Rapid Ortho-to-para Nuclear Spin Conversion of H₂ on a Silicate Dust Surface

Abstract

The H2 molecule has two nuclear spin isomers, the so-called ortho and para isomers. Nuclear spin conversion (NSC) between these states is forbidden in the gas phase. The energy difference between the lowest ortho and para states is as large as 14.7 meV, corresponding to ∼170 K. Therefore, each state of H2 differently affects not only the chemistry but also the macroscopic gas dynamics in space; thus, the ortho-to-para abundance ratio (OPR) of H2 has significant impacts on various astronomical phenomena. For a long time, the OPR of nascent H2 upon formation on dust grains has been assumed to have a statistical value of 3 and to gradually equilibrate in the gas phase at the temperature of the circumstances. Recently, the NSC of H2 was experimentally revealed to occur on water ice at very low temperatures and thus incorporated into gas–dust chemical models. However, H2 molecules should form well before dust grains are coated by water ice. Information about how the OPR of H2 behaves on bare silicate dust before ice-mantle formation is lacking. Knowing the influence of the OPR of H2 is desirable, if the OPR changes even on a bare silicate surface within an astronomically meaningful timescale. We report the first laboratory measurements of the NSC of H2 physisorbed on amorphous silicate (Mg2SiO4) at temperatures up to 18 K. The conversion was found to occur very rapidly.

The Isotopic Links from Planet Forming Regions to the Solar System

“…Even when the H 2 OPR is small, ∼10 −3 , H 2 rather than CO is the main destroyer of H 2 D + and thus deuterium fractionation can be slowed down (Furuya et al 2015). When H 2 is formed by grain surface reactions, the H 2 OPR corresponds to the statistical value of three (Watanabe et al 2010), but its value can fall over time via nucluear spin conversion in the gas phase, through proton transfer reactions (e.g., o-H 2 + H + p-H 2 + H + ) (Hugo et al 2009;Honvault et al 2011), and on grain surfaces (Ueta et al 2016;Tsuge et al 2019Tsuge et al , 2021. Astrochemical models with nucluear spin conversion of H 2 at the formation stage of molecular clouds predict that the H 2 OPR is already much lower than the statistical value of three, when the main component of the gas becomes H 2 molecules (Furuya et al 2015;Lupi et al 2021).…”

Graph-|Q⟩⟨C|, a Graph-Based Quantum/Classical Algorithm for Efficient Electronic Structure on Hybrid Quantum/Classical Hardware Systems: Improved Quantum Circuit Depth Performance

“…In order to gauge the accuracy and reduction in computational complexity arising from our Graph-| Q ⟩⟨ C | method presented above, we have applied this approach to a range of hydrogen molecular cluster problems. These systems are critical for applications related to energy storage. − In particular, the safe and efficient storage ,− of molecular hydrogen is of paramount importance to potential developments in new fuel cell technologies. − Furthermore, the study of ortho- and para-hydrogen − at low-temperatures has been a fundamental challenge that has implications toward the study of exotic new states of matter that may have important applications in low-temperature physics. − …”

On the low ortho-to-para H2 ratio in star-forming filaments

“…Recent experimental works both on amorphous solid water (Ueta et al 2016) and on bare silicates (Tsuge et al 2021), show the efficiency of the ortho-to-para H 2 conversion on the surface of dust grains. To consistently follow this process within our simulations and evaluate its overall impact on the OPR, we employ state-of-the-art frameworks (Bovino et al 2017;Furuya et al 2019) in which the conversion rates in units of s −1 are defined as…”

“…where the desorption time is calculated as the minimum between the thermal desorption time and the cosmic-ray induced desorption time, τ conv is the experimental conversion time (Tsuge et al 2021) fitted as τ conv = 6.3 × 10 4 T −1.9 dust , and γ = 9 exp (−170.5/T dust ) is the thermalised value of the H 2 OPR, assuming the energy difference between o-H 2 and p-H 2 on the grains is the same as that in the gas phase. This allows us to include both the ortho-to-para conversion and its inverse process (para-to-ortho) in the H 2 rate equations.…”