Formation of molecular hydrogen on analogues of interstellar dust grains: experiments and modelling

Molecular hydrogen has an important role in the early stages of star formation as well as in the production of many other molecules that have been detected in the interstellar medium. In this review we show that it is now possible to study the formation of molecular hydrogen in simulated astrophysical environments. Since the formation of molecular hydrogen is believed to take place on dust grains, we show that surface science techniques such as thermal desorption and time-of-flight can be used to measure the recombination efficiency, the kinetics of reaction and the dynamics of desorption. The analysis of the experimental results using rate equations gives useful insight on the mechanisms of reaction and yields values of parameters that are used in theoretical models of interstellar cloud chemistry.Comment: 23 pages, 7 figs. Published in the J. Phys.: Conf. Se

Ab initio simulations of the kinetic properties of the hydrogen monomer on graphene

The understanding of the kinetic properties of hydrogen (isotopes) adatoms on graphene is important in many fields. The kinetic properties of hydrogen-isotope (H, D and T) monomers were simulated using a composite method consisting of density functional theory, density functional perturbation theory and harmonic transition state theory. The kinetic changes of the magnetic property and the aromatic $\pi$ bond of the hydrogenated graphene during the desorption and diffusion of the hydrogen monomer was discussed. The vibrational zero-point energy corrections in the activation energies were found to be significant, ranging from 0.072 to 0.205 eV. The results obtained from quantum-mechanically modified harmonic transition state theory were compared with the ones obtained from classical-limit harmonic transition state theory over a wide temperature range. The phonon spectra of hydrogenated graphene were used to closely explain the (reversed) isotope effects in the prefactor, activation energy and jump frequency of the hydrogen monomer. The kinetic properties of the hydrogen-isotope monomers were simulated under conditions of annealing for 10 minutes and of heating at a constant rate (1.0 K/s). The isotope effect was observed; that is, a hydrogen monomer of lower mass is desorbed and diffuses more easily (with lower activation energies). The results presented herein are very similar to other reported experimental observations. This study of the kinetic properties of the hydrogen monomer and many other involved implicit mechanisms provides a better understanding of the interaction between hydrogen and graphene.Comment: Accepted by J. Phys. Chem.

Hydrogen-induced chemical erosion of amorphous hydrogenated carbon thin films: Structure and reactivity

In the present study, we investigated the effects of annealing of amorphous hydrogenated carbon (a-C:H) films, particularly with respect to structural changes of the carbon network and their impact on the hydrogen-atom-induced erosion. The a-C:H films were deposited at 300 K on a Pt(111) single crystal using the ion-beam deposition (IBD) method. Electron energy loss spectroscopy (EELS) and high-resolution electron energy loss spectroscopy (HREELS) were employed to monitor structural changes of the carbon network as a function of the annealing temperature. A transition from an sp(3)-rich carbon network toward an sp(2) dominated, "graphitic" structure was observed around 900 K. The hydrogen-induced erosion of as-deposited and annealed a-C:H films was investigated by in situ mass spectrometry. The postdeposition annealing did not change the overall erosion rates of a-C:H films; however, the product distribution indicates the growth of existing graphitic structures as a consequence of annealing at temperatures above 900 K

Abstraction of sulfur from Pt(111) surfaces with thermal H atoms toward adsorbed and gaseous H2S

Sulphur layers on Pt(1 1 1) surfaces with coverages of 0.25 and 0.33 were prepared by H2S adsorption at 85 K and subsequent annealing. If,S adsorption on Pt, S/Pt and H/Pt surfaces and S adsorbate layers were characterized by Auger electron and thermal desorption spectroscopies. Admission of thermal H atoms to S covered Pt(I 1 1) at 85 K leads to formation of gaseous (80%) as well as adsorbed H2S (20%). The yield of adsorbed H2S decreases due to its isothermal desorption above 90 K. The interaction of H(g) with S(a) involves three reaction steps: 1. H(g) + S(a) --> SH(a), 2. H (9) + SH(a) --> H2S(g, a), and 3. H(g) + SH(a) --> H,(g) + S(a) with apparent cross-sections sigma = 0.3 Angstrom(2), sigma(2) = 0.6 Angstrom(2) and sigma(3) = 0.03 Angstrom(2). Above 140 K the hydrogenation of SH toward H2S(a,g) is blocked by thermal decomposition of H2S. Impact of D on coadsorbed S, SH, and H'S leads to desorption of H2S. (C) 2002 Elsevier Science B.V. All rights reserved

Abstraction of sulfur from Pt(111) surfaces with thermal H atoms toward adsorbed and gaseous H2S

Sulphur layers on Pt(1 1 1) surfaces with coverages of 0.25 and 0.33 were prepared by H2S adsorption at 85 K and subsequent annealing. If,S adsorption on Pt, S/Pt and H/Pt surfaces and S adsorbate layers were characterized by Auger electron and thermal desorption spectroscopies. Admission of thermal H atoms to S covered Pt(I 1 1) at 85 K leads to formation of gaseous (80%) as well as adsorbed H2S (20%). The yield of adsorbed H2S decreases due to its isothermal desorption above 90 K. The interaction of H(g) with S(a) involves three reaction steps: 1. H(g) + S(a) --> SH(a), 2. H (9) + SH(a) --> H2S(g, a), and 3. H(g) + SH(a) --> H,(g) + S(a) with apparent cross-sections sigma = 0.3 Angstrom(2), sigma(2) = 0.6 Angstrom(2) and sigma(3) = 0.03 Angstrom(2). Above 140 K the hydrogenation of SH toward H2S(a,g) is blocked by thermal decomposition of H2S. Impact of D on coadsorbed S, SH, and H'S leads to desorption of H2S. (C) 2002 Elsevier Science B.V. All rights reserved