METHYL INTERNAL-ROTATION OF PHOTOEXCITED CHLOROTOLUENE MOLECULES

Laser-induced fluorescence excitation and dispersed fluorescence spect ra of jet-cooled chlorotoluene molecules were measured. Weak bands wer e 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 state s. Internal rotational levels observed in the spectra were well reprod uced 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 quit e 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 pote ntial barrier height increases, revealing that the methyl rotor is hin dered in the S-1 state. For o-chlorotoluene, no internal rotational ba nds were observed, probably because of the large steric hindrance and/ or the electronic properties of the Cl substituent. The contribution o f the methyl internal rotation to the nonradiative processes is discus sed using a comparison between the observed intensities and the Franck -Condon factors.