Subject: Absorption Cross Section

From: <gustavo$at$mercury.chem.pitt.edu>
Date: Sun, 19 Oct 2003 22:28:32 -0400 (EDT)

Dear CCL readers,
     I have read several papers about the calculation of the cross
sections for two-photon absorption (d). The value of d as a function
of the frequency of the incident photon (w) is related to the
imaginary part of the second hyperpolarizability gamma(-w;w,w,-w)
through

       d=(8*pi^2*hbar*w^2)/(n*c)^2*Im[gamma(-w;w,w,-w)]

where n is a refractive index and c is the speed of light.
     None of the papers I have read so far say where the equation
above came from. So my question is: Does anybody on this list know
the origin of the equation above? Reference? I am also curious to
know if there is a similar equation relating the imaginary part of
the polarizability alpha(-w;w) to a cross section for one-photon
absorption. Is this cross section simply the oscillator strength
for the one photon transition?
     Thank you very much in advance. I will summarize if I receive
any answer.
                                 Gustavo Moura
                           gustavo^at^mercury.chem.pitt.edu