We have explored the effect of alloying an unreactive metal, Sn, on the dynamics of D2 dissociative chemisorption at Pt(111). By comparing D2 sticking and recombinative desorption on Pt(111) with that on the ordered p(2×2) Sn/Pt(111) and (∛×∛)R30° Sn/Pt(111) surface alloys, we examine the influence of the local surface composition on reactivity. The energy dependence of D2 sticking S(E) has been measured for all three surfaces using a hyperthermal beam. We find that the activation barrier for dissociative chemisorption is low on the p(2×2) alloy, but the sticking probability is reduced, compared to Pt(111), by an increase in the steric constraint on the dissociation site. Sticking on the (∛×∛)R30° alloy is inefficient at thermal energies with a threshold of ∼280 meV, below which the sticking probability falls exponentially. The increase in the barrier to D2 dissociation occurs as the stable, high coordination Pt3–D binding sites are lost by formation of the (∛×∛)R30° alloy. Despite the large activation barrier, sticking is dominated by the vibrational ground state with the barrier occurring in the entrance channel, before the D2 bond has stretched. Departures from a normal energy scaling indicate that the dissociation site is localized in the unit cell and we suggest favorable dissociation sites on the alloy surfaces. Estimates for the heats of adsorption, obtained by comparing activation energies to adsorption and desorption, indicate an abrupt decrease in the D binding energy as the Pt3 sites are lost. We show that sticking and desorption parameters are consistent with an increasing steric constraint for adsorption/desorption on the alloy surfaces as the Sn content is increased and an increase in the barrier to dissociation as the stable Pt3 sites are lost by alloying.

Initial sticking probabilities for D2 dissociative chemisorption at a Ag(111) surface have been measured for translational energies in the range Ei=220–500 meV, as a function of incident angle θi and gas temperature, using seeded molecular beams. Sticking probabilities are dependent on the D2 internal state distribution and scale with the normal component of the translational energy. The data has been fit by assuming that dissociation is independent of molecular rotation, being the sum of contributions from molecules in different vibrational states v with a sticking function S0(Ei,θi,v)=A/2{1+tanh[Ei cos2 θi−E0(v)]/w(v)}, in a manner similar to the behavior on copper. Sticking parameters E0, the translational energy required for S0 to reach half of its maximum value, are determined with good precision (±25 meV) for levels v=3 (328 meV) and v=4 (170 meV) with width parameters w=54 and 63 meV, respectively, while the barriers for levels v=1 and 2 are close to the upper limit of the sticking data and E0 is estimated as 700±100 and 510±70 meV, respectively. Parameters for the vibrational ground state (v=0) could not be obtained, since sticking of this state is negligible at translational energies less than 500 meV. No dissociation could be observed at thermal energies Ei⩽70 meV either on a flat or a defective surface.

We have observed efficient translational to rotational energy transfer for H 2 ͑y 1, J 2 √ 0͒ scattered from Cu(111), the cross section increasing rapidly from near threshold to reach a maximum at a translational energy of 140 meV. Above this energy a change in the behavior of the H 2 ͑y 1, J 2 √ 0͒ cross section was seen, coinciding with the abrupt onset of H 2 ͑y 1, J͒ removal by coupling of translational and vibrational coordinates. Vibrational relaxation competes with rotational excitation, both processes occurring preferentially at the same impact sites and geometry. [S0031-9007 (97)02334-X] PACS numbers: 79.20.Rf, 68.35.JaThe potential energy surfaces (PES) and dynamics of activated dissociative chemisorption have become the focus of much attention recently as experiments become available which provide dynamical insight in a relatively direct way [1]. One source of information is energy transfer measurements which can be compared with scattering calculations to investigate the characteristics of the PES. Here we report state selected measurements of rotational energy transfer for H 2 at Cu(111) which is a model system of both experimental and theoretical interest [2]. Dissociation of H 2 at Cu(111) is highly activated and is enhanced by vibrational excitation of H 2 , the translational energy barrier to dissociation decreasing with increasing vibrational state [1,3,4]. This state dependence arises from the location of the barrier to dissociation at extended H-H separations in the exit channel leading to separated atoms. Calculations of the dissociation dynamics on low dimensional model potential energy surfaces [2] show sticking probabilities which depend on vibrational level, the sticking probability following an "S" shaped curve with a rapid increase in sticking as some threshold translational energy is reached. This picture is supported both by deconvolution of the sticking probabilities of individual vibrational levels from seeded beam measurements [4] and by application of microscopic reversibility to recombinative desorption data [1]. Strong coupling between the vibrational and translational coordinates is also evident for H 2 (D 2 ) scattering from Cu(111) [5,6]. For D 2 vibrational relaxation of ͑y 1, J͒ occurs at 0.35 eV, while at translational energies above 0.65 eV efficient vibrational excitation of ground state molecules to the excited vibrational level occurs. For D 2 it was shown that the vibrational excitation process scaled with normal energy [5], while the gap between the translational energy thresholds corresponds to the energy separation D 2 ͑y 1 √ y 0͒. Applying microscopic reversibility implies that the lower threshold is indeed due to T √ V transfer and cannot be attributed simply to dissociation. For H 2 ͑y 1͒ Rettner, Auerbach, and Michelsen observed a loss of flux near 0.3 eV, and vibrational excitation ͑y 1 √ y 0͒ was again seen at energies above 0.6 eV, just above the threshold for dissociative chemisorption of the ground state [6]. Scattering calculations on model two dimensiona...

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