Intramolecular energy transfer rates in photoexcited cluster ions: The photodissociation dynamics of CO−3⋅H2O and CO−3⋅CO2

Abstract

The photodissociation dynamics of CO−3⋅H2O and CO−3⋅CO2 have been investigated at photon energies of 2.13, 2.41, 2.54, and 2.71 eV. Experiments were conducted by crossing a mass-selected, 8 kV ion beam with a linearly polarized laser beam, and measuring the kinetic energy distributions of the charged photodissociation products. By varying the angle between the ion beam and the laser polarization vector, product angular distributions were obtained. The only ionic product observed from both systems was CO−3. The average energy partitioned into relative translation of the photofragments was determined to be ∼0.1 eV for CO−3⋅H2O and ∼0.07 eV for CO−3⋅CO2. In both cases, these kinetic energy release values were found to be nearly independent of photon energy. The small fraction of the available energy partitioned into kinetic energy of the photofragments indicates that the upper states of the transitions leading to photodissociation are bound, and that a substantial fraction of the available energy must be channeled into internal energy of the dissociating fragments. The angular distributions of CO−3 photoproducts from both CO−3⋅H2O and CO−3⋅CO2 were found to be extremely isotropic. Modeling the experimental data using statistical phase space theory shows that dissociation occurs prior to complete energy randomization, and provides a measure of the extent of energy randomization prior to dissociation. Comparison of theory and experiment indicates the photodissociation processes proceed by the following mechanism: (1) Photon absorption occurs via a transition localized on CO−3 moiety: CO−3 (2B1)⋅X+hν→CO−3 (2A1)⋅X, where X=CO2 or H2O. (2) CO−3(2A1)⋅X internally converts to CO−3(2B1)⋅X, with a high degree of vibrational excitation being localized on the CO−3 moiety. (3) The vibrational excitation localized on the CO−3 moiety begins to slowly randomize throughout the cluster ion. (4) Before complete energy randomization has occurred, CO−3(2B1)⋅X dissociates to CO−3 and X, with the nascent CO−3 product containing a significant amount of internal energy (∼1.4 eV). The time required for approximately 1.0 eV of vibrational energy localized in CO−3(2B1) to randomize throughout the CO−3⋅X cluster is at least 10−9±1 s.