trying to find the definition of the cross-section and understanding it. I am still clueless why the dimensions are what they are: 10-50cm4.s.photon-1molecules-1 so if you can help out it would be great. V8rik 20:02, 7 May 2007 (UTC)
Well, (almost a year later!) I've done that for you. • TheBendster (talk) 28 April 2008, 07:34 (UTC)

[edit] Expert tag

I wouldn't say that this article is great, but I don't think it needs the Expert tag at this point. Any dissenters to the idea of removing it? • TheBendster (talk) 28 April 2008, 07:35 (UTC)

[edit] Expert tag

Howdy Bendster:

I do research with TPA and TPEF so this is right up my alley, but I have not done editing in wikipedia before. first - the TPA cross section is usual a lower case delta or with a subscript 2. The units are correct... the cross section takes into account the idea that two separate events must occur in a given region for the extinction of photon in the process (this happens twice though, which is accounted for later in the constitutive equation) - it is sometime best to think of it as "the likelihood of the photon in questions consumption over a give region x the likelihood that another will be there and go with it x the inverse of photons provided/second" so we get cm^2 x cm^2 x (hv/s)^-1 = cm^4 sec / photon. Then the whole mess is per molecule... sometimes per mol or (when MW is high) per repeat unit.

I'm finishing up my phd in molecular optics, and I've only published twice in the area... not sure if this qualifies me as an expert. What is the criteria..? Anyway, hope that helped. J.e.raymond (talk) 17:47, 8 May 2008 (UTC)

Hi there. If you think that you can make a better rationalization for the units of cross-section than I did, then please go right ahead. As to the symbol for moleucular cross-section, although delta is very common, sigma is also seen a lot, particularly among chemists. As far as I know there's no accepted "correct" symbol, but correct me (and the article) if I'm wrong. Beta is generally used for the bulk cross-section (i.e. the 2-photon equivalent to OD). In general, any additions you can make to the article would be great. You're certainly expert enough, and anyway the community is here to correct any errors. If you wat to go further, you could start a new article on TPEF... WP could certainly do with one, and I'll pledge to help with it if you can put up the bones. • TheBendster (talk) 9 May 2008, 06:09 (UTC)

[edit] Selection Rules

I think the discussion of selection rules is misleading as written:

"The selection rules for TPA are therefore different than for one-photon absorption (OPA), which is dependent on the first-order susceptibility. For example, in a centrosymmetric molecule, one- and two-photon allowed transitions are mutually exclusive. In quantum mechanical terms, this difference results from the need to conserve spin. Since photons have spin of ±1, one-photon absorption requires excitation to involve an electron changing its molecular orbital to one with a spin different by ±1. Two-photon absorption requires a change of +2, 0, or −2."

Only circularly polarized light carries angular momentum. Linearly polarized light does not, but there are still different selection rules for one- and two-photon absorption. These selection rules have to do with the symmetry of the states under inversion (or parity). The spin of the states is the same for both processes.

Perhaps the article could be amended so that the example refers explicitly to circularly polarized light, or generalized to include other examples of one- and two-photon processes that probe states with different symmetries.

Jmkinder1 (talk) 20:34, 3 June 2008 (UTC)