Temperature dependence of binary and ternary recombination of H3+ ions with electron

TL;DR

This study investigates how binary and helium-assisted ternary recombination rates of H3+ ions with electrons vary with temperature, providing experimental data and theoretical calculations for temperatures between 77 K and 330 K.

Contribution

It presents the first comprehensive measurement and theoretical analysis of ternary He-assisted recombination of H3+ ions across a broad temperature range, revealing differences between ortho- and para-H3+.

Findings

Binary and ternary recombination rates are characterized over 77-330 K.

Ternary recombination involves formation of a long-lived H3* molecule and subsequent helium collision.

Significant differences in recombination rates are found between ortho- and para-H3+ at low temperatures.

Abstract

We study binary and the recently discovered process of ternary He-assisted recombination of H3+ ions with electrons in a low temperature afterglow plasma. The experiments are carried out over a broad range of pressures and temperatures of an afterglow plasma in a helium buffer gas. Binary and He-assisted ternary recombination are observed and the corresponding recombination rate coefficients are extracted for temperatures from 77 K to 330 K. We describe the observed ternary recombination as a two-step mechanism: First, a rotationally-excited long-lived neutral molecule H3* is formed in electron-H3+ collisions. Second, the H3* molecule collides with a helium atom that leads to the formation of a very long-lived Rydberg state with high orbital momentum. We present calculations of the lifetimes of H3* and of the ternary recombination rate coefficients for para and ortho-H3+. The calculations show a large difference between the ternary recombination rate coefficients of ortho- and para-H3+ at temperatures below 300 K. The measured binary and ternary rate coefficients are in reasonable agreement with the calculated values.