Transition between mechanisms of laser-induced field-free molecular orientation

TL;DR

This paper investigates the transition between two mechanisms of laser-induced molecular orientation in CO, identifying hyperpolarizability-driven orientation at lower intensities and ionization depletion at higher intensities, with implications for calibrating ionization rates.

Contribution

It experimentally distinguishes and characterizes two regimes of laser-induced molecular orientation in CO, linking them to specific physical mechanisms and proposing a method for calibrating ionization rates.

Findings

Hyperpolarizability causes orientation up to 8 x 10^13 W/cm^2.

At higher intensities, ionization depletion dominates orientation.

Measurement of orientation revival times matches theoretical predictions.

Abstract

The transition between two distinct mechanisms for the laser-induced field-free orientation of CO molecules is observed via measurements of orientation revival times and subsequent comparison to theoretical calculations. In the first mechanism, which we find responsible for the orientation of CO up to peak intensities of 8 x 10^13 W/cm^2, the molecules are impulsively oriented through the hyperpolarizability interaction. At higher intensities, asymmetric depletion through orientation-selective ionization is the dominant orienting mechanism. In addition to the clear identification of the two regimes of orientation, we propose that careful measurements of the onset of the orientation depletion mechanism as a function of the laser intensity will provide a relatively simple route to calibrate absolute rates of non-perturbative strong-field molecular ionization.