Vacuum UV spectroscopy and dynamics of CHF₃: analysis of emissions and evidence for parent ion emission

Abstract

The fluorescence observed following electron impact and vacuum UV photon excitation of fluoroform (CHF3) are studied using three techniques: (a) electron beam excitation of a supersonic molecular beam with dispersed fluorescence detection; (b) He* and Ne* rare-gas metastable excitation at room temperature with dispersed fluorescence detection; (c) photon excitation at room temperature using tunable vacuum UV radiation from a synchrotron source with undispersed fluorescence detection. Using the first two techniques, the spectra are dominated by CF2 Ã-[Xtilde] fluorescence. The spectra are continuous owing to the high density of rovibrational transitions, but a vibrational progression in the bending mode is observed in the rare-gas metastable experiment at high gas pressure. Using photons in the energy range 11–35 eV, three distinct fluorescence decay channels are observed: (1) CF3 fluorescence for photon energies 11–13 eV; (2) CF2 fluorescence for energies 14–18 eV; (3) CHF+ 3 fluorescence for energies greater than 20 eV. The shapes of the excitation functions confirm that (1) and (2) are resonant processes via Rydberg states of CHF3, but (3) is a non-resonant photoionisation process. The emitting state in CHF+ 3 is the [Dtilde]2A1 state, with a vertical ionisation potential of 20·5 eV. Using the synchrotron in its single-bunch mode, radiative lifetimes of the fluorescing electronic states of CF3, CF2 and CHF+ 3 have been measured to be 17, 52 and 80 ns respectively.