Hydroxylated Mg-rich amorphous silicates as catalysts for molecular hydrogen formation in the interstellar medium

TL;DR

This study investigates how hydroxylated Mg-rich amorphous silicates catalyze molecular hydrogen formation in space, combining experiments and quantum calculations to reveal barrierless reactions relevant to interstellar environments.

Contribution

It provides new experimental and theoretical insights into hydrogen formation mechanisms on Mg-rich amorphous silicates, highlighting barrierless reactions and catalytic processes in space.

Findings

Deuteroxyl groups form with a cross section of 4.2×10^{-18} cm^2.

H2 formation occurs via barrierless reactions on silicate surfaces.

Results are relevant to hydrogen chemistry in interstellar medium environments.

Abstract

We present results from an experimental study on the interaction of atomic deuterium with Mg-rich amorphous enstatite and forsterite type silicates. Infrared spectroscopy was used to examine the process. During D atom exposure, deuteroxyl group formation was observed. The cross section for OD group formation, estimated from the OD stretching band intensity with D atom fluence, is 4.2.10^{-18} cm^2 for both silicates. HD (D2) molecules form via D atom abstraction of chemisorbed H(D) atoms from OH (OD) groups, with a cross section of 7.0.10^{-18} cm^2. Quantum chemical calculations on enstatite and forsterite amorphous nano-clusters were used to analyze chemisorption and abstraction energies of H atoms. The formation of OH groups on forsterite is barrierless, while enstatite has a small energy barrier. H2 abstraction from hydroxyl groups is barrierless in both silicates. The results support our interpretation of IR spectral changes during D atom exposure as addition and exchange reactions, with catalytic molecular deuterium formation. These findings, obtained at 300 K, are relevant to astrophysical environments like photodissociation regions and interstellar clouds at lower temperatures. Amorphous silicates, like carbon grains, undergo hydrogenation in the diffuse interstellar medium due to interactions with atomic hydrogen and UV photons. The detection of these components on comet 67P/Churyumov-Gerasimenko connects primitive solar system objects to interstellar dust, offering new insights into their evolution.