Multiphoton ionization of a molecule calculated by means of a nonorthogonal system of wave functions

A study is made of the transition of a molecular electron from a bound to a free state under the influence of an alternating electric field produced by radiation whose photon energy is less than the adiabatic ionization potentials of the molecule but greater than or of the order of the energy of the normal vibrational transitions. In this case the adiabatic approximation for the separation of the electron and the vibrational motion is still valid. The electron motion is represented by means of a system of nonorthogonal wave functions, the effect of the external field on the free-electron states being allowed for exactly. The Hamiltonian of the vibrational motion is obtained in this representation and it is shown that allowance for this motion leads to a molecule-specific ionization effect caused by the joint tunneling of an electron and nuclei through the vibrational potential barrie r formed in the external field.