TIME-RESOLVED STUDIES OF DYNAMICS IN MOLECULAR AND CLUSTER ANIONS

Femtosecond photoelectron spectroscopy (FPES) is used to study the tim e-resolved photodissociation dynamics of I-2(-)(CO2)(n=4,16) clusters excited at 780 nm. The FPES experiment on I-2(-)(CO2)(4) shows that th e I- fragment formed by excitation to the A'(2) Pi(1/2,g) repulsive st ate of I-2(-) 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 I-2(-) is known to be complete, FPES probes the recombination dynamics of the I-2(-) in considerable detai l. Specifically, vibrational relaxation on the (I2-X2) Sigma(u)(+) sta te and the accompanying evaporation on CO2 molecules can be followed i n real time. Vibrational relaxation is essentially complete by 10 ps, whereas solvent evaporation is not entirely complete by 200 ps. The sp ectra also show evidence for short-lived recombination on the I(2)(-)A (2) Pi(3/2,g) state. The results are compared to previous experimental results for I-2(-)(Ar)(n) clusters and recent simulations of cluster dynamics.