Scattering resonances in slow NH${}_3$-He collisions

TL;DR

This paper presents a detailed quantum mechanical study of slow NH3-He collisions, revealing pronounced scattering resonances and providing insights into their observation in future experiments.

Contribution

It introduces a new four-dimensional potential energy surface and analyzes scattering resonances in NH3-He collisions with quantum close-coupling calculations.

Findings

Pronounced shape and Feshbach resonances identified

Resonant structures analyzed via wavefunctions and phase shifts

Prospects for experimental observation discussed

Abstract

We theoretically study slow collisions of NH$_3$ molecules with He atoms, where we focus in particular on the observation of scattering resonances. We calculate state-to-state integral and differential cross sections for collision energies ranging from 10${}^{-4}$ cm$^{-1}$ to 130 cm$^{-1}$, using fully converged quantum close-coupling calculations. To describe the interaction between the NH${}_3$ molecules and the He atoms, we present a four-dimensional potential energy surface, based on an accurate fit of 4180 {\it ab initio} points. Prior to collision, we consider the ammonia molecules to be in their antisymmetric umbrella state with angular momentum $j=1$ and projection $k=1$, which is a suitable state for Stark deceleration. We find pronounced shape and Feshbach resonances, especially for inelastic collisions into the symmetric umbrella state with $j=k=1$. We analyze the observed resonant structures in detail by looking at scattering wavefunctions, phase shifts, and lifetimes. Finally, we discuss the prospects for observing the predicted scattering resonances in future crossed molecular beam experiments with a Stark-decelerated NH$_3$ beam.