H₂ Ortho–Para Spin Conversion on Inhomogeneous Grain Surfaces

Abstract

We investigate the evolution of the ortho-to-para ratio of overall (gas + ice) H 2 via the nuclear spin conversion on grain surfaces coated with water ice under physical conditions that are relevant to star-and planet-forming regions. We utilize the rate equation model that considers adsorption of gaseous H 2 on grain surfaces, which have a variety of binding sites with a different potential energy depth, thermal hopping, desorption, and the nuclear spin conversion of adsorbed H 2 . It is found that the spin conversion efficiency depends on the H 2 gas density and the surface temperature. As a general trend, enhanced H 2 gas density reduces the efficiency, while the temperature dependence is not monotonic; there is a critical surface temperature at which the efficiency is the maximum. At low temperatures, the exchange of gaseous and icy H 2 is inefficient (i.e., adsorbed H 2 does not desorb and hinders another gaseous H 2 to be adsorbed), while at warm temperatures, the residence time of H 2 on surfaces is too short for the spin conversion. Additionally, the spin conversion becomes more efficient with lowering the activation barriers for thermal hopping. We discuss whether the spin conversion on surfaces can dominate over that in the gas-phase in star-and planet-forming regions. Finally, we establish a simple, but accurate way to implement the H 2 spin conversion on grain surfaces in existing gas-ice astrochemical models.

Adsorption of H2on amorphous solid water studied with molecular dynamics simulations

“…Theoretical sticking coefficients at different gas temperatures. In red (3) and in blue (5). Temperature of the surface is 10 K Figure 9 Theoretical sticking probabilities as a function of the angle of incidence (θ ) for the H 2 /ASW system at 10 K.…”

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 k i ads = S v i σ dust are the adsorption rates of the species on the surface of grains, with S being the sticking coefficient (here assumed to be 1 for simplicity), v i the thermal gas speed, and σ dust the distribution-averaged grain geometrical cross-section. The efficiency of the process is regulated by the factor η i which represents the competition between the conversion process and desorption, and it is defined as (Furuya et al 2019)…”

“…However, since the inverse process is, in general, negligible at low temperatures (Bovino et al 2017), we opt for completely neglecting it in our simulations, which in practice corresponds to assuming γ = 0 (Bovino et al 2017). We note that our approach is based on a single average binding energy approximation, in particular E b = 600 K as reported in the literature (Perets & Biham 2006;Vidali & Li 2010;He & Vidali 2014), and refer to other recent works (Furuya et al 2019) for a more comprehensive treatment, which also includes the thermal hopping between different adsorption sites. The effect of these processes on the evolution of the H 2 OPR is discussed in Appendix C.…”

Experimental Approach to Physicochemical Hydrogen Processes on Cosmic Ice Dust

“…Another astronomically important issue related to hydrogen is the ortho-to-para ratio (OPR) of H2 molecules. [35][36][37][38][39][40] A hydrogen molecule consists of two protons, and thus the total nuclear spin becomes 0 or 1, which are referred to as the para and ortho states, respectively. According to the Pauli exclusion principle, the ortho and para hydrogen can only take odd and even numbers for the rotational state, respectively.…”