Mechanisms of two-color laser-induced field-free molecular orientation

TL;DR

This paper compares two mechanisms of laser-induced molecular orientation using two-color lasers, showing that ionization depletion dominates at higher intensities and affects the timing of molecular revivals, which can be used to identify the mechanism.

Contribution

It provides a detailed comparison of hyperpolarizability and ionization depletion mechanisms for molecular orientation, highlighting the dominant role of ionization at practical intensities.

Findings

Ionization depletion leads to substantial molecular orientation.

Hyperpolarizability mechanism produces only small orientation effects.

Revival timings differ between the two mechanisms and can be used experimentally.

Abstract

Two mechanisms of two-color (\omega + 2\omega) laser-induced field-free molecular orientation, based on the hyperpolarizability and ionization depletion, are explored and compared. The CO molecule is used as a computational example. While the hyperpolarizability mechanism generates small amounts of orientation at intensities below the ionization threshold, ionization depletion quickly becomes the dominant mechanism as soon as ionizing intensities are reached. Only the ionization mechanism leads to substantial orientation (e.g. on the order of |<cos\theta>| > 0.1). For intensities typical of laser-induced molecular alignment and orientation experiments, the two mechanism lead to robust, characteristic timings of the field-free orientation wave-packet revivals relative to the the alignment revivals and the revival time. The revival timings can be used to detect the active orientation mechanism experimentally.