Nonadiabatic Laser-Induced Alignment Dynamics of Molecules on a Surface

TL;DR

This study investigates how a sodium dimer on a helium droplet surface responds to a laser pulse, revealing surface-induced confinement effects that alter its rotational alignment dynamics compared to free molecules.

Contribution

It demonstrates the nonadiabatic laser-induced alignment of molecules on a surface and introduces a 2D quantum rotor model to describe the surface-constrained rotational dynamics.

Findings

Alignment dynamics are qualitatively different from gas phase molecules.

Surface interaction confines the molecule to a tangential plane.

A 2D quantum rotor model accurately describes the observed behavior.

Abstract

We demonstrate that a sodium dimer, Na$_{2}$($1^3\Sigma_{u}^+$), residing on the surface of a helium nanodroplet, can be set into rotation by a nonresonant 1.0 ps infrared laser pulse. The time-dependent degree of alignment measured, exhibits a periodic, gradually decreasing structure that deviates qualitatively from that expected for gas phase dimers. Comparison to alignment dynamics calculated from the time-dependent rotational Schr\"{o}dinger equation shows that the deviation is due to the alignment dependent interaction between the dimer and the droplet surface. This interaction confines the dimer to the tangential plane of the droplet surface at the point where it resides and is the reason that the observed alignment dynamics is also well-described by a 2D quantum rotor model.