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

The vacuum-ultraviolet (VUV) absorption and fluorescence spectroscopy of CF 2X2 (X=H, Cl, Br) in the range 190-690 nm is reported. Tunable vacuum-UV ra diation in the range 8-22 eV from synchrotron sources at either Daresbury, U.K. or BESSY1, Germany is used to excite the titled molecules. Fluorescenc e excitation spectra, with undispersed detection of the fluorescence, were recorded at Daresbury with a resolution of 0.1 nm. VUV absorption spectra a t a resolution of 0.08 nm, and dispersed emission spectra with an optical r esolution of 8 nm were recorded at BESSY1. Action spectra, in which the VUV energy is scanned with detection of the fluorescence at a specific wavelen gth, were also recorded at BESSY1 with a resolution of 0.3 nm; appearance e nergies for production of a particular emitting state of a fragment are the n obtained. Using the single-bunch mode of BESSY1, lifetimes of all emittin g states that fall in the range similar to3-80 ns have been measured. The p eaks in the VUV absorption spectra of CF2X2 are assigned to Rydberg transit ions. For CF2H2 below 11 eV, there is good agreement between the absorption and the fluorescence excitation spectra, whereas above 11 eV and for the w hole range 8-22 eV for CF2Cl2 and CF2Br2 there is little similarity. This s uggests that photodissociation to emitting states of fragment species repre sent minor channels. In the range 8-15 eV, emission is due mainly to CF2 (A ) over tilde B-1(1)-(X) over tilde (1)A(1) and weakly to CFX (A) over tilde (1)A"-(X) over tilde (1)A'. These products form by photodissociation of Ry dberg 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 (A) over tilde B-1(1) can only form with H-2, whereas fo r CF2Cl2 and CF2Br2 it is not possible to say whether the other products ar e 2X or X-2. For energies above similar to 15 eV, emission is due to diatom ic fragments; CF B (2)Delta and A (2)Sigma (+), CCl A (2)Delta, CH B (2)Sig ma (-) and A (2)Delta, Cl-2 and Br-2 D' 2 (3)Pi (g), and possibly CBr A (2) Delta. From their appearance energies, there is evidence that with the exce ption of CF B (2)Delta /CF2H2 where the ground state of HF must form, the e xcited state of CF, CCl, or CH forms in association with three atoms. Our r esults yield no information whether the three bonds in CF2X2* break simulta neously or sequentially. We suggest that the anomalous behavior of CF2H2, i n forming H-H or H-F bonds in unimolecular photofragmentation processes, re lates to the small size of the hydrogen atom, and hence, the unimportance o f steric effects in the tightly constrained transition state. In no cases i s emission observed from excited states of either the CF2X free radical or the parent molecular ion, CF2X2 . (C) 2001 American Institute of Physics.