Catalytic formation of H2 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 H2 formation on coronene films at temperatures from 10 to 250 K. The model uses a three-dimensional amorphous carbon lattice with heterogeneous physisorption (45 ± 5 meV) and chemisorption (1.75 ± 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 nH = 10–104 cm−3, the model predicts three distinct regimes for the formation efficiency ε, the fraction of impinging H atoms released as H2. At 10 K diffusion is slow and ε ≈ 0.06. Between 20 K and 80 K, LH dominates and ε ≈ 0.28. Above 150 K, an ER plateau at ε = 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 per cent density-dependent stochastic enhancement, which rate-equation models cannot capture. At Tdust = 60 K, nH = 103 cm−3 we find the ratio of H2 formation to free-fall time $t_{{\rm H}_2}/t_{\rm ff} \approx 0.93$, so dust-catalysed H2 chemistry can keep pace with gravitational collapse in high-redshift star-forming environments.