Towards state selective recombination of H3+ under astrophysically relevant conditions

Abstract

We present studies on the thermalisation of H₃⁺ ions in a cold He/Ar/H₂ plasma at temperatures 30–70 K. We show that we are able to generate a rotationally thermalised H₃⁺ ensemble with a population of rotational and nuclear spin states corresponding to a particular ion translational temperature. By varying the para-H₂ fraction used in the experiment we are able to produce para-H₃⁺ ions with fractional populations higher than those corresponding to thermodynamic values. At 35 K, only the lowest rotational states of para and ortho H₃⁺ are populated. This is the first step towards experimental studies of electron–molecular ion recombination processes with precisely specified quantum states at astrophysically relevant temperatures.