Probing the field-free orientation dynamics of polar molecules using laser-induced THz wave generation

Abstract

ABSTRACTHere, we propose a novel approach to probing the field-free orientation dynamics of an ensemble of polar molecules. This approach implies the measurement of the THz signal produced in a molecular target exposed to an intense femtosecond laser pulse. High sensitivity of the efficiency of ionisation-induced THz wave production to the molecular angular distribution is shown to be due to a strong dependence of the residual current produced in ionisation of a polar molecule on the angle θ between the electric field vector of the probe pulse and the permanent dipole moment of a molecule. The physics behind this dependence is the angle-dependent subcycle asymmetry of ionisation of a polar molecule combined with the effect of the Coulomb potential on the escaping electron. Using a CO molecule as a computational example, we show that when measured as a function of delay between pump and probe pulses, the mean residual current generated in molecular ensemble strongly correlates with the ⟨cosθ⟩ observable commonly used to characterise the degree of molecular orientation.