Quantum resonances in selective rotational excitation of molecules with a sequence of ultrashort laser pulses

TL;DR

This paper demonstrates how quantum resonance enhances rotational excitation in diatomic molecules using ultrashort laser pulse trains, enabling isotope and spin-selective control of molecular rotation.

Contribution

It experimentally shows the effect of quantum resonance on rotational energy transfer and introduces a method for selective excitation of isotopologues and nuclear spin isomers.

Findings

Energy transfer peaks at quantum resonance conditions.

Selective rotational excitation of nitrogen isotopologues achieved.

Spin-selective excitation of para- and ortho- nitrogen isomers demonstrated.

Abstract

We investigate experimentally the effect of quantum resonance in the rotational excitation of the simplest quantum rotor - a diatomic molecule. By using the techniques of high-resolution femtosecond pulse shaping and rotational state-resolved detection, we measure directly the amount of energy absorbed by molecules interacting with a periodic train of laser pulses, and study its dependence on the train period. We show that the energy transfer is significantly enhanced at quantum resonance, and use this effect for demonstrating selective rotational excitation of two nitrogen isotopologues, $ ^{14}N_2$ and $ ^{15}N_2$. Moreover, by tuning the period of the pulse train in the vicinity of a fractional quantum resonance, we achieve spin-selective rotational excitation of para- and ortho-isomers of $ ^{15}N_2$.