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 + hν → 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 Do(H3C–X) and Do(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.