Long-lasting field-free alignment of large molecules inside helium nanodroplets

Lars Christiansen,Benjamin Shepperson,Henrik Stapelfeldt,Adam S Chatterley,Constant Schouder,Mette Heidemann Rasmussen

doi:10.1038/s41467-018-07995-0

Lars Christiansen, Benjamin Shepperson + Show 4 more

Open Access

https://doi.org/10.1038/s41467-018-07995-0

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Journal: Nature Communications	Publication Date: Jan 11, 2019
Citations: 42	License type: open-access

Affiliation: Aarhus University

Abstract

Molecules with their axes sharply confined in space, available through laser-induced alignment methods, are essential for many current experiments, including ultrafast molecular imaging. For these applications the aligning laser field should ideally be turned-off, to avoid undesired perturbations, and the strong alignment should last long enough that reactions and dynamics can be mapped out. Presently, this is only possible for small, linear molecules and for times less than 1 picosecond. Here, we demonstrate strong, field-free alignment of large molecules inside helium nanodroplets, lasting >10 picoseconds. One-dimensional or three-dimensional alignment is created by a slowly switched-on laser pulse, made field-free through rapid pulse truncation, and retained thanks to the impeding effect of the helium environment on molecular rotation. The opportunities field-free aligned molecules open are illustrated by measuring the alignment-dependent strong-field ionization yield of dibromothiophene oligomers. Our technique will enable molecular frame experiments, including ultrafast excited state dynamics, on a variety of large molecules and complexes.

Highlights

Molecules with their axes sharply confined in space, available through laser-induced alignment methods, are essential for many current experiments, including ultrafast molecular imaging
If the laser field is switched on slowly as compared to the rotational period(s) of the molecules, it is possible to obtain one-dimensional (1D) alignment, where the most polarizable molecular axis is fixed in space, by a linearly polarized field or three-dimensional (3D) alignment[7], and where all three principal polarizability axes are fixed in space with an elliptically polarized field
For linear or symmetric top molecules, the degree of 1D alignment can be high during the revivals and, thereby, offers an opportunity to conduct molecular frame experiments under field-free conditions, which has been exploited in a range of studies[1,3,4,10,11]

Summary

Introduction

Molecules with their axes sharply confined in space, available through laser-induced alignment methods, are essential for many current experiments, including ultrafast molecular imaging For these applications the aligning laser field should ideally be turned-off, to avoid undesired perturbations, and the strong alignment should last long enough that reactions and dynamics can be mapped out. We show that unlike the gas-phase case, where such truncation leads to fast loss of alignment[18,19], the molecules inside He droplets remain aligned for 10 ps or longer owing to the impeding effect of the He environment on molecular rotation observed earlier[17,20,21] This period should be enough time for many potential dynamics processes. As the alignment pulse turns off, we observe that the parent ion Intensity (TW cm–2)

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Results

Conclusion