Strong-field approximation for above-threshold ionization of polyatomic molecules
We consider above-threshold ionization of polyatomic molecules by a strong laser field within the molecular strong-field approximation (MSFA). A polyatomic molecule is modeled by an $(N+1)$-particle system, which consists of $N$ heavy atomic (ionic) centers and an electron. After the separation of the center-of-mass coordinate, the dynamics of this system is reduced to the relative electronic and nuclear coordinates. Two forms of the MSFA, one with the field-free and the other with the field-dressed initial molecular bound state, are derived. For neutral polyatomic molecules the ionization amplitude takes a simple form which allows interpretation as a multiple-slit-type interference. This is illustrated by a numerical example for the ozone molecule.