Formation of H-2 on an olivine surface: a computational study.
MON NOT R ASTRON SOC
1403 - 1407.
The formation of H-2 on a pristine olivine surface [forsterite (010)] is investigated computationally. Calculations show that the forsterite surface catalyzes H-2 formation by providing chemisorption sites for H atoms. The chemisorption route allows for stepwise release of the reaction exothermicity and stronger coupling to the surface, which increases the efficiency of energy dissipation. This suggests that H-2 formed on a pristine olivine surface should be much less rovibrationally excited than H-2 formed on a graphite surface. Gas-phase H atoms impinging on the surface will first physisorb relatively strongly (E-phys = 1240 K). The H atom can then migrate via desorption and re-adsorption, with a barrier equal to the adsorption energy. The barrier for a physisorbed H atom to become chemisorbed is equal to the physisorption energy, therefore there is almost no gas-phase barrier to chemisorption. An impinging gas-phase H atom can easily chemisorb (E-chem = 12 200 K), creating a defect where a silicate O atom is protonated and a single electron resides on the surface above the adjacent magnesium ion. This defect directs any subsequent impinging H atoms to chemisorb strongly (39 800 K) on the surface electron site. The two adjacent chemisorbed atoms can subsequently recombine to form H-2 via a barrier (5610 K) that is lower than the chemisorption energy of the second H atom. Alternatively, the adsorbed surface species can react with another incoming H atom to yield H-2 and regenerate the surface electron site. This double chemisorption 'relay mechanism' catalyzes H-2 formation on the olivine surface and is expected to attenuate the rovibrational excitation of H-2 thus formed.
|Title:||Formation of H-2 on an olivine surface: a computational study|
|Keywords:||astrochemistry, molecular processes, ISM: molecules, MOLECULAR-HYDROGEN FORMATION, ELEY-RIDEAL MECHANISM, GRAPHITE SURFACE, THERMOCHEMICAL KINETICS, INTERSTELLAR CONDITIONS, QUANTUM DYNAMICS, ICY MANTLES, WATER, DUST, RECOMBINATION|
|UCL classification:||UCL > School of BEAMS
UCL > School of BEAMS > Faculty of Maths and Physical Sciences
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