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

Abstract

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 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.