Vacuum-UV negative photoion spectroscopy of CH(3)F, CH(3)Cl and CH(3)Br.

Using tunable vacuum-UV radiation from a synchrotron, negative ions are detected by quadrupolar mass spectrometry following photoexcitation of three gaseous halogenated methanes CH(3)X (X = F, Cl, Br). The anions X(-), H(-), CX(-), CHX(-) and CH(2)X(-) 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 + hnu--> A(-) + B(+) (+ neutrals). Absolute cross sections for ion-pair formation are obtained by calibrating the signal intensities with those of F(-) from both SF(6) and CF(4). The cross sections for formation of X(-) + CH(3)(+) are much greater than for formation of CH(2)X(-) + H(+). In common with many quadrupoles, the spectra of m/z 1 (H(-)) anions show contributions from all anions, and only for CH(3)Br 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 CH(2)X(-) are used to calculate upper limits to 298 K bond dissociation energies for D(o)(H(3)C-X) and D(o)(XH(2)C-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 CH(3)F at 13.4 eV, where its width and lack of structure suggest it may correspond to a direct ion-pair transition.