Electronic Excitation and Dynamic Promotion of a Surface Reaction

D. N. Denzler, C. Frischkorn, C. Hess, M. Wolf, and G. Ertl

Phys. Rev. Lett. 91, 226102 – Published 26 November 2003

Abstract

The mechanism of recombinative desorption of hydrogen from a Ru(0001) surface induced by femtosecond-laser excitation has been investigated and compared to thermally initiated desorption. For the laser-driven process, it is shown that hot substrate electrons mediate the reaction within a few hundred femtoseconds resulting in a huge isotope effect between $H_{2}$ and $D_{2}$ in the desorption yield. In mixed saturation coverages, this ratio crucially depends on the proportions of H and D. Deviations from second order desorption kinetics demonstrate that the recombination is dynamically promoted by excitation of neighboring, but nonreacting adatoms. A concentration dependent rate constant which accounts for the faster excitation of H versus D is proposed.

Received 17 April 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.226102

Authors & Affiliations

D. N. Denzler¹, C. Frischkorn², C. Hess¹, M. Wolf², and G. Ertl¹

¹Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
²Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany

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Issue

Vol. 91, Iss. 22 — 28 November 2003

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Images

Figure 1
(a) TD spectra for $H_{2}$ , HD, and $D_{2}$ from a saturation coverage of HD. Spectra are scaled according to detection sensitivity and offset for clarity. (b) The fs-laser induced TOF spectra for $H_{2}$ (offset) and $D_{2}$ . Mean kinetic energies are obtained by fitting modified Maxwell-Boltzmann distributions (solid lines) to the experimental data. $E_{k i n} = (2110 \pm 400) K$ and $(1720 \pm 290) K$ for $H_{2}$ and $D_{2}$ , respectively.Reuse & Permissions
Figure 2
Two-pulse correlation (pulse intensity ratio $54 ∶ 46$ ) of the laser-induced desorption yield of hydrogen from Ru(0001). The yield of the first shot out of a series of laser pulses is shown as a function of the pulse-pulse delay ( $D_{2}$ data offset). Dashed lines mark the overall yield of both individual pulses infinitely separated in time. Solid lines are obtained using the electronic friction model of Ref. 11 with a FWHM of 1.1 and 1.5 ps for $H_{2}$ and $D_{2}$ , respectively. The dip at very short delays originates from the competition between the heat transport into the bulk and electron-phonon coupling [12].Reuse & Permissions
Figure 3
Energy transfer from substrate to adsorbate: (a) Based on the two-temperature model [13], the calculated time profiles of the electron and the phonon temperature $T_{e l}$ and $T_{p h}$ at the substrate surface are plotted for two laser pulses with a pulse separation of $Δ t = 1 p s$ . Assuming a purely electron-mediated energy transfer [11], the adsorbate temperatures $T_{a d s}$ for an H and D saturation layer, respectively, result. In the lower section, the corresponding rates and their time integrals, the desorption yields, are displayed. (b) Illustration of the electronic processes after fs-laser excitation.Reuse & Permissions
Figure 4
$H_{2}$ , $D_{2}$ , and HD desorption yield from a hydrogen-saturated Ru(0001) surface as a function of the D species concentration on the surface. (a) Thermal desorption by resistive heating. Solid lines are based on second order rate equations numerically integrated with the coupling continuity equations (see text). (b) Desorption induced by fs-laser excitation, reflecting the isotope effect of $10 ∶ 1$ between isotopically pure coverages. Dashed lines are guides to the eye.Reuse & Permissions
Figure 5
(a) Comparison of fs-laser induced $H_{2}$ and $D_{2}$ desorption yield with the outcome of second order rate equation calculations based on time-dependent rate constants (see text). (b) Top: In a cold matrix realized by a D surrounding, $D_{2}$ recombination occurs between two excited reactants. Bottom: The more rapid excitation of H atoms creates a hot matrix, in which the recombination of $D_{2}$ molecules is enhanced.Reuse & Permissions

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