Mass-spectrometric investigations of gas evolution

Method of mass-spectrometry with time-of-flight recording of the desorbed products was used to study the gas evolution of impurities from the subsurface layer of Si crystals molten by the electron beam (of ~2 mm² area) in the vacuum of 10⁻⁵ – 10⁻⁷ Pa. It is shown that irrespective of vacuum level, oxygen (m = 32) and hydrogen (m = 2) in the molecular state as well as Si atoms (m = 28) are registered as the main components of gas evolution in the mass-spectrum in melting. With longer time of the subsurface layer exposure in the molten state, an indication of CO evolution (fragment peak m = 12) appears in the mass-spectrum. There is, however, a ground to believe that this is the consequence of gas evolution from the fixtures, and not from the Si sample. Features of gas evolution were revealed at the initial stage of heating and melting of Si sample, depending on the previous heat-treatment of the sample. If melting the subsurface zone proceeds after contact with the atmosphere, initial peaks of evolution of oxygen and hydrogen molecules and Si atoms are observed. These are partially weakened with further keeping the sample in the molten state. In our opinion, such a peak is due to contamination of the surface at such a contact. A long-term exposure in vacuum of a sample cooled after melting does not lead to appearance of the above peak at subsequent melting

MICROSCOPIC MODEL OF ASSOCIATIVE DESORPTION FOR HYDROGEN ON Mo(110)

Adsorbed hydrogen layers on the Mo(110) surface have been investigated both experimentally by temperature programmed desorption (TPD) method and theoretically by means of DFT-based optimization of surface structures. We suggest a novel microscopic model of the associative hydrogen desorption, which explains essential features of the process. In this model, the process of hydrogen desorption can be described as association of hydrogen atoms on the surface, but molecular formation is actually accomplished while the molecule moves away from the surface. We also suggest a new algorithm for realistic Monte Carlo simulations of associative desorption, which implements the microscopic description of the association of hydrogen adatoms into a molecule with activation energy, found from the DFT calculations. Good agreement between simulated and experimental TPD spectra gives insight into different behavior of the spectra, obtained for low and high hydrogen coverages on the Mo(110) surface.Hydrogen, desorption, DFT calculations, Monte Carlo simulations, lateral interaction

ABSENCE OF CO DISSOCIATION ON Mo(110): TPD AND DFT STUDY

The problem of the CO dissociation on Mo(110) has been addressed by means of temperature-programmed desorption (TPD) and density-functional (DFT) calculations. The TPD spectra show a first-order CO desorption, which indicates the desorption from a "virgin" state, not a recombinative form of desorption. The height of the potential barrier for the dissociation (2.75 eV), estimated from DFT calculations, substantially exceeds the energy of CO chemisorption (2.1 eV), which makes the thermally induced CO dissociation on Mo improbable. Monte Carlo simulations of TPD spectra, performed using estimated chemisorption energies, are in good agreement with experiment and demonstrate that the two-peak shape of the spectra can be explained without involving the CO dissociation. Thus, the results of the present study finally refute the concept of a dissociative form of CO adsorption on Mo surfaces.Carbon oxide, dissociation, density functional calculations, Monte Carlo simulations