# question on vibronic coupling calculation

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
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Sergiusz Kwasniewski
LUC SBG/TS
Universitaire Campus Gebouw D
3590 Diepenbeek
BELGIUM
tel(direct): 032 (0)11/268315
fax : 032 (0)11/268301
email : sergiusz.kwasniewski.,at,.luc.ac.be
http://www.luc.ac.be/Research/TheoChem
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