Multiphoton ionization of the hydrogen molecule H₂

Generalized cross sections for two-, three-, and four-photon ionization of H₂ by linearly polarized laser light have been calculated. Three different situations are considered: the laser polarization being parallel or perpendicular to the internuclear axis and the case of an isotropical distribution of H₂ molecules. The main features of the spectra are discussed. In order to assess the reliability of the results, two completely different ab initio methods have been developed and used for the parallel orientation; good agreement is found between them. The impact of the present results on the question of alignment of molecules in laser fields is discussed.