Photochemistry of phosgene in the solid phase: State-resolved dissociation dynamics

Abstract

The translational, rotational, and vibrational state distributions of CO (g) resulting from the single photon photodissociation of Cl2CO in the condensed phase at ∼90 K have been determined by time-of-flight (TOF) distribution measurement and resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The TOF distribution of CO (g) is bimodal. Internal state characterization of the slow channel reveals a completely thermalized origin, with a rotational temperature of Trot=88±5 K, which is equal to the translational temperature as well as the substrate temperature. We believe these slow CO molecules originate from photodissociation below the topmost surface of the molecular film and achieve thermal equilibrium with the substrate before escaping into the gas phase. Internal state characterization of the fast channel shows, on the other hand, an energetic origin: at hν=5.0 eV, the rotational distribution, with an overall flux-weighted mean rotational energy of 〈Erot〉=0.12±0.01 eV, is non-Boltzmann and can be approximated by a bimodal distribution with rotational temperatures of 210±40 K at low J″(s) and 2200±300 K at high J″(s); the relative vibrational population is Nν=1/Nν=0=0.33±0.05. Both rotational and translational distributions of fast CO show positive correlation with photon energy. These CO molecules must be promptly ejected into the gas phase, carrying nascent energetic information from the photodissociation reaction on the surface of the molecular film. For electronic excitation events that result in photodissociation, 74% of the excess excitation energy is distributed in the translational and internal motions of products (CO and Cl); only 26% of the available energy is converted to motions of surrounding molecules.