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 + h\(v\) \(\rightarrow\) 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.