Directional spinning of molecules with sequences of femtosecond pulses

TL;DR

This paper compares two ultrafast laser pulse techniques for controlling the direction of molecular rotation, demonstrating their effectiveness in inducing and manipulating rotational directionality and selectivity in molecules.

Contribution

It introduces and experimentally validates two methods—double kick and chiral pulse train—for directional molecular rotation control with quantum state selectivity.

Findings

Both methods produce significant rotational directionality.

Increasing pulses enhances control and state selectivity.

Counter-rotation of different molecular isomers is achievable.

Abstract

We present an analysis of two experimental approaches to controlling the directionality of molecular rotation with ultrashort laser pulses. The two methods are based on the molecular interaction with either a pair of pulses (a "double kick" scheme) or a longer pulse sequence (a "chiral pulse train" scheme). In both cases, rotational control is achieved by varying the polarization of and the time delay between the consecutive laser pulses. Using the technique of polarization sensitive resonance-enhanced multi-photon ionization, we show that both methods produce significant rotational directionality. We demonstrate that increasing the number of excitation pulses supplements the ability to control the sense of molecular rotation with quantum state selectivity, i.e. predominant excitation of a single rotational state. We also demonstrate the ability of both techniques to generate counter-rotation of molecular nuclear spin isomers (here, ortho- and para-nitrogen) and molecular isotopologues (here, 14N_2 and 15N_2).