Visible-ultraviolet photodissociation of three chlorofuoromethanes

Chlorofluorocarbons (CFC's; with a general formula: C{dollar}sb{lcub}rm x{rcub}{dollar}F{dollar}sb{lcub}rm y{rcub}{dollar}Cl{dollar}sb{lcub}rm z{rcub}{dollar}) have posed a potential threat to the earth's precious ozone layer that protects us from harmful solar UV radiation. The destructive property of CFC's comes from their photodissociation product--the Cl atom--as a result of absorbing solar UV/VUV radiation. The atomic Cl fragments consume ozone through cyclic chain reactions. Since a single Cl atom can be involved in many chain reactions before it is consumed by other means, the destruction capability of CFC's is tremendous. At the altitudes where ozone destruction occurs, the gas density is sufficiently low such that single collision conditions exist. Consequently, factors that influence the efficiency of the binary collision, i.e., the translational energy and the electronic state of the Cl atom, should be relevant to the reaction cross section.; Photodissociation of three widely used chlorofluoromethanes: CFC-11 (CFCl{dollar}sb3{dollar}), CFC-12 (CF{dollar}sb2{dollar}Cl{dollar}sb2{dollar}), and CFC-13 (CF{dollar}sb3{dollar}Cl) was investigated at three VUV wavelengths (118 nm, 125 nm, and 187 nm). The atomic Cl fragments are probed by (2+1) resonance enhanced multiphoton ionization to identify their electronic states. Both spin-orbit states of Cl are observed (Cl: {dollar}sp2Psbsp{lcub}3/2{rcub}{lcub}circ{rcub}{dollar}; Cl{dollar}sp*: sp2Psbsp{lcub}1/2{rcub}{lcub}circ{rcub}).{dollar} The initial translational energies, resulting from photodissociation and carried by Cl{dollar}sp+{dollar}, are analyzed by TOF (Time-of-Flight) mass spectrometry. A forward convolution method is employed in simulating TOF mass spectra to extract these initial translational energy distributions. At 118 nm (10.5 eV) CF{dollar}sb3{dollar}Cl undergoes a parallel transition and the excited molecule dissociates via four channels. Two of them produce CF{dollar}sbsp{lcub}3{rcub}{lcub}*{rcub}(1Asbsp{lcub}1{rcub}{lcub}prime{rcub}){dollar} and Cl/Cl{dollar}sp*{dollar} fragments with 1.33/1.47 eV c.m. (center-of-mass) translational energy. The other two produce CF{dollar}sbsp{lcub}3{rcub}{lcub}*{rcub}(2Asbsp{lcub}1{rcub}{lcub}prime{rcub}{dollar} or {dollar}2Asbsp{lcub}2{rcub}{lcub}primeprime{rcub}{dollar}/1E{dollar}spprime){dollar} and Cl/Cl{dollar}sp*{dollar} fragments with almost zero translational energy. With decreasing excitation energy the former two channels become major processes at 125 nm (9.92 eV). Angular distributions indicate that the excitation is characterized by a perpendicular transition. There are no Cl fragments observed at 187 nm (6.63 eV). Photodissociation of both CF{dollar}sb2{dollar}Cl{dollar}sb2{dollar} and CFCl{dollar}sb3{dollar} at 118 nm and 125 nm showed Cl fragments with broad translational energy distributions characteristic of three-body dissociation. At 187 nm, the translational energy distributions of the Cl fragments are simulated by nearly isotropic angular distributions which suggest indirect dissociation processes.