Predominance of CF-stretch vibrational excitation in state resolved inelastic scattering of protons from 11 small fluorohydrocarbon molecules

Abstract

Time-of-flight spectra of H+ scattered from all ten different fluorinated hydrocarbon molecules of type CHmFn and C2HmFn (m+n=4) as well as C2F6 have been measured for small scattering angles (θlab≤15°) at 9.8 eV collision energy. For the pure hydrocarbons the spectra show a complicated structure suggesting unresolved excitation of several modes and a significant effect of charge transfer collisions. In contrast all the spectra for fluorine containing molecules exhibit more or less sharp maxima indicating high overtone excitation of predominantly one vibrational mode with ωv≂1200 cm−1 corresponding to a CF-stretch vibration. At some scattering angles the overtone transition probabilities deviate somewhat from a Poisson distribution suggesting small unresolved contributions from additional vibrational modes. Over- all, however, there is good agreement for the low symmetry molecules, some of which have sizable dipole moments, with the predictions of the simple forced oscillator model previously tested only for the spherically symmetric molecules CF4 and SF6. The relative magnitudes of the observed energy transfers are consistent with absolute infrared intensities where these are available from the literature.