Photodlssoclation and Geminate Recombination Dynamics of I , ' in Mass - Selected 12 - ( C 02 ) n Cluster Ions

We report size-dependent photodissociation and geminate recombination dynamics of the 1 1 chromophore in mass-selected I2-(CO2)" cluster ions, 0 I n I 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: 12--based photofragment ions, in which the photodissociated 12chromophore 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 I, is strongly dependent on the cluster ion size, varying from zero to unity over the size range studied. Picosecond pumpprobe studies reveal a transient bleach in the sequential two-photon absorption of these 12-(C02), cluster ions. A single laser pulse time-resolved experiment indicates that the time scale for absorption recovery is =30 ps for cluster ions with 9 I n I 13 and -15 ps for n = 15 and 16, with a dramatic decrease in absorption recovery time over the size range 13 I n I 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.