Angular momentum changing transitions in proton-Rydberg atom collisions

TL;DR

This paper presents a precise, non-perturbative calculation of angular momentum changing collision rates between protons and Rydberg hydrogen atoms, crucial for astrophysical and plasma modeling, validated by classical simulations.

Contribution

It provides the first accurate expression for l-changing collision rates beyond dipole-allowed transitions, improving upon previous overestimations.

Findings

New rate coefficients are accurate for non-perturbative regimes.

Previous models overestimated rates by a factor of six.

Classical trajectory simulations confirm the new results.

Abstract

Collisions between electrically charged particles and neutral atoms are central for understanding the dynamics of neutral gases and plasmas in a variety of physical situaziones of terrestrial and astronomical interest. Specifically, redistribution of angular momentum states within the degenerate shell of highly excited Rydberg atoms occurs efficiently in distant collisions with ions. This process is crucial in establishing the validity of the local thermal equilibrium assumption and may also play a role in determining a precise ionization fraction in primordial recombination. We provide an accurate expression for the non-perturbative rate coefficient of collsions between protons and H(n_l) ending in a final state H(n_l'), with n being the principal quantum number and l,l' the initial and final angular momentum quantum numbers, respectively. The validity of this result is confirmed by results of classical trajectory Monte Carlo simulations. Previous results, obtained by Pengelly and Seaton only for dipole-allowed transitions, l--->l+-1, overestimate the l-changing collisional rate approximately by a factor of six, and the physical origin of this overestimation is discussed.