Dissociative photoionisation of H₂: proton kinetic energy spectra

The kinetic energy spectra of the protons produced in the dissociative photoionisation of H 2 and directed predominantly along the electric vector of a photon beam have been obtained at certain wavelengths between 19.8 and 40.8 eV. The time-of-flight method described employed a specially constructed wide aperture analyser with multichannel analysis to extract the very weak signals available. The system has been used to measure the energy distributions of the protons formed through the ground state of H 2 + and at higher energies, where two-electron excited states of H 2 and other states of H 2 + also contribute to proton production. Protons identified with the lowest 1 Sigma u + state (predominantly 2p sigma u 2s sigma g ) exhibit a peak in their kinetic energy distribution which appears to move to higher energies as the photon energy is increased. The contributions from direct and autoionising processes are assessed and related to other published results including electron impact data.