Photochemistry of phosgene in the solid phase: Dissociation, ejection, and thermal desorption

Understanding photochemistry and energy transfer mechanisms in molecular solid films is of interest to many scientific issues, ranging from matrix-assisted laser desorption ionization mass spectrometry to photochemical processes on polar stratospheric cloud particles. We present a study of a model system: the photochemistry (hv= 1.2-6.4 eV) of a molecular ClCO solid film at low laser power density, 10 μJ-1 mJ/cm for ∼10 ns pulses. At hv≥3.5 eV, photon absorption by ClCO leads to a major photodissociation channel resulting in CO (g) and Cl (g) and a minor molecular ClCO ejection channel. Both photodissociation and molecular ejection are observed at the lowest laser power density and their yields depend linearly on pulse energy. This result establishes a single photon photoexcitation mechanism. The electronically excited ClCO in the surface region of the solid film can either dissociate or convert its electronic energy to translational motion in ClCO. The translational energy distribution of CO (g) from the photodissociation channel is bimodal: the flux-weighted mean translational energy of the fast channel is photon energy dependent (〈E〉=210, 135, and ∼90 meV at hv=6.4, 5.0, and 3.5 eV, respectively), while the slow channel is independent of photon energy and corresponds to completely thermalized CO molecules (〈E/2k〉=84±3 K). The mean translational energy of photoejected ClCO is 〈E〉=220±20 meV. In addition to photoejection, there is also a distinctively different thermal desorption channel due to transient laser heating. © 1996 American Institute of Physics.