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 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 + hν → 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. Schematic of apparatus used to measure yields of anions following tunable vacuum-UV photoexcitation of an effusive jet of CH 3 X (X = F, Cl, Br).