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Quantum-state-specific dynamics of the dissociative adsorption and associative desorption of H at a Cu(111) surface

The Journal of Chemical Physics, ISSN: 0021-9606, Vol: 102, Issue: 11, Page: 4625-4641
1995
  • 264
    Citations
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  • 56
    Captures
  • 1
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  • Citations
    264
    • Citation Indexes
      264
  • Captures
    56
  • Mentions
    1
    • News Mentions
      1
      • News
        1

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Article Description

We have determined the dependence of the dissociative adsorption probability in the zero coverage limit, S, for H on Cu(111) as a function of translational energy, E, and incidence angle, θ, vibrational state, v, and rotational state, J. We have also obtained information on the effect of surface temperature, T , on this probability. These results have been obtained by combining the findings of two separate experiments. We have obtained the form of the dependence of S on E at T=925 K for a range of quantum states from desorption experiments via the principle of detailed balance. We have obtained absolute S values from direct molecular beam adsorption experiments, which reveal that S scales with the so-called "normal energy," E=E cos θ. The desorption experiments provide detailed information for J=0 to 10 of H(v=0) and for J=0 to 7 of H(v=1). The beam experiments additionally provide information on the adsorption of H (v=2), averaged over J. All measurements are consistent with adsorption functions with an s-shaped form, which can be described by S =A (1+erf(x))/2, where x = (E-E)/W. Values of W are ∼0.16 and 0.13 eV for v=0 and v=1, respectively, at T =925 K, falling by about 0.05 eV for T=120 K, and with only a slight dependence on J. Values of A are insensitive to v and J, with a value of ∼0.25. S(E,v,J) curves are thus similar for different v and J, but shifted in E. In contrast, we find that the values of E, which determine the mid-point of the curves, have a strong dependence on v and J. Specifically, E for H(v=0) molecules is about 0.6 eV, falling to 0.3 and 0.1 eV for H(v=1) and H(v=2), respectively. Translational energy is thus about twice as effective as vibrational energy in promoting dissociation. E rises with increasing J at low J, before falling at high J, indicating that rotational motion hinders adsorption for low rotational states (J<4), and enhances adsorption for high rotational states (J>4). Results are compared with similar studies on the D/Cu(111) system and with recent calculations. Finally, these results are used to predict the dependence of the rate of dissociation on temperature for a "bulb" experiment with ambient hydrogen gas in contact with a Cu(111) surface. This simulation yields an activation energy of 0.47 eV for temperatures close to 800 K, compared to a literature value of 0.4 eV from experiment. Analysis of the temperature dependence reveals that the dominant reason for the increase in rate at high temperature is the increase in population of the high energy tail of the translational energy distribution. © 1995 American Institute of Physics. © 1995 American Institute of Physics.

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