The Interstellar Formation and Spectra of the Noble Gas, Proton-Bound HeHHe+, HeHNe+, & HeHAr+ Complexes

TL;DR

This study uses quantum chemical methods to predict the spectra and stability of helium-containing proton-bound complexes, identifying potential astrophysical observation targets and their spectral features.

Contribution

It provides detailed spectral predictions and stability analysis for HeHHe+, HeHNe+, and HeHAr+ complexes, highlighting their astrophysical relevance.

Findings

HeHHe+ is strongly bound and has a bright IR mode at 7.4 microns.

HeHNe+ exhibits a similar IR mode near 6.9 microns.

HeHAr+ is less interactive but its fundamental frequency is near the argonium cation's observed frequency.

Abstract

The sheer interstellar abundance of helium makes any bound molecules or complexes containing it of potential interest for astrophysical observation. This work utilizes high-level and trusted quantum chemical techniques to predict the rotational, vibrational, and rovibrational traits of HeHHe+, HeHNe+, and HeHAr+. The first two are shown to be strongly bound, while HeHAr+ is shown to be more of a van der Waals complex of argonium with a helium atom. In any case, the formation of HeHHe+ through reactions of HeH+ with HeH3+ is exothermic. HeHHe+ exhibits the quintessentially bright proton-shuttle motion present in all proton-bound complexes in the 7.4 micron range making it a possible target for telescopic observation at the mid-IR/far-IR crossover point and a possible tracer for the as-of-yet unobserved helium hydride cation. Furthermore, a similar mode in HeHNe+ can be observed to the blue of this close to 6.9 microns. The brightest mode of HeHAr+ is dimmed due the reduced interaction of the helium atom with the central proton, but this fundamental frequency can be found slightly to the red of the Ar-H stretch in the astrophysically detected argonium cation.