The interstellar formation and spectra of the noble gas, proton-bound HeHHe+, HeHNe+ and HeHAr+ complexes

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 HeH 3 + 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. INTRODUCTIONHelium and hydrogen make up nearly all of the observable matter in the universe leaving chemists to squabble over the remaining scraps. These scraps are what compose the planets, our bodies, and most other things engineered by human beings. Nearly all other processes depend upon atoms much more interesting than the first two on the periodic table. Even so, helium and hydrogen can engage in chemistry with one another almost certainly combining to make HeH + (Hogness & Lunn 1925). This cation should be produced in detectable amounts if for no other reason than the sheer abundance of the constituents in the interstellar medium (ISM) (Roberge & Dalgarno 1982). However, such an interstellar observation of this diatomic cation has yet to be reported in the literature. It was the analogous ArH + that has been observed toward various astronomical sources (Barlow et al. 2013;Schilke et al. 2014;Roueff, Alekseyev & Bourlot 2014;Neufeld & Wolfire 2016) making the argonium and not helonium (helium hydride) cation the first noble gas molecule detected in nature. The smaller and more abundant helium and even neon hydride cations have not been observed, yet.The chemistry of helium is likely the least voluminous for any of the elements between hydrogen and iron even in controlled laboratory conditions. However, helium will make complexes and form some bonds. Helium cationic clusters have been predicted, HemHn + clusters have been synthesized, dication complexes observed, and even hydrogen-like replacement structures analyzed (Frenking & Cremer 1990;Roth, Dopfer & Maier 2001;Grandinetti 2004;Savic et al. 2015;Zicler et al. 2016). In all cases, the issue is that the helium cation binding in any of these complexes is relatively weak making long-lifetime molecules and high enough abundances for observable interstellar spectra of such chemical co...

Cationic Noble-Gas Hydrides: From Ion Sources to Outer Space

“…Are there prospects to detect the Ng-H-Ng + and Ng-H-Ng' + in outer space (Fortenberry, 2017;Stephan and Fortenberry, 2017)? Assuming that the extreme conditions of some dense regions of the interstellar medium and planetary atmospheres may be sufficient to produce not only ArH + but also NeH + , Fortenberry (2017) suggested the conceivable formation of Ar-H-Ar + , Ne-H-Ne + , and Ar-H-Ne + from the exothermic reaction of NgH + with the H + 3 (Ng) (Ng = Ne, Ar) (vide infra) or with Ne or Ar atoms absorbed on polycyclic aromatic hydrocarbons (PAHs) (Rodríguez-Cantano et al, 2017).…”

A Molecular Candle Where Few Molecules Shine: HeHHe+

“…While there are a surprising number of possible hydro-helium species [11,13,14], none are as tantalizing as the triatomic molecule, HeHHe + . Even if this molecule also has no permanent dipole moment precluding any rotational observation, it has recently been shown quantum chemically to possess an exceptionally bright vibrational frequency (the antisymmetric stretch) at 1345.2 cm −1 /7.43 µm [15]. In fact, the astrophysical detection of the related HeH + molecule was actually done in the infrared even if for a rotational signature [8].…”

Formation of argon cluster with proton seeding

“…Charged clusters are on the other hand, easily studied using mass spectrometric techniques [15,16]. The first noble gas molecule observed in nature is the simple argon-proton cation (argonium, Ar-p) [17]. Argonium has been detected in the interstellar medium (ISM) toward various astronomical objects [18,19].…”