Signatures of a quantum diffusion limited hydrogen atom tunneling reaction

We are studying the details of hydrogen atom (H atom) quantum diffusion in highly enriched parahydrogen (pH2) quantum solids doped with chemical species in an effort to better understand H atom transport and reactivity under these conditions. In this work we present kinetic studies of the 193 nm photo-induced chemistry of methanol (CH3OH) isolated in solid pH2. Short-term irradiation of CH3OH at 1.8 K readily produces CH2O and CO which we detect using FTIR spectroscopy. The in situ photochemistry also produces CH3O and H atoms which we can infer from the post-photolysis reaction kinetics that display significant CH2OH growth. The CH2OH growth kinetics indicate at least three separate tunneling reactions contribute; (i) reactions of photoproduced CH3O with the pH2 host, (ii) H atom reactions with the CH2O photofragment, and (iii) long-range migration of H atoms and reaction with CH3OH. We assign the rapid CH2OH growth to the following CH3O + H2 → CH3OH + H → CH2OH + H2 two-step sequential tunneling mechanism by conducting analogous kinetic measurements using deuterated methanol (CD3OD). By performing photolysis experiments at 1.8 and 4.3 K, we show the post-photolysis reaction kinetics change qualitatively over this small temperature range. We use this qualitative change in the reaction kinetics with temperature to identify reactions that are quantum diffusion limited. While these results are specific to the conditions that exist in pH2 quantum solids, they have direct implications on the analogous low temperature H atom tunneling reactions that occur on metal surfaces and on interstellar grains.