Catalytic formation of H_2 on carbonaceous dust grains - implications for interstellar observations

Abstract

We use kinetic Monte Carlo (KMC) simulations to study molecular hydrogen formation on carbonaceous dust grain surfaces, validated against recent laboratory measurements of H${}_2$ formation on coronene films at temperatures from 10 to 250 K. The model uses a three-dimensional amorphous carbon lattice with heterogeneous physisorption ($45\pm 5$ meV) and chemisorption ($1.75\pm 0.25$ eV) sites, and tracks both Langmuir--Hinshelwood (LH) and Eley--Rideal (ER) formation channels within a stochastic Gillespie event-driven framework. The model reproduces the measured efficiency curve within the experimental uncertainties, including the isothermal (constant surface temperature) measurements at 100 - 250 K. The simulations correctly describe the phase boundary between the LH and ER driven processes as functions of grain temperature and the observed crossover. Under interstellar medium conditions, 10 - 250 K and n = 10 - 10${}^4$ cm${}^3$, the model predicts three distinct regimes for the formation efficiency $\epsilon$, the fraction of impinging H atoms released as H${}_2$. At 10 K diffusion is slow and $\epsilon \approx 0.06$. Between 20 K and 80 K, LH dominates and $\epsilon \approx 0.28$. Above 150 K, an ER plateau at $\epsilon =0.19$ is sustained by chemisorption-trapped H atoms. The LH-to-ER crossover occurs between 100 and 120 K. At 100 K we observe a 16\% density-dependent stochastic enhancement, which rate-equation models cannot capture. At T${}_{dust}$ = 60 K, n = 10${}^3$ cm${}^3$ we find the ratio of H${}_2$ formation to free-fall time $t_{{\mathrm{H}}_2}/t_{\mathrm{ff}}\approx 0.93$, so dust-catalysed H${}_2$ chemistry can keep pace with gravitational collapse in high-redshift star-forming environments.

21 pages, 18 figures