General characterization of partially oriented polar molecules by the time-frequency profile of high-order harmonic generation

Abstract

For polar molecules, the degree of orientation is an essential variable with many interesting manifestations in strong-field phenomena. However, there is no direct way to measure the orientation degree in experiments. On the other hand, current indirect methods based on high-order harmonic generation (HHG) require specific laser conditions for characterization. In this study, we propose a general method to probe the whole range of orientation degrees by calibrating the time profile of harmonics. To develop the method, we first study the sensitivity of the harmonic time profile to the degree of orientation and then present a detailed procedure to extract this quantity from the harmonic time profile, which in turn is obtained from the HHG measurements. In our study, the ``experimental'' HHG data emitted from CO molecules are simulated by numerically solving the time-dependent Schr\odinger equation. A strong point of the suggested method is its universal applicability with arbitrary laser pulse parameters. Moreover, not only the magnitude but also the sign of the degrees of orientation can be extracted accurately.