Vacuum ultraviolet photolysis of hydrogenated amorphous carbons

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 H 2 molecules in the bulk of the dust particles can diffuse out to the gas phase and contribute to the total H 2 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 H 2 or D 2 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. −0.0023 cm 2 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 photoproduced H 2 molecules in cold photon-dominated regions, where destruction of the aliphatic component in hydrogenated amorphous carbons most probably takes place.

Interstellar Dust Analogues: Structure, Survivability and Physical Processes

“…Unfortunately, we are not able to prove this affirmation experimentally, since our experimental set up configuration do not allow to detect the signal of the released H 2 molecules in the mass spectrometer. Nonetheless experimental results by other groups confirm this description Martín-Doménech, Dartois, and Muñoz Caro, 2016) We gather in Table 4.4 our obtained values for the main parameters of each model. As advanced before, we do not find a clear dependence of our values with the temperature, and the values for the corresponding destruction cross sections vary within the experimental error.…”

“…Equation 4.5 provide an exceptional fitting of bibliographic results using both high energy ions and electrons , and the consequences of this will be further discussed later. It is also worth noticing that considering H 2 to be the only recombinable species introduces an error since more complex hydrocarbons can be formed from the breaking of more complex functional groups Alata et al, 2015;Martín-Doménech, Dartois, and Muñoz Caro, 2016).…”

Characterizing Interstellar Medium, Planetary Surface and Deep Environments by Spectroscopic Techniques Using Unique Simulation Chambers at Centro de Astrobiologia (CAB)

“…Our collaboration with the group of E. Dartois at the Institut d’Astrophysique Spatiale (IAS) and Institut des Sciences Moléculaires d’Orsay (ISMO) (France) employs the French facilities and ISAC to simulate in more detail the processes that occur in interplanetary dust particles and bare interstellar carbon grains observed in the diffuse medium [19,20,21,22,23,24,25,26,27,28,29,30,31]. These carbon grains are composed of hydrogenated amorphous carbon and comprise an important fraction of the carbon budget in galaxies including our Milky Way.…”