Intra- and intermolecular vibrational energy relaxation of C-H overtone excited benzonitrile, para-difluorobenzene, and pyrazine in solution

Femtosecond IR-pump-UV-probe spectroscopy allows us to directly observe the intramolecular vibrational energy redistribution (IVR) and the intermolecular vibrational energy transfer (VET) of selectively excited aromatic molecules in solution (CF2ClCFCl2). In this article, we report global IVR and VET rate coefficients for benzonitrile, p-difluorobenzene, and pyrazine in a weakly interacting (nonpolar) solvent which are excited in overtones or combination bands of C-H stretch vibrations. The experimental findings are compared with recent results for benzene, toluene, and alpha,alpha,alpha-trifluorotoluene. While we have found a characteristic variation of the relaxation dynamics of the aromatic systems upon chemical substitution, the remarkable similarity of the time scales of IVR for a large group of molecules investigated is striking. Furthermore, the intermolecular vibrational relaxation is characteristically different for different chemical substitutions; however, the correlation of VET rates with the presence of low-frequency modes in the molecules is poor.