Photodissociation of chlorine molecules adsorbed on amorphous and crystalline water ice films

The photodissociation dynamics of chlorine molecules adsorbed on amorphous and crystalline water ice films was investigated at 351 nm. The ice films were prepared on a gold polycrystalline substrate at 80-140 K. Time-of-flight spectra of the photofragment chlorine atoms, measured with the resonance-enhanced multiphoton ionization technique, were simulated with a composite of two translational energy distributions: a Gaussian distribution and a flux-weighted Maxwell-Boltzmann distribution. For both amorphous and crystalline ice films, the Gaussian distribution is characterized by the average energy 〈E_t〉 = 0.38 ± 0.02 eV, while the Maxwell-Boltzmann one by 〈E_t〉 = 0.12 ± 0.01 eV. The Gaussian distribution is attributable to the chlorine atoms produced from the direct photodissociation of Cl₂, while the Maxwell-Boltzmann characterizes those having undergone strong relaxation processes. The observed translational energy distributions for amorphous and crystalline ice films were similar to each other, but the relative contribution of the two energy distributions as well as the photodissociation yield of Cl atoms depend on the states of the ice films. Free OH groups and surface morphology of an ice film surface have a strong influence on the photodissociation quantum yield. The adsorbate-water interaction for Cl₂ and an ice surface is discussed on the basis of the measurements of time-of-flight, infrared absorption, and temperature-programmed desorption spectra.