From chemistry-request@ccl.net Sun Sep 22 14:56:30 1991
Date: Sun, 22 Sep 91 14:30:00 EDT
From: Michael A. Lee <lee@nematic.kent.edu>
To: chemistry@ccl.net
Subject: optics,electronics, chemistry question
Status: R

Hello Chemistry list.

I have a question about chemistry which may not be particularly 
computational, but I need it for something computational.

I need to know about the characteristic times of mechanisms
for taking electromagnetic energy and converting it to thermal energy.

Let me describe the problem:
We do experimental and theoretical research on nonlinear optics of
liquid crystals.

Green laser light enters an organic liquid ( MW about 150, couple of
benzene rings with a cyano group and an alkyl tail ). The material
has a small linear absorption coefficient. It has a large nonlinear
absorption coefficient, possibly associated with 2 photon absorption.
In the optical energy range of 2 photons (uv) the material is
strongly absorbing.  About 20% of the energy from the laser goes into
absorption of some type which we are not sure we know.  This much
energy would raise the temperature of the material by a few degrees.

My question is, how fast can one expect to get energy through an electronic
absorption into vibrational and hence thermal modes.  The vibrational 
absorption energies in this material are all lower than the laser energy and
hence visually the material is essentially clear.

Heating effects in these materials are manifested as optical nonlinearities.
There appears to be some nonlinearity that acts in a few nanoseconds or less.
Heating is one of two candidates for explaining this fast nonlinearity.
Some of my colleagues say it is impossible to get energy out of an 
electronic state in nanoseconds, so "heating" is not the operative nonlinear
mechanism in the above experiments.  I know that "typical" atomic states
have such lifetimes, but I do not know about such states in liquids.

I would appreciate opinions and experiemntal or theoretical information
sources so I know whether I should give up looking into the thermal
mechanism.

Thanks,
Mike Lee (lee@nematic.kent.edu)
Physics, Kent State