Subject: question on vibronic coupling calculation

From: sergiusz kwasniewski <[email-masked]>
Date: Mon, 25 Mar 2002 14:54:58 +0100 (MET)

Hi everyone,

I'm trying to write a program to account for vibronic coupling on
UV/Visible absorption spectra. For the vibronic progression with
only one symmetric vibrational mode and up to 3 quanta, the Frank-Condon
approximation is fairly easy to implement. However, if I want to
include more vibrational modes, I encounter a small problem:

I(lambda) ~ SUM ( PRODUCT ( F.C.'s) * D )

where lambda is given in wavelength (nm), the sum is over the quanta (0-3),
and the product over the Frank-Condon factors corresponding to the different
vibrational modes.

Now: in order to describe the band shape factor D, I have to calculate
a factor (lambda - lambda(max))^2. How does this factor change while including
more than one vibrational frequency ?

What I found is: (lambda - (lambda(max) + SUM nu * omega))^2

where nu is the quantum (which is a constant in this summation) and omega
is the energy of the vibration, but then instead of having a series of 
6 additional peaks besides your lambda(max) (i.e. 3 per vibrational modes), 
these modes are coupled now 2 by 2 in order to give 3 additional peaks 
but with an almost doubled spacing (depending on the energies of the vibrations
off course).

Does anyone know if this is correct, the literature I found about vibronic
coupling doesn't explain this thoroughly.

I appreciate any kind of help !!

Serge

___________________________________________________

	Sergiusz Kwasniewski
	LUC SBG/TS
	Universitaire Campus Gebouw D
	3590 Diepenbeek
	BELGIUM

	tel(direct): 032 (0)11/268315
	fax	   : 032 (0)11/268301
	email      : [email-masked]
	http://www.luc.ac.be/Research/TheoChem
___________________________________________________