Vacuum–ultraviolet absorption and fluorescence spectroscopy of CF2H2, CF2Cl2, and CF2Br2 in the range 8–22 eV

Abstract

The vacuum–ultraviolet (VUV) absorption and fluorescence spectroscopy of CF2X2 (X=H, Cl, Br) in the range 190–690 nm is reported. Tunable vacuum–UV radiation in the range 8–22 eV from synchrotron sources at either Daresbury, U.K. or BESSY1, Germany is used to excite the titled molecules. Fluorescence excitation spectra, with undispersed detection of the fluorescence, were recorded at Daresbury with a resolution of 0.1 nm. VUV absorption spectra at a resolution of 0.08 nm, and dispersed emission spectra with an optical resolution of 8 nm were recorded at BESSY1. Action spectra, in which the VUV energy is scanned with detection of the fluorescence at a specific wavelength, were also recorded at BESSY1 with a resolution of 0.3 nm; appearance energies for production of a particular emitting state of a fragment are then obtained. Using the single-bunch mode of BESSY1, lifetimes of all emitting states that fall in the range ∼3–80 ns have been measured. The peaks in the VUV absorption spectra of CF2X2 are assigned to Rydberg transitions. For CF2H2 below 11 eV, there is good agreement between the absorption and the fluorescence excitation spectra, whereas above 11 eV and for the whole range 8–22 eV for CF2Cl2 and CF2Br2 there is little similarity. This suggests that photodissociation to emitting states of fragment species represent minor channels. In the range 8–15 eV, emission is due mainly to CF2 Ã 1B1–X̃ 1A1 and weakly to CFX Ã 1A″−X̃ 1A′. These products form by photodissociation of Rydberg states of CF2X2, and the thresholds for their production, therefore, relate to energies of the Rydberg states of the parent molecule. For CF2H2 below 11.18 eV CF2 Ã 1B1 can only form with H2, whereas for CF2Cl2 and CF2Br2 it is not possible to say whether the other products are 2X or X2. For energies above ∼15 eV, emission is due to diatomic fragments; CF B 2Δ and A 2Σ+, CCl A 2Δ, CH B 2Σ− and A 2Δ, Cl2 and Br2 D′ 2 3Πg, and possibly CBr A 2Δ. From their appearance energies, there is evidence that with the exception of CF B 2Δ/CF2H2 where the ground state of HF must form, the excited state of CF, CCl, or CH forms in association with three atoms. Our results yield no information whether the three bonds in CF2X2* break simultaneously or sequentially. We suggest that the anomalous behavior of CF2H2, in forming H–H or H–F bonds in unimolecular photofragmentation processes, relates to the small size of the hydrogen atom, and hence, the unimportance of steric effects in the tightly constrained transition state. In no cases is emission observed from excited states of either the CF2X free radical or the parent molecular ion, CF2X2+.