Resonant dissociative photoionization of H2

Abstract

The absorption of photons with an energy of 26.9 eV by H₂ produces H⁺ ions with kinetic energies ranging up to 4.4 eV. We attribute the energetic H⁺ ions to absorption into a ¹Σ_u⁺ resonance state of H₂ which autoionizes into a continuum corresponding to a free electron moving in the field of the 1sσ_g core of H₂⁺. We present calculations of the real and imaginary parts of the complex potential energy surface of the resonance state and of the transition dipole moment between the ground state of H₂ and the resonance state. The resulting cross section for the production of energetic H⁺ ions and the calculated ratio of H⁺ to H₂⁺ ions produced are in agreement with measurements. The energy distribution of the H⁺ ions has a maximum at about 3.2 eV. The maximum cannot be reproduced by a classical description of the nuclear motion in the autoionizing resonance state, but it does arise when a quantal treatment is used, in which the nuclear motion is governed by the real part of the complex potential.