Rotational coherence spectroscopy of molecules in helium nanodroplets: Reconciling the time and the frequency domains

TL;DR

This study measures and analyzes the rotational alignment of molecules in helium nanodroplets using time-resolved spectroscopy, confirming known constants for some molecules and providing new data for others, with results matching theoretical calculations.

Contribution

It introduces a rotational spectroscopy method for molecules in helium droplets that aligns experimental results with theoretical models and reports new rotational constants for CS$_2$ and I$_2$.

Findings

Rotational constants for OCS match IR spectroscopy values.

First experimental rotational constants for CS$_2$ and I$_2$ in helium droplets.

Alignment dynamics agree with gas-phase rotational Schrödinger equation calculations.

Abstract

Alignment of OCS, CS$_2$ and I$_2$ molecules embedded in helium nanodroplets is measured as a function of time following rotational excitation by a non-resonant, comparatively weak ps laser pulse. The distinct peaks in the power spectra, obtained by Fourier analysis, are used to determine the rotational, B, and centrifugal distortion, D, constants. For OCS, B and D match the values known from IR spectroscopy. For CS$_2$ and I$_2$, they are the first experimental results reported. The alignment dynamics calculated from the gas-phase rotational Schr\"{o}dinger equation, using the experimental in-droplet B and D values, agree in detail with the measurement for all three molecules. The rotational spectroscopy technique for molecules in helium droplets introduced here should apply to a range of molecules and complexes.