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As far as I know, the effect, on the one hand, depends on the timing of the subsequent photons to be absorbed. They have to reach a fluorophore within a short time interval of sub-femtoseconds to a few femtoseconds, i.e. quasi simultaneously. On the other hand, it depends on the symmetry of a fluorophore, which, I think, determines the transition matrix between energy levels. Then it all comes down to whether a transition is allowed or not by the selection rules, which constrain transition for example by the need to conserve an electron's angular momentum. What exactly is going on, I don't know (does anybody?), but one interpretation is indeed, that the first photon induces a transition from the ground state to a virtual excited state at an intermediate energy level (what's wrong with that? It's just a model). This is thought to be close to a real state that can be occupied according to the selection rules. The second photon carries the system from the virtual state to the final state, that originally might have been forbidden.

Best,
Marcus


-----Original Message-----
From: Confocal Microscopy List [mailto:[log in to unmask]] On Behalf Of Martin Wessendorf
Sent: Monday, April 07, 2014 4:24 AM
To: [log in to unmask]
Subject: two-photon absorption

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Dear List--

Is there a physicist out there who can offer an intuitive explanation of how 2-photon absorption occurs?  I expect we all know that it isn't that there isn't any half-excited state that allows one photon to boost an electron half-way to the excited state, and the next photon to finish the job.  My sense is that it has to do with time-energy uncertainty (a la Heisenberg) but my quantum mechanics is elementary-school level.

Thanks!

Martin Wessendorf

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