Orientation and Alignment Echoes

TL;DR

This paper introduces a simple classical system of free rotors that exhibits multiple orientation and alignment echoes after impulsive excitation, supported by experimental demonstration in a gas of CO2 molecules.

Contribution

It provides a clear mechanism for echo formation via phase space filamentation and presents the first experimental observation of classical alignment echoes in a thermal gas.

Findings

Multiple echoes and fractional echoes observed after impulsive kicks.

Phase space filamentation explains the echo formation.

Experimental demonstration in CO2 gas confirms theoretical predictions.

Abstract

We present what is probably the simplest classical system featuring the echo phenomenon - a collection of randomly oriented free rotors with dispersed rotational velocities. Following excitation by a pair of time-delayed impulsive kicks, the mean orientation/alignment of the ensemble exhibits multiple echoes and fractional echoes. We elucidate the mechanism of the echo formation by kick-induced filamentation of phase space, and provide the first experimental demonstration of classical alignment echoes in a thermal gas of CO_2 molecules excited by a pair of femtosecond laser pulses.