Chemical desorption of methanol induced by hydrogen abstraction–addition cycles on cold grain surfaces

Abstract

We report ultra-high vacuum experiments on the non-thermal desorption of methanol (CH3OH) from cold dust-grain analogues under hydrogen-atom bombardment. Monolayer methanol films on polycrystalline gold, compact amorphous solid water (ASW, 10 ML) and 13CO ice (2 ML) are exposed to H atoms at 3.3 × 1012 cm−2 s−1 and Ts = 8–12 K for up to 40 min. Films are monitored in situ by reflection absorption infrared spectroscopy (RAIRS) and post-exposure by temperature-programmed desorption (TPD). The methanol content drops rapidly, plateauing after ~20 min. Because the elevated H2 background precludes gas-phase detection, the depletions are upper limits to the chemical desorption efficiency: 52 ± 8 per cent on gold (12 K) and 12 ± 3 per cent on ASW. Subtracting the maximum plausible contribution from undetected ice-phase products (≲ 15–20 per cent cumulatively) brackets the true efficiency to ~32–52 per cent on gold and ≲ 12 per cent on ASW. On 13CO, concurrent formation of 13CH3OH from CO hydrogenation prevents a quantitative measurement. We attribute the depletion to repeated H-abstraction and re-addition cycles on the methyl group. The process is most efficient on gold, where the physisorbed, closed-shell molecule couples only weakly to the metal’s phonon and electronic dissipation channels, and is suppressed on ASW, where the hydrogen-bonded network rapidly drains the reaction energy. Methyl-bearing interstellar complex organic molecules (iCOMs) are susceptible to analogous abstraction–addition loops, but are also efficiently formed on grain surfaces, so desorption selectivity cannot be separated from formation efficiency with the present data.