Molecular tunneling ionization of the carbonyl sulfide molecule by double-frequency phase-controlled laser fields

Abstract

We have investigated the orientation-selective molecular tunneling ionization of carbonyl sulfide (OCS) molecules induced by linearly polarized double-frequency phase-controlled laser fields consisting of a fundamental and a second-harmonic light with a pulse duration of 130 fs and an intensity of 5 \ifmmode\times\else\texttimes\fi{} 10${}^{13}$ W/cm${}^{2}$. We performed simultaneous measurements using gas mixtures of OCS and carbon monoxide to calibrate the relative phase difference of the phase-controlled fields and to verify the mechanism of the tunnel ionization. It is demonstrated that there is a definite correlation between the orientation of ionized molecules and the structure of the highest occupied molecular orbital. We have discussed the experimental results by means of the weak field asymptotic theory. In addition, we have analyzed the quantum dynamics of photoelectrons in simultaneous ion-electron detection. The experimental results can be explained by a two-step model including the interaction with the parent ion. The recollision process plays a minor role for determining the preferable directions of polar molecules in the tunneling ionization at the experimental laser intensity used.