DIRECT MEASUREMENTS OF HYDROGEN ATOM DIFFUSION AND THE SPIN TEMPERATURE OF NASCENT H ₂ MOLECULE ON AMORPHOUS SOLID WATER

Abstract

Physicochemical processes (H-atom sticking, diffusion, recombination, and the nuclear spin temperature of nascent H2 molecules) important in the formation of molecular hydrogen have been experimentally investigated on amorphous solid water (ASW). A new type of experiment is performed to shed light on a longstanding dispute. The diffusion rate of H atom is directly measured at 8 K and is found to consist of a fast and a slow component due to the presence of at least two types of potential sites with the energy depths of ∼20 and >50 meV, respectively. The fast diffusion at the shallow sites enables efficient H2 formation on interstellar ice dust even at 8 K, while H atoms trapped in the deeper sites hardly migrate. The spin temperature of nascent H2 formed by recombination on ASW has been obtained for the first time and is higher than approximately 200 K. After formation, H2 molecules are trapped and their spin temperature decreases due to the conversion of spin states on ASW.