Vacuum ultraviolet photolysis of hydrogenated amorphous carbons

III. Diffusion of photo-produced H₂ as a function of temperature

¹ Centro de Astrobiología, INTA-CSIC, Carretera de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
e-mail: rmartin@cab.inta-csic.es
² Institut d’Astrophysique Spatiale, UMR 8617, CNRS/Université Paris-Sud, Université Paris-Saclay, Université Paris-Sud, 91405 Orsay, France

Received: 12 March 2016
Accepted: 10 May 2016

Abstract

Context. Hydrogenated amorphous carbon (a-C:H) has been proposed as one of the carbonaceous solids detected in the interstellar medium. Energetic processing of the a-C:H particles leads to the dissociation of the C-H bonds and the formation of hydrogen molecules and small hydrocarbons. Photo-produced H₂ molecules in the bulk of the dust particles can diffuse out to the gas phase and contribute to the total H₂ abundance.

Aims. We have simulated this process in the laboratory with plasma-produced a-C:H and a-C:D analogs under astrophysically relevant conditions to investigate the dependence of the diffusion as a function of temperature.

Methods. Experimental simulations were performed in a high-vacuum chamber, with complementary experiments carried out in an ultra-high-vacuum chamber. Plasma-produced a-C:H and a-C:D analogs were UV-irradiated using a microwave-discharged hydrogen flow lamp. Molecules diffusing to the gas-phase were detected by a quadrupole mass spectrometer, providing a measurement of the outgoing H₂ or D₂ flux. By comparing the experimental measurements with the expected flux from a one-dimensional diffusion model, a diffusion coefficient D could be derived for experiments carried out at different temperatures.

Results. Dependence on the diffusion coefficient D with the temperature followed an Arrhenius-type equation. The activation energy for the diffusion process was estimated (E_D(H₂) = 1660 ± 110 K, E_D(D₂) = 2090 ± 90 K), as well as the pre-exponential factor (D₀(H₂) = 0.0007^+0.0013_-0.0004 cm² s^-1, D₀(D₂) = 0.0045^+0.005_-0.0023 cm² s^-1).

Conclusions. The strong decrease of the diffusion coefficient at low dust particle temperatures exponentially increases the diffusion times in astrophysical environments. Therefore, transient dust heating by cosmic rays needs to be invoked for the release of the photo-produced H₂ molecules in cold photon-dominated regions, where destruction of the aliphatic component in hydrogenated amorphous carbons most probably takes place.