Following the excited state relaxation dynamics of indole and 5-hydroxyindole using time-resolved photoelectron spectroscopy

Time-resolved photoelectron spectroscopy was used to obtain new information about the dynamics of electronic relaxation in gas-phase indole and 5-hydroxyindole following UV excitation with femtosecond laser pulses centred at 249 nm and 273 nm. Our analysis of the data was supported by ab initio calculations at the coupled cluster and complete-active-space self-consistent-field levels. The optically bright (1)L(a) and (1)L(b) electronic states of (1)ππ∗ character and spectroscopically dark and dissociative (1)πσ∗ states were all found to play a role in the overall relaxation process. In both molecules we conclude that the initially excited (1)L(a) state decays non-adiabatically on a sub 100 fs timescale via two competing pathways, populating either the subsequently long-lived (1)L(b) state or the (1)πσ∗ state localised along the N-H coordinate, which exhibits a lifetime on the order of 1 ps. In the case of 5-hydroxyindole, we conclude that the (1)πσ∗ state localised along the O-H coordinate plays little or no role in the relaxation dynamics at the two excitation wavelengths studied.