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How to understand the negative two-photon transition strengt

Posted: 19 May 2014, 03:23
by lyzhao
Dear all,
How to understand the negative rotationally averaged two-photon transition strength (DeltaTPA)
I have calculated the two-photon absorption (TPA) of a system using the CCSD response approach. My system has a C2v symmetry. TPAs of three excited states for each irreducible representation have been calculated. For most of the excited states considered, I got the positive DeltaTPAs, while for some excited states, I got the negative DeltaTPAs. The negative DeltaTPA will lead to the negative TPA cross section, then to the negative TPA absorption coefficient (Beta). My question is:
whether the negative Beta can be related to the nonlinear saturable absorption process.

Any references and comments are appreciated.

Best regards.