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...

A complete set of state-to-state rotational energy transfer rate constants has been measured for acetylene–acetylene collisions at room temperature under single collision conditions. Initial rotational states (Ji=5,7,...,25) were prepared and final states (Jf=1,...,25) interrogated. The measurements were carried out in a typical gas phase pump and probe arrangement. The initial vibrationally excited state was prepared by stimulated Raman pumping using strong Q-branch transitions. State preparation via this branch produces an isotropic spatial distribution of the excited state which is important for data analysis. Narrow bandwidth lasers ensure single rotational state selectivity. The rotational distribution after collisions is monitored by time-delayed laser-induced fluorescence via the Ã 1Au(ν3′ = 1)←X̃ 1Σg(ν2″ = 1) transition. In general, the rate constants decrease exponentially with the transferred rotational energy. The complete rate constant matrix can consistently be described by a simple parameter set within the dynamical infinite order sudden power approximation. In addition to this general behavior a significant ΔJ propensity of the rate constants is observed. Using the energy corrected sudden approximation with a power law basis an excellent match, reproducing the ΔJ propensities, to the rate constant matrix is obtained, again with a single set of parameters.

The effects of acetylene self-collisions on alignment have been studied in detail. Strongly aligned samples of C2H2 were prepared in several rotational states of the 2l level by stimulated Raman pumping. Laserinduced fluorescence verified the initial degree of alignment and was used to follow its decrease via collisions. Measurements of the initially excited alignment agree well with theoretical calculations of that achievable by stimulated Raman pumping, and the decay can be well modeled by a simple kinetic scheme.

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