Abstract
We examine establishing of a thermal equilibrium in a H3+-dominated plasma (produced by a microwave discharge ignited in He-Ar-H2 gas mixture) and in its afterglow at temperatures 77-220 K by Cavity Ring-Down Spectroscopy. With respect to the observed population of lowest rotational states of the ground vibrational state of H3+ ion, we conclude that the rotational temperature is well defined and stable.
It is equal to the He buffer gas temperature and to the kinetic temperature of the ions. By using para-enriched H2 gas, we achieve high population of para nuclear spin states of H3+ ions in the discharge.
This population is not constant in time and evolves towards a steady state (asymptotic) value which is not equal to the thermodynamic equilibrium value for a given temperature. The fact that the value of the steady state para-H3+ fraction does not vary with the concentration of para-enriched H2 indicates that degradation of para-enrichment of H2 in the discharge is negligible.