The vacuum ultraviolet photodissociation of the chlorofluorocarbons. Photolysis of CF3Cl, CF2Cl2, and CFCl3 at 187, 125, and 118 nm

Abstract

Photofragmentation of the chlorofluorocarbons, CF3Cl, CF2Cl2, and CFCl3, was investigated at 187, 125, and 118 nm using VUV harmonic generation techniques and (2+1) resonantly enhanced multiphoton ionization detection of ground Cl(2P3/20) and excited Cl*(2P1/20) state fragment atoms. Product translational energy and angular distributions were derived from Cl+ arrival time distributions obtained by time-of-flight mass spectrometry. Photolysis of CF3Cl at 125 and 118 nm takes place via the 4s(a1) and 4p(e) Rydberg states, respectively, and two primary fragmentation channels are observed. A ‘‘slow’’ channel with a most probable center-of-mass (c.m.) translational energy near zero is assigned to the production CF*3 radicals in the 2A′1, 2A″2, and 1E′ electronically excited states. The second Cl/Cl* fragmentation channel has a c.m. translational energy distribution peaked at Etr≥1 eV and is tentatively assigned to a sequential dissociation process in which rapid C–Cl single bond rupture is followed by a secondary fragmentation of CF*3 to CF2+F. The time-of-flight (TOF) spectra for CF2Cl2 and CFCl3 following excitation at 125 and 118 nm suggest that concerted three-body fragmentation involving the loss of two Cl/Cl* atoms is the dominant dissociation process. By contrast, photolysis of CF2Cl2 and CFCl3 at 187 nm results in structured Cl+ arrival time distributions which are used to derive translational energy distributions and asymmetry parameters. Simulations of the TOF spectra suggest the presence of three Cl/Cl* fragment channels, with the highest energy channel clearly attributable to single C–Cl bond rupture leading to internally excited molecular fragments. The contribution of sequential and simultaneous two-Cl loss processes to the low translational energy channels is also discussed.