We report the first measurement of the preferential steric orientation of D 2 molecules associatively desorbing from a metal surface. The flux of D 2 desorbing from Pd(100) is probed by laser induced fluorescence with linearly polarized tunable vacuum ultraviolet radiation in the B 1 S 1 u ͑y 0 , J 0 , M 0 ͒ √ X 1 S 1 g ͑y 00 , J 00 , M 00 ͒ Lyman bands. In ͑y 00 0͒ an increasing positive alignment with rotational quantum number is observed up to J 00 6, establishing the preferred helicopter motion of the molecules. In ͑y 00 1͒ and also for (y 00 0, J 00 7 and 8) an isotropic J vector distribution is measured. PACS numbers: 68.45.Da, 33.20.Ni, 79.20.Rf, The dynamical processes which govern the associative desorption and its counterpart, dissociative adsorption, of hydrogen on transition metal surfaces are of general interest for the understanding of catalytic reactions on surfaces. Molecular beam studies of the dissociative adsorption behavior on palladium [1,2], tungsten [3,4], and platinum [5] single-crystal surfaces show that the initial sticking probability exhibits a minimum as the kinetic energy of the hydrogen molecules is increased. This behavior is often interpreted as precursor mediated adsorption. Internal state selective studies revealed an enhanced vibrational population for desorption from Pd (100) [6], similar to the desorption of hydrogen from copper single-crystal surfaces [7,8]. This behavior could be reconciled by quantum mechanical calculations showing a late barrier in the dissociative adsorption potential, although with a considerably lower barrier height than in copper [6]. The rotational population distribution in the desorption flux has been measured earlier in our group [9]. It always showed lower rotational temperatures than the surface temperature. An interpretation of this behavior predicts a specific steric orientation of the molecular axis during desorption [10]. Similarly, interpreting the rotational state dependence of the velocity of H 2 and D 2 desorbing from Cu (111) [11], combined with the observed rotationaltranslational coupling, leads to the prediction of a preferential orientation of the molecular rotational axis parallel to the surface normal.Very recently a potential energy surface for the H 2 ͞Pd(100) system has been derived from density functional theory [12]. It shows activated as well as nonactivated pathways for dissociative adsorption, without any molecular precursor potential well. On this potential energy surface, Gross, Wilke, and Scheffler [13] performed the first six-dimensional quantum calculation for the dynamical behavior of a hydrogen/metal system. They could reproduce the main experimentally observed features very well: the decrease of the initial sticking coefficient with kinetic energy and the rotational cooling in desorption. A key element of this potential energy surface and their dynamical calculation is the steering of slow hydrogen molecules onto adsorption paths which do not show a barrier. In this calculation Gross, Wilke, and Scheffler predicted...

We report rotationally state-resolved measurements of the average steric orientation of D2 molecules desorbing from a Cu(111) surface. The desorbing flux of D2 molecules is probed by laser-induced fluorescence with linearly polarized tunable vacuum ultraviolet radiation in theMolecules desorbing in the vibrational ground state show for all rotational states measured (J = 1-8) a quadrupole alignment factor, A(2) 0 , compatible with a spatially isotropic distribution. A strong azimuthal, polar, and lateral corrugation of the dissociative interaction potential can cause this behavior.

The rotational and vibrational state distribution of D 2 molecules desorbing from Pd͑100͒ is determined by tunable vacuum ultraviolet laser ionization spectroscopy. For the deuterium supply a permeation source is used operating at surface temperatures in the range from 400 to 850 K. The molecules are detected in rovibrational states up to (vЉ,JЉ)ϭ(0,12) and ͑1,8͒. A significant rotational cooling is observed together with a strong vibrational heating. These experimental results are found to be in good agreement with quantum-mechanical calculations based on an ab initio potential-energy surface. A consistent microscopic picture of the hydrogen reaction dynamics on Pd͑100͒ is established and thus a recent controversy about the application of the principle of detailed balance in the system D 2 /Pd(100) can be settled.

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The influence of sulfur adsorbates on the stereodynamics of surface reactions is evaluated for the associative desorption of D2 from Pd(100). The experimental results show that translational and rotational steering is more effective than at the clean surface, which implies that the potential energy surface (PES) for the reaction shows a larger spatial extension into the vacuum than for the clean surface. A significant polar corrugation of the PES becomes evident at high J" states. At J" = 8 a very strong coverage dependence of the alignment with A(2)0 = 0.07+/-0.06 at the clean surface and A(2)0 = 0.60+/-0.11 at Theta(S) = 0.5 monolayer is observed. This coverage dependence indicates a shift of the reaction site from the bridge to the c-type hollow position at a sulfur coverage of more than Theta(S) = 0.25 monolayer.

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The rotational alignment of 0, molecules desorbing from a Pd(tOO) single crystal surface is measured by laserinduced fluorescence via the B I:; e-. x' I:; Lyman bands around A = 106 nm and 110 nm. Different rotational states in the vibrational ground state v" = 0 are probed. For low rotational states, J" = 2, an isotropic distribution of the J vector is found. For higher rotational states J"~3 a preference of the desorbing molecules for helicoptering over cartwheeling motion is observed, in agreement with recent theoretical predictions.

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