The process of two-photon absorption is similar to ordinary single photon absorption.  In this process an electron absorbs two photons at approximately the same time and achieves an excited state that corresponds to the algebraic sum of the energy of the incident photons.  The atom is excited to a "virtual state" which does not correspond to any electronic or vibrational energy eigenstate.  

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Once electrons have absorbed two photons and are at a high energy level, they can be 'ripped' from the Cs atom if struck with a third photon.  The Nd-YAG laser provides such a powerful output of photons so that this process can be achieved. 

With the neutral density filters placed in the optical path, we would expect the resultant ion current to be reduced by a greater amount than the reduction in the incident optical power.

With varying temperature, we would expect to see a squared dependence.

Also, click here to see a table of known energy levels for Cs.

References:

http://www.phy.davidson.edu/jimn/Welcome.html


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