Probing the Kinetics of a Nonadiabatic Transition Initiating Out of Vibrationally Excited as Well as Ground State Modes with Femtosecond Time-Resolved Transient Gratings

Abstract

Femtosecond time-resolved population gratings are employed for observing the kinetics of vibrational cooling and internal conversion in the S1 state of β-carotene. Two independent laser pulses, which are tuned resonant to the S2 ← S0 optical transition induce transient population gratings into solutions of β-carotene. Because the S2 state lies approximately 6000 cm-1 above the S1 potential, the subsequent internal conversion to the S1 state generates a population grating of hot vibrational modes on the S1 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 S1 state population gratings under Bragg conditions. When the probe laser pulse is tuned to different wavelengths within the red flank of the Sn ← S1 absorption profile, excited vibrational states as well as the vibrational ground state modes in the S1 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 S1/S0 internal conversion increases for hot vibrational modes.