Abstract

We report the first direct femtosecond infrared pump-probe experiments in higher alcohols on the s→p transition of the equilibrated solvated electron e_S (itself, a transient species). We employ a three laser pulse sequence (previously used for the hydrated electron),¹,² where a UV synthesis pulse generates excess electrons and, after a few nanosecond delay, an NIR pump pulse promotes the equilibrated ground-state electron to the excited p-state, with a tunable probe pulse monitoring subsequent spectral dynamics. Representative pump-probe absorption transients of solvated electrons in water and various alcohols are shown (Fig. 1-2). Transients obtained probing on the blue edge of the ground state absorption band (600 – 740 nm) show a fast, instrument-limited reduction in optical density (bleach) followed by a bleach recovery that occurs on two timescales, having both a fast, sub-picosecond component and a longer, > 1 ps component. Transients probing on the red edge of the ground state absorption band (950 nm) show a transient increase in optical density. Finally, transients probing in the intermediate spectral region (860 nm) show complicated non-monotonic behavior. The recovery of the initial bleach “overshoots” to yield a transient increased absorption, which decays on a picosecond timescale. The non-monotonic behavior of these transients indicates that other processes besides electronic relaxation, namely solvation dynamics, play a role in the observed spectral dynamics. These data contain information on the p-state to s-state internal conversion mechanism as well as the energy disposal and relaxation dynamics of the solvent molecules that are coupled to the electron. It is important to distinguish the three laser pulse experiment from the complimentary two-pulse approach of previous femtosecond experimental work on the solvated electron which involved an ultraviolet (UV) pump pulse to produce photoelectrons in the conduction band and a probe pulse to monitor the spectral dynamics due to trapping and subsequent relaxation.

© 1996 Optical Society of America