Journal of Physical Chemistry A, Vol.102, No.45, 8727-8733, 1998
Methyl internal rotation of photoexcited chlorotoluene molecules
Laser-induced fluorescence excitation and dispersed fluorescence spectra of jet-cooled chlorotoluene molecules were measured. Weak bands were observed near the origin for p- and m-chlorotoluene, and assigned to be the internal rotation of the methyl group in the S-0 and S-1 states. Internal rotational levels observed in the spectra were well reproduced by periodic potential with a free rotor basis set. The potential barrier heights for the S-0 and S-1 states of p-chlorotoluene are quite low, suggesting that the methyl rotor is almost free in both states. When m-chlorotoluene changes from the S-0 to the S-1 states, the potential barrier height increases, revealing that the methyl rotor is hindered in the S-1 state. For o-chlorotoluene, no internal rotational bands were observed, probably because of the large steric hindrance and/or the electronic properties of the Cl substituent. The contribution of the methyl internal rotation to the nonradiative processes is discussed using a comparison between the observed intensities and the Franck-Condon factors.
Keywords:INTRAMOLECULAR VIBRATIONAL REDISTRIBUTION, P-FLUOROTOLUENE;PHOTOFRAGMENTATION, ACCELERATION, FRAGMENT