Time-resolved studies of dynamics in molecular and cluster anions

Abstract

Femtosecond photoelectron spectroscopy (FPES) is used to study the time-resolved photodissociation dynamics of I₂^-(CO₂)_n=4,16 clusters excited at 780 nm. The FPES experiment on I₂^-(CO₂)₄ shows that the I^- fragment formed by excitation to the A′²Π_1/2,g repulsive state of I₂^- initially pulls away from the cluster, but by 0.2 ps it is drawn back to complex with more of the solvent molecules. In the n=16 cluster, where caging of the I₂^- is known to be complete, FPES probes the recombination dynamics of the I₂^- in considerable detail. Specifically, vibrational relaxation on the I₂^- X²Σ_u⁺ state and the accompanying evaporation on CO₂ molecules can be followed in real time. Vibrational relaxation is essentially complete by 10 ps, whereas solvent evaporation is not entirely complete by 200 ps. The spectra also show evidence for short-lived recombination on the I₂^- A²Π_3/2,g state. The results are compared to previous experimental results for I₂^-(Ar)_n clusters and recent simulations of cluster dynamics.