From populations to coherences and back again: a new insight about rotating dipoles

TL;DR

This paper provides experimental evidence linking free-induction decay (FID) emission in molecules to a reduction in rotational energy, revealing a new aspect of molecular coherence and energy transfer in spectroscopic processes.

Contribution

It demonstrates experimentally that terahertz FID emission causes a measurable decrease in rotational populations, offering new insights into molecular coherence and energy dynamics.

Findings

FID emission reduces excited rotational populations

Energy radiated via FID manifests as population decrease

Results applicable to broader coherent spectroscopy

Abstract

The process in which light is absorbed by an ensemble of molecules obeys the fundamental law of conservation of energy - the energy removed from the light resides in the molecular degrees of freedom. In the process of coherent emission from excited molecules known as free-induction decay (FID), spectroscopic measurements of the emitted radiation are often conducted in order to gain insight into molecular structure and behavior. However, the direct influence of the FID emission on its molecular source is not measured directly. In this work we present experimental evidence from the molecular rotor perspective of the consequences of terahertz-frequency FID emission from rotationally excited molecules. We show that when gas phase molecules transiently orient under field-free conditions, the energy radiated via FID is manifest as an abrupt reduction in excited rotational populations. The connection between coherent FID emission and stored energy leaves a particularly distinct signature in our measurements, but the results are generalizable throughout coherent spectroscopy and coherent control.