H2 formation via the UV photo-processing of a-C:H nano-particles

TL;DR

This study investigates how UV photo-processing of hydrogenated amorphous carbon nano-particles in space can lead to the formation of molecular hydrogen, highlighting a potential pathway for H2 production in interstellar environments under certain radiation conditions.

Contribution

It provides a detailed mechanism and rate estimate for H2 formation via UV-driven processing of a-C(:H) dust, a novel insight into interstellar molecule synthesis.

Findings

Photon-driven H2 formation rate constant estimated at 2x10^-17 cm^3 s^-1.

H2 formation is viable in low to moderate radiation fields (G_0 between 1 and 100).

Intense radiation fields favor grain fragmentation over H2 production.

Abstract

Context. The photolysis of hydrogenated amorphous carbon, a-C(:H), dust by UV photon-irradiation in the laboratory leads to the release of H2 as well as other molecules and radicals. This same process is also likely to be important in the interstellar medium. Aims. To investigate molecule formation arising from the photo-dissociatively-driven, regenerative processing of a-C(:H) dust. Methods. We explore the mechanism of a-C(:H) grain photolysis leading to the formation of H2 and other molecules/radicals. Results. The rate constant for the photon-driven formation of H2 from a-C(:H) grains is estimated to be 2x10^-17 cm^3 s^-1. In intense radiation fields photon-driven grain decomposition will lead to fragmentation into daughter species rather than H2 formation. Conclusions. The cyclic re-structuring of arophatic a-C(:H) nano-particles appears to be a viable route to formation of H2 for low to moderate radiation field intensities (1 < G_0 < 10^2), even when the dust is warm (T ~ 50 - 100 K).