Interactions of Atomic and Molecular Hydrogen with a Diamond-like Carbon Surface: H₂ Formation and Desorption

Abstract

The interactions of atomic and molecular hydrogen with bare interstellar dust grain surfaces are important for understanding H2 formation at relatively high temperatures (> 20 K). We investigate the diffusion of physisorbed H-atoms and the desorption energetics of H2 molecules on an amorphous diamondlike carbon (DLC) surface. From temperature-programmed desorption experiments with a resonance-enhanced multiphoton ionization (REMPI) method for H2 detection, the H2 coverage-dependent activation energies for H2 desorption are determined. The activation energies decrease with increasing H2 coverage and are centered at 30 meV with a narrow distribution. Using a combination of photostimulated desorption and REMPI methods, the time variations of the surface number density of H2 following atomic and molecular hydrogen depositions are studied. From these measurements, we show that H2 formation on a DLC surface is quite efficient, even at 20 K. A significant kinetic isotope effect for H2 and D2 recombination reactions suggests that H-atom diffusion on a DLC surface is mediated by quantum mechanical tunneling. In astrophysically relevant conditions, H2 recombination due to physisorbed H-atoms is unlikely to occur at 20 K, suggesting that chemisorbed H-atoms might play a role in H2 formation at relatively high temperatures.

Experimental Approach to Physicochemical Hydrogen Processes on Cosmic Ice Dust

“…69,70) Recently, we successfully produced a hydrogen-free DLC surface by the laser ablation method and investigated the H (D) atom diffusion processes on it. 34) We found that the H atoms on DLC cannot be detected by the PSD-REMPI method, in contrast to those demonstrated for ASW and CO surfaces. 30,33) Instead, we decided to monitor the surface number density of H2 upon H atom deposition to extract information on the H atom diffusion.…”

“…Recently, using a combination of photostimulated desorption (PSD) 29) and resonance-enhanced multiphoton ionization (REMPI) methods, we were successful in monitoring hydrogen on ASW, 30,31) polycrystalline ice (PCI), [30][31][32] pure solid CO, 33) and diamond-like carbon. 34) In the following sections, a summary of these experiments is provided after the description of the PSD-REMPI method. Additionally, as our new experimental findings relevant to the hydrogen diffusion, a proton-hole transfer that delivers a negative charge through ice is presented (Sect.…”

H2 Ortho–Para Spin Conversion on Inhomogeneous Grain Surfaces

“…We obtained the binding energy distribution of H 2 by considering zero-point energy difference between D 2 and H 2 , 3.15 meV (Amiaud et al 2015). Note that laboratory experiments have found that o-D 2 is bound to surfaces slightly more strongly than p-D 2 (∼1 meV) (Amiaud et al 2008;Tsuge et al 2019), but we neglect the difference in this work for simplicity. The hopping activation energy from a site with the binding energy of E b to another site with the binding energy of E b (E hop ) is given as follows (Cazaux et al 2017, see their Fig.…”

Behaviour of radicals on interstellar dust analogues

“…Because diamond-like carbon (DLC) is a good analogue for potential C-dominated ISM dust, we investigated the behaviour of H atoms on DLC (Tsuge et al 2019). The DLC surface was prepared on the aluminium substrate by the laser ablation of a graphite disk in situ within the experimental chamber to avoid contamination in air.…”

“…For deuterium, the D 2 intensities for D atom deposition are always below those for D 2 deposition, indicating that the diffusion of D atoms is sufficiently slower than that of H atoms. A more detailed analysis has been presented elsewhere (Tsuge et al 2019).…”