Probing the kinetics of a nonadiabatic transition initiating out of vibrationally excited as well as ground state modes with femtosecond time-resolved transient gratings

Femtosecond time-resolved population gratings are employed for observing the kinetics of vibrational cooling and internal conversion in the S-1 state of beta-carotene. Two independent laser pulses, which are tuned resonant to the S-2 <-- S-0 optical transition induce transient population gratings into solutions of beta-carotene. Because the S-2 state lies approximately 6000 cm(-1) above the S-1 potential, the subsequent internal conversion to the S-1 state generates a population grating of hot vibrational modes on the S-1 potential. This is followed by vibrational cooling and an internal conversion to the electronic ground state. The kinetics associated with these processes are interrogated by scattering a third, time-variable probe pulse off the S-1 state population gratings under Bragg conditions. When the probe laser pulse is tuned to different wavelengths within the red flank of the S-n <-- S-1 absorption profile, excited vibrational states as well as the vibrational ground state modes in the S-1 potential can selectively be monitored. This is verified by characterizing the contribution of vibrational cooling to the acquired kinetics. With a comparison between the kinetics observed for excited and ground state vibrational modes, it can be shown that the rate of the S-1/S-0 internal conversion increases for hot vibrational modes.