Doubly excited states of molecular hydrogen: theoretical absorption and photodissociation cross sections

We report absorption and photodissociation cross sections of H 2 from the ground to the repulsive doubly excited, or superexcited, states Q 1 1 Σ u + (1), Q 1 1 Σ u + (2), Q 1 1 Π u (1) and Q 2 1 Π u (1). Ground and excited electronic wavefunctions and the excited potential energy functions were obtained at the configuration-interaction level from Hartree-Fock molecular orbitals of the corresponding valence state of lowest symmetry. The electronic dipole transition moments for each transition were calculated at the internuclear distance range 1.0⩽ R ⩽2.4 a 0 for the first time, and thus the Franck-Condon approximation was not invoked. The doubly excited states of hydrogen present a competition between direct dissociation and autoionization processes. Employing published resonance widths and a semiclassical approximation, we estimated the survival probability of each state. Combining the survival probability and the absorption cross sections we computed the dissociation cross sections for H(2 l ) production, and the latter were compared with available experimental results. Our dissociation cross sections are higher than the absolute experimental results in the range of the maximum, and possible reasons for this discrepancy are discussed.