Nuclear spin temperatures of hydrogen and water molecules on amorphous solid water

Abstract

To clarify the meaning of the nuclear spin temperatures of H2 and H2O molecules associated with various astronomical targets, it is important to understand the mechanisms that could alter these temperatures; i.e., the molecules' ortho/para nuclear-spin ratio (OPR). We have performed a series of experiments to investigate how the OPRs of H2 and H 2O behave on the surface of amorphous solid water (ASW), which is analogous to cosmic ice dust. The OPR of H2 initially shows a high temperature limit of 3 upon its formation through H-H recombination at ∼10 K and gradually decreases toward lower temperatures on the surface. The spin temperatures of H2O molecules that are thermally desorbed from various types of ASW at ∼10 K always return the high-temperature limit. © 2013 AIP Publishing LLC.