Penetration of Non-energetic Hydrogen Atoms into Amorphous Solid Water and their Reaction with Embedded Benzene and Naphthalene

TL;DR

This study investigates how non-energetic hydrogen atoms penetrate amorphous solid water and react with embedded benzene and naphthalene, revealing significant penetration depths and potential for hydrogenation within ice mantles in space.

Contribution

It extends previous work by demonstrating hydrogen atom penetration and reactivity with aromatic molecules embedded in amorphous ice, suggesting new pathways for chemical evolution in molecular clouds.

Findings

Hydrogen atoms penetrate >50 monolayers in porous ice

Hydrogen atoms penetrate ~10 monolayers in non-porous ice

Embedded benzene and naphthalene can be fully hydrogenated in space environments

Abstract

Chemical processes on the surface of icy grains play an important role in the chemical evolution in molecular clouds. In particular, reactions involving non-energetic hydrogen atoms accreted from the gaseous phase have been extensively studied. These reactions are believed to effectively proceed only on the surface of the icy grains; thus, molecules embedded in the ice mantle are not considered to react with hydrogen atoms. Recently, Tsuge et al. (2020) suggested that non-energetic hydrogen atoms can react with CO molecules even in ice mantles via diffusive hydrogenation. This investigation was extended to benzene and naphthalene molecules embedded in amorphous solid water (ASW) in the present study, which revealed that a portion of these molecules could be fully hydrogenated in astrophysical environments. The penetration depths of non-energetic hydrogen atoms into porous and non-porous ASW were determined using benzene molecules to be >50 and ~10 monolayers, respectively (1 monolayer ~ 0.3 nm).