Strong-field ionization of polar molecules: Stark-shift-corrected strong-field approximation

We extend the molecular strong-field approximation for ionization, in the tunneling limit, to include systematically the linear and quadratic static Stark shifts of the ionizing molecular orbital. This approach, simple to implement, is capable of describing the essential physics of the process of strong-field ionization of oriented polar molecules by circularly polarized laser pulses. The modification of the molecular orbitals in strong fields is also discussed and in cases of extreme polarization of inner electrons a simple model is devised. We find very good agreement between the results from our model and those obtained in the recent experiment on ionizaton of oriented carbonyl sulphide and benzonitrile molecules [L. Holmegaard et al., Nature Phys. 6, 428 (2010)].