Behavior of molecular hydrogen exposed to strong dc, ac, or low-frequency laser fields: I. Bond softening and enhanced ionization

A three-dimensional, fully correlated ab initio method for calculating the ionization rates and Stark shifts of molecular hydrogen oriented parallel to an external electric or low-frequency electromagnetic field is presented. With its aid, the occurrence of bond softening and enhanced ionization in H-2 is investigated. Based on a simple quantum-mechanic explanation for those effects, their occurrence is also predicted for any neutral covalently bound molecule. The ab initio results are used to investigate the applicability of simpler numerical models for predicting those phenomena in larger molecules.