The molecular and dissociative photoionization of ethane, propane, and n-butane: absolute oscillator strengths (10–80 eV) and breakdown pathways

Abstract

Absolute partial photoionization oscillator strengths for the molecular and dissociative photoionization channels of the linear alkanes, C 2H 6, C 3H 8, and C 4H 10, have been determined using dipole (e, e+ion) coincidence spectroscopy (1 eV fwhm) at equivalent photon energies from the first ionization threshold up to 80 eV to C 2H 6, and up to 50 eV for C 4H 10. These have been derived from the triple product of previously published absolute photoabsorption oscillator strengths. (J.W. Au et al., Chem. Phys. 173 (1993) 209), and the photoionization efficiencies and ionic photofragmentation branching ratios obtained from time-of-flight mass spectra reported in the present work. The broad nature of some of the fragment ion peaks at higher equivalent photon energies suggests that they arise from Coulomb explosion processes involving the energetic dissociation of short-lived doubly charged ions. Consideration of the presently reported data together with ionization potentials from photoelectron and electron momentum spectroscopies yields quantitative information on the breakdown pathways of C 2H 6, C 3H 8, and C 4H 10 following absorption of radiation in the VUV and soft X-ray regions. The molecular ion and the larger fragments arise mainly from outer valence ionization. The smaller fragment ions are found to arise from decomposition of the inner valence ion states, with some contributions evidently arising from higher energy many-body (satellite) states.