Nonsymmetric molecules driven by intense few-cycle laser pulses: Phase and orientation dependence of enhanced ionization

The ionization of nonsymmetric heteronuclear diatomic molecules by intense few-cycle laser pulses linearly polarized along the internuclear axis has been investigated. It is found that enhanced ionization (EI) occurs in nonsymmetric molecules and is accompanied by enhanced excitation (EE). We show that the nonsymmetric distribution of the electron cloud between the two nuclei leads to a strong dependence of EI and EE on the carrier envelope phase of few-cycle pulses, and on the orientation of the molecule parallel or antiparallel with the peak electric field of the pulse. This effect is as strong as the pulse duration is short and disappears with increasing pulse duration. The field ionization model, and mainly the energy level crossing mechanism, are used to explain these phase effects. The newly proposed energy level crossing mechanism, which is relevant to nonsymmetric molecules, occurs as the driving field moves the dressed ground and excited states closer to each other until their energy levels cross, leading to an enhancement of excitation and ionization. A semiclassical nonadiabatic model derived to interpret the level crossing mechanism also predicts the critical internuclear distance R-c at which EI, EE, and energy crossings occur as a function of charge asymmetry and laser intensity, in good agreement with quantum-mechanical simulations.