Controlling the sense of molecular rotation: classical vs quantum analysis

TL;DR

This paper compares classical and quantum models of laser-induced molecular rotation control, showing they agree well under experimental conditions but diverge near quantum revival times, with both approaches effectively predicting ensemble properties.

Contribution

It provides a detailed comparison of classical and quantum descriptions for controlling molecular rotation with laser pulses, highlighting their agreement and differences.

Findings

Classical and quantum models agree on optimal pulse parameters and induced angular momentum.

Both approaches predict similar time-averaged molecular distributions.

Quantum and classical analyses diverge near quantum revival times.

Abstract

Recently, it was predicted theoretically and verified experimentally that a pair of delayed and cross-polarized short laser pulses can create molecular ensembles with a well defined sense of rotation (clockwise or counterclockwise). Here we provide a comparative study of the classical and quantum aspects of the underlying mechanism for linear molecules and for symmetric tops, like benzene molecules, that were used for the first experimental demonstration of the effect. Very good quantitative agreement is found between the classical description of the process and the rigorous quantum mechanical analysis at the relevant experimental conditions. Both approaches predict the same optimal values for the delay between pulses and the angle between them, and deliver the same magnitude of the induced oriented angular momentum of the molecular ensemble. As expected, quantum and classical analysis substantially deviate when the delay between pulses is comparable with the period of quantum rotational revivals. However, time-averaged characteristics of the excited molecular ensemble are equally good described by the these two approaches. This is illustrated by calculating the anisotropic time-averaged angular distribution of the double-pulse excited molecules, which reflects persistent confinement of the molecular axes to the rotation plane defined by two polarization vectors of the pulses.