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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...

The advantages of the cross-correlation time of flight (TOF) method when applied to surface scattering experiments under ultrahigh vacuum conditions are demonstrated in comparison with the conventional TOF method. The principle of the cross-correlation TOF technique including the deconvolution procedure and statistical accuracy given by this method is discussed. The main parts of the spectrometer: chopper blade, magnetic rotor suspension and drive, and data acquisition system are described followed by the test of the TOF spectrometer, and by an illustration of the efficiency of this technique by means of TOF distributions measured for hydrogen molecules desorbing from metal surfaces.

We present energy-resolved He-scattering data on the thermal behavior of the clean Cu(l 10) surface. At variance with a recent x-ray study we have observed no evidence for a proliferation of steps, i.e., for thermal roughening, up to 7 7 =900 K. The analysis points to an anomalous increase of the mean-square displacement of the surface atoms at temperatures r> 550 K which might be ascribed to an enhanced surface anharmonicity.