Investigation of electronically excited indole relaxation dynamics via photoionization and fragmentation pump-probe spectroscopy

Abstract

The studies herein investigate the involvement of the low-lying 1La and 1Lb states with 1ππ* character and the 1πσ* state in the deactivation process of indole following photoexcitation at 201 nm. Three gas-phase, pump-probe spectroscopic techniques are employed: (1) Time-resolved photoelectron spectroscopy (TR-PES), (2) hydrogen atom (H-atom) time-resolved kinetic energy release (TR-KER), and (3) time-resolved ion yield (TR-IY). Each technique provides complementary information specific to the photophysical processes in the indole molecule. In conjunction, a thorough examination of the electronically excited states in the relaxation process, with particular focus on the involvement of the 1πσ* state, is afforded. Through an extensive analysis of the TR-PES data presented here, it is deduced that the initial excitation of the 1Bb state decays to the 1La state on a timescale beyond the resolution of the current experimental setup. Relaxation proceeds on the 1La state with an ultrafast decay constant (<100 femtoseconds (fs)) to the lower-lying 1Lb state, which is found to possess a relatively long lifetime of 23 ± 5 picoseconds (ps) before regressing to the ground state. These studies also manifest an additional component with a relaxation time of 405 ± 76 fs, which is correlated with activity along the 1πσ* state. TR-KER and TR-IY experiments, both specifically probing 1πσ* dynamics, exhibit similar decay constants, further validating these observations. © 2014 AIP Publishing LLC.