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 12-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 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.

Pump-probe techniques are used in conjunction with a tandem time-of-flight mass spectrometer to investigate the I* * *I-cage recombination dynamics following IF photodissociation in sizeselected 11 (CO,), cluster ions. The absorption recovery, which reflects the recombination and vibrational relaxation of the photodissociated IT, exhibits a strong cluster size dependence in the range of n= 13-15. Over this limited cluster size range, the absorption recovery time decreases from-40 ps (n<12) to-10 ps (n) 15). In addition, a recurrence is observed at ~2 ps in the absorption recovery of the larger clusters (II = 14-17). This feature results from coherent 1. * *Imotion following photodissociation. Measurement of the absorption recovery with both parallel and perpendicular pump-probe polarizations demonstrates that the pump and probe transition dipoles lie in the same direction. Analysis of the 1, transition dipole directions shows that the coherent motion takes place on the first two repulsive excited potential surfaces. The two-photon photofragment distribution reflects the solvent cage structure as a function of pump-probe delay time.

We report time-resolved photodissociation and geminate recombination dynamics of I2− in size-selected I2− Arn and I2−(CO2)n cluster ions by using ultrafast pump–probe techniques at 790 nm in conjunction with a tandem time-of-flight mass spectrometer. The absorption recovery, which reflects the time scale for photodissociation followed by recombination and vibrational relaxation of I2− inside the cluster, shows a strong dependence on the composition of the surrounding cluster solvent. The absorption recovery time for I2−(CO2)16 is ∼1 ps, whereas for I2−Ar20 it is ∼130 ps. This difference is discussed in terms of electrostatic and hard sphere interactions. We also observe the time dependence of the destruction of the Ar solvent cage for I2−Ar16. Finally, absorption recovery data for I2−(CO2)n cluster ions taken with 790 nm pump–probe wavelengths are compared with the greater energy release 720 nm data.

We report the photoionization of ion beam desorbed amino acids using femtosecond laser pulses at 195 and 260 nm. Ionization of ion-desorbed glycine, alanine, valine, leucine, isoleucine, and phenylalanine using 195 nm laser pulses is found to produce almost exclusively a decarboxylated ion fragment, while tyrosine and tryptophan produce a functional group cation. The adiabatic ionization potentials for the former amino acids correspond to the removal of an electron from the amine nitrogen atom, while for tyrosine and tryptophan it corresponds to the removal of an electron from the aromatic functional group. We find that fragmentation is initiated by the formation of a radical site which leads to an α-cleavage reaction and, depending upon which electron is removed, results in the formation of a decarboxylated ion or a functional group ion. These results indicate that a significant fraction of the amino acids are desorbed intact, but are fragmented when ionized. Except for leucine, isoleucine, and phenylalanine, the mass spectra produced by 260 nm irradiation are similar to the mass spectra produced by 195 nm irradiation. When using these two wavelengths, the desorbed amino acids exhibit a 2- to 8-fold higher ion yield than is found using only secondary ions produced directly by the incident ion beam. Tyrosine exhibits up to a 40-fold increase in signal using 195 nm irradiation.