PHOTODISSOCIATION AND GEMINATE RECOMBINATION DYNAMICS OF I2- IN MASS-SELECTED I2-(CO2)N CLUSTER IONS

We report size-dependent photodissociation and germinate recombination dynamics of the I2- chromophore in mass-selected I2-(CO2)n cluster ions, 0 less-than-or-equal-to n less-than-or-equal-to 22, by using nanosecond and picosecond pulsed laser sources in conjunction with a tandem time-of-flight mass spectrometer. Photoexcitation of these cluster ions results in the formation of two types of photofragment ions: I2--based photofragment ions, in which the photodissociated I2- chromophore has recombined and vibrationally relaxed, and I--based photofragment ions, in which an iodine atom has escaped the cluster ion. The "caging fraction", or quantum yield for formation of photofragment ions containing recombined I2-, is strongly dependent on the cluster ion size, varying from zero to unity over the size range studied. Picosecond pump-probe studies reveal a transient bleach in the sequential two-photon absorption of these I2-(CO2)n cluster ions. A single laser pulse time-resolved experiment indicates that the time scale for absorption recovery is almost-equal-to 30 ps for cluster ions with 9 less-than-or-equal-to n less-than-or-equal-to 13 and almost-equal-to 15 ps for n = 15 and 16, with a dramatic decrease in absorption recovery time over the size range 13 less-than-or-equal-to n less-than-or-equal-to 15. These results are interpreted in terms of recent Monte Carlo simulations performed in this laboratory and molecular dynamics studies performed by Amar and Perera.