Effect of vibration and internuclear axis orientation on multiphoton ionization of H2

Multiphoton processes in H2+ induced by a linearly polarized monochromatic laser have been studied within the framework of lowest-order perturbation theory. To illustrate the convergence of the one-centre B-spline expansion used, we present results for fixed internuclear distance. In a further step the vibrational motion has been taken into account using the reflection approximation. The frequency dependence of the ionization cross section has been calculated for the parallel as well as perpendicular orientation of the molecule with respect to the polarization vector of the laser field. It is found that in the multiphoton, in contrast to the tunnelling regime, ionization occurs dominantly in the perpendicular orientation, if vertical transitions from low-lying vibronic states are considered.