NEW ACCURATE MEASUREMENT OF ³⁶ ArH ⁺ AND ³⁸ ArH ⁺ RO-VIBRATIONAL TRANSITIONS BY HIGH RESOLUTION IR ABSORPTION SPECTROSCOPY

Abstract

The protonated argon ion, 36 ArH + , was recently identified in the Crab Nebula from Herschel spectra. Given the atmospheric opacity at the frequency of its J = 1-0 and J = 2-1 rotational transitions (617.5 and 1234.6 GHz, respectively), and the current lack of appropriate space observatories after the recent end of the Herschel mission, future studies on this molecule will rely on mid-infrared observations. We report on accurate wavenumber measurements of 36 ArH + and 38 ArH + rotation-vibration transitions in the v = 1-0 band in the range 4.1-3.7 μm (2450-2715 cm −1 ). The wavenumbers of the R(0) transitions of the v = 1-0 band are 2612.50135 ± 0.00033 and 2610.70177 ± 0.00042 cm −1 (±3σ ) for 36 ArH + and 38 ArH + , respectively. The calculated opacity for a gas thermalized at a temperature of 100 K and with a linewidth of 1 km s −1 of the R(0) line is 1.6 × 10 −15 × N ( 36 ArH + ). For column densities of 36 ArH + larger than 1 × 10 13 cm −2 , significant absorption by the R(0) line can be expected against bright mid-IR sources.

ArH2+ and NeH2+ as global minima in the Ar+/Ne+ + H2 reactions: energetic, spectroscopic, and structural data

“…Single deuteration of the terminal hydrogen significantly reduces the anharmonicities, on the order of a half, and the ArDH + ν2 stretches are almost harmonic in comparison. Also, the ν2 Ar−H stretch is significantly lower in ArH2 + than it is in ArH + where the frequency is 2592.65 cm −1 or 3.857 microns (Cueto et al 2014) indicating that the Ar−H bond is stronger in the diatomic cation than it is in ArH2 + . However, the 590.5 cm −1 ν2 Ar−H stretch is notably higher than the comparable 350 cm −1 Ar−H2 stretch observed for the ArH2 neutral van der Waals complex (McKellar 1996).…”

“…It is unlikely that noble gas compounds containing krypton and xenon will be observed since these atoms are both larger than iron and exist in substantially smaller abun-dances. However, the large dipole moments exhibited by noble gas hydride cations (Cueto et al 2014) brought about from the drastic difference between the centre-of-mass (very close to the noble gas atomic centre) and the centre-ofcharge (roughly midway between the two atoms) make them very rotationally bright aiding in their potential detection as a class. Even so, other compounds containing He, Ne, and Ar are more likely to be observed in the ISM than those containing the heavier noble gas atoms.…”

“…The reasons for the difference in abundances of the two isotopes have been well documented (Cueto et al 2014), but inclusion of all three forms of argon is warranted in this study for the sake of completeness. The structural, spectroscopic, and vibrational values for the twelve different isotopologues (including deuteration) are given in Tables 5, 6, and 7.…”

“…From Table 4, the F11 force constant corresponds to the Ar−H stretch and is 1.895 279 mdyn/Å. Using the harmonic approximation from the vibrational frequency derived by Cueto et al (2014) (Roth et al 2008). Regardless of the strength of the bonds and the degree to which electrons are shared between the Ar atom and the H2 + cation, the rotational constants and other spectroscopic data for ArH2 + as provided in Tables 5, 6, and 7 can be applied to spectral analysis of the ISM or simulated laboratory experiments.…”

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

“…The ν1 proton shuttle motion now is actually more correctly described as the Ar−H stretch since the He−H stretching component coefficient in defining the potential (Cueto et al 2014). The frequency of the ν2 bend and the ν3 He−H stretch drop significantly relative to the other complexes further highlighting the relative weakness of the helium interactions with the argonium core.…”