Vacuum ultraviolet photolysis of hydrogenated amorphous carbons

Abstract

Context. The interstellar hydrogenated amorphous carbons (HAC or a-C:H) observed in the diffuse medium are expected to disappear in a few million years, according to the destruction time scale from laboratory measurements. The existence of a-C:H results from the equilibrium between photodesorption, radiolysis, hydrogenation and resilience of the carbonaceous network. During this processing, many species are therefore injected into the gas phase, in particular H 2, but also small organic molecules, radicals or fragments. Aims. We perform experiments on interstellar a-C:H analogs to quantify the release of these species in the interstellar medium. Methods. The vacuum ultraviolet (VUV) photolysis of interstellar hydrogenated amorphous carbon analogs was performed at low (10 K) to ambient temperature, coupled to mass-spectrometry detection and temperature-programed desorption. Using deuterium isotopic substitution, the species produced were unambiguously separated from background contributions. Results. The VUV photolysis of hydrogenated amorphous carbons leads to the efficient production of H 2 molecules, but also to small hydrocarbons. Conclusions. These species are formed predominantly in the bulk of the a-C:H analog carbonaceous network, in addition to the surface formation. Compared with species made by the recombination of H atoms and physisorbed on surfaces, they diffuse out at higher temperatures. In addition to the efficient production rate, it provides a significant formation route in environments where the short residence time scale for H atoms inhibits H 2 formation on the surface, such as PDRs. The photolytic bulk production of H 2 with carbonaceous hydrogenated amorphous carbon dust grains can provide a very large portion of the contribution to the H 2 molecule formation. These dust grains also release small hydrocarbons (such as CH 4) into the diffuse interstellar medium, which contribute to the formation of small carbonaceous radicals after being dissociated by the UV photons in the considered environment. This extends the interstellar media environments where H 2 and small hydrocarbons can be produced.