ABSTRACT
Photoionization of H2 plays an important role in interstellar clouds, planetary atmospheres and plasma physics. In this process, the photon energy can be shared between electronic and nuclear degrees of freedom. This is at variance with atomic photoionization where the photon energy is entirely absorbed by the electrons. Put in a different way, the outgoing electron may absorb part of the total radiant energy, leaving Lhe rest to the residual ion that can be now in an excited vibrational state. For large enough photon energies, the residual ion may also dissociate into H + H 1 , thus leading to the process called dissociative photoionization. Although contribution of the latter process to the total photoionization cross section is very small (-:::: 5% or less of the total cross section), there has been a significant effort to understand this dissociative process (sec, for instance, Refs. [1, 2, 3, 4, 5, G, 7, 8] and references therein). Afi expected, when the photon energy is large enough to populate the don Illy-excited states of the molecule, the kinetic energy distribution (KED) of ejected protons exhibit resonant peaks. This is the consequence of the coupling between doubly-excited states and the non resonant background. An additional complication is the interference with the nuclear motion, which leads to resonance peaks whose shapes and positions are difficult to predict, and has led quite often to incorrect assigments of the structures observed experimentally.
