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

The studies herein investigate the involvement of the low-lying L-1(a) and L-1(b) states with (1)pi pi* character and the (1)pi sigma* 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 (1)pi sigma* state, is afforded. Through an extensive analysis of the TR-PES data presented here, it is deduced that the initial excitation of the B-1(b) state decays to the L-1(a) state on a timescale beyond the resolution of the current experimental setup. Relaxation proceeds on the L-1(a) state with an ultrafast decay constant (<100 femtoseconds (fs)) to the lower-lying L-1(b) 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 (1)pi sigma* state. TR-KER and TR-IY experiments, both specifically probing (1)pi sigma* dynamics, exhibit similar decay constants, further validating these observations. (C) 2014 AIP Publishing LLC.