Vacuum-UV negative photoion spectroscopy of CH3F, CH3Cl and CH3Br

Using tunable vacuum-UV radiation from a synchrotron, negative ions are detected by quadrupolar mass spectrometry following photoexcitation of three gaseous halogenated methanes CH3X (X = F, Cl, Br). The anions X-, H-, CX-, CHX- and CH2X- are observed, and their ion yields recorded in the range 8-35 eV. The anions show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically described as AB + h nu -> A(-) + B+ (+ neutrals). Absolute cross sections for ion-pair formation are obtained by calibrating the signal intensities with those of F-from both SF6 and CF4. The cross sections for formation of X- + CH3+ are much greater than for formation of CH2X- + H+. In common with many quadrupoles, the spectra of m/z 1 (H-) anions show contributions from all anions, and only for CH3Br is it possible to perform the necessary subtraction to obtain the true H-spectrum. The anion cross sections are normalised to vacuum-UV absorption cross sections to obtain quantum yields for their production. The appearance energies of X-and CH2X- are used to calculate upper limits to 298 K bond dissociation energies for D-o(H3C-X) and D-o(XH2C-H) which are consistent with literature values. The spectra suggest that most of the anions are formed indirectly by crossing of Rydberg states of the parent molecule onto an ion-pair continuum. The one exception is the lowest-energy peak of F- from CH3F at 13.4 eV, where its width and lack of structure suggest it may correspond to a direct ion-pair transition.