Vacuum-UV fluorescence spectroscopy of CCl3F, CCl3H and CCl3Br in the range 8–30 eV
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Colleges, School and Institutes
The fluorescence spectroscopy of CCl3X (X=F, H, Br) in the range 200–700 nm is reported, using vacuum-UV radiation in the range 8–30 eV from a synchrotron as a tunable photoexcitation source. Excitation spectra, with undispersed detection of the fluorescence, have been recorded at the Daresbury UK source with a resolution of 0.1 nm, corresponding to an average energy resolution of ca. 0.015 eV. Dispersed emission spectra in the range 200–700 nm have been recorded at the BESSY 1 Germany source with an optical resolution of 8 nm, following photoexcitation at the energies of the peaks in the excitation spectra. Action spectra, in which the vacuum-UV energy is scanned with detection of the fluorescence at a specific wavelength, have also been recorded at BESSY 1 with a resolution of 0.3 nm; thresholds for production of a particular excited state of a fragment are then obtained. Using single-bunch mode, lifetimes of all the emitting states that fall in the range ca. 3–100 ns have been measured. For photon energies in the range 8–12 eV, emission is due to both CCl2 Ã1B1–1A1 and CXCl Ã1A″–1A′. These products form by photodissociation of low-lying Rydberg states of CCl3X, and the thresholds for their production therefore relate to energies of the Rydberg states of the parent molecule. It is not possible to say whether the other products form as two halogen atoms or a diatomic molecule. For energies in the range 13–17 eV, emission is due to diatomic fragments; CCl A2Δ, CF B2Δ, CH B2Σ- and A2Δ, CBr A2Δ, and Cl2D′ 23Πg. From their threshold energies, there is now accumulated evidence that the excited state of CCl or CX forms in association with three isolated atoms. Our results yield no information on whether the three bonds in CCl3X* break simultaneously or sequentially. In the range 13–17 eV, Cl2* almost certainly forms in conjunction with ground-state CX+Cl. This ion-pair state of Cl2 also forms at higher excitation energies around 20 eV, probably with atomic products C+X+Cl. In no cases is emission observed from excited states of either the CCl3radical or the parent molecular ion, CCl3X+.
|Journal||Physical Chemistry Chemical Physics|
|Publication status||Published - 13 Jan 1999|