Rotational Alignment in Associative Desorption ofD2(v′′

Abstract

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...

Hydrogen dissociation on metal surfaces - a model system for reactions on surfaces

“…The results of these calculations have been compared to a number of independent experiments [16,[39][40][41], and they are at least in semi-quantitative agreement with all of these experiments This shows that ab initio dynamics calculations are indeed capable of adequately describing the hydrogen dissociation on transition metal surfaces.…”

The effect of the exchange-correlation functional on H2 dissociation on Ru(0001)

“…Examples of such experiments include associative desorption experiments, which by application of detailed balance may yield initial state-resolved reaction probabilities (measured for, for instance, H 2 60 and D 2 93, 98 dissociation on Cu(111)). Alternatively, such experiments may also yield rotational state populations (as measured for H 2 + Pd(100) 99 ), average translational energies for H 2 desorbing from the surface in particular (v, J) states (as measured for H 2 + Cu(100) 20 ) and initial rotational quadrupole alignment parameters describing the orientational dependence of reaction (measured for, for instance, H 2 + Cu(111), 100-102 H 2 + Cu(100), 20 and H 2 + Pd(100) 103 ). Additional valuable information can perhaps be obtained from experiments that use laser excitation and detection using resonance enhanced multi-photon ionization (REMPI) to determine probabilities for rotationally inelastic scattering, like the experiments performed earlier for H 2 + Cu(111), 104 H 2 , 105 HD, 106 and D 2 107 + Cu(100) and H 2 , 108 HD, 106 and D 2 107 + Pd(111).…”

Signatures of site-specific reaction of H2 on Cu(100)

“…Associative desorption experiments can also determine the influence on reaction of the molecule's alignment with respect to the surface by measuring the rotational quadrupole alignment A 0 (2) (v, j) of the angular momentum of the desorbing molecules. [13][14][15][16] Theory can now also make important contributions. Reduced-dimensionality theoretical studies on similar systems and models 25,34,[37][38][39][42][43][44][45][46][47][48][49][50][51][52][55][56][57][58]69,72 have revealed interesting effects like the vibrational enhancement of the reaction, 40,41,46,52 the effect on the reaction of the corrugation of the potential energy surface ͑PES͒, 47 and the effects of the alignment of the incident H 2 molecule.…”