Rotational corrugation in associative desorption of D 2 from Cu(111)

Baerends

1998

We present results of six-dimensional ͑6D͒ quantum wave-packet calculations for the dissociative adsorption of (ϭ0,jϭ4,m j ) H 2 on Cu͑100͒. The potential-energy surface is a fit to points calculated using density-functional theory ͑DFT͒, with the generalized gradient approximation ͑GGA͒, and a slab representation for the surface. New aspects of the methodology we use to adapt the wave function to the symmetry of the surface, which relate to calculations for initial rotational states with odd m j ͑the magnetic quantum number͒, are explained. Invoking detailed balance, we calculate the quadrupole alignment for H 2 as it would be measured in an associative desorption experiment. The reaction of the helicopter (ϭ0,jϭ4,m j ϭ4) state is preferred over that of the (ϭ0,jϭ4,m j ϭ0) cartwheel state for all but the lowest collision energies considered here. The energy dependence of the quadrupole alignment that we predict for (ϭ0,jϭ4) H 2 desorbing from Cu͑100͒ is in good qualitative agreement with velocity-resolved associative desorption experiments for D 2 ϩCu͑111͒. The vibrational excitation probability P(ϭ0,j→ϭ1) is much larger for j ϭ4 than for jϭ0, and the m j -dependence of P(ϭ0,jϭ4,m j →ϭ1) is markedly different from that of the initial-state-resolved reaction probability. For all but the highest collision energies, vibrational excitation from the (ϭ0,jϭ4) state is accompanied by loss of rotational energy, in agreement with results of molecular beam experiments on scattering of H 2 and D 2 from Cu͑111͒.

Section: ͑15͒mentioning

confidence: 51%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Dissociative adsorption of H2 on Cu(100): Fixed-site calculations for impact at hollow and top sites

Mowrey

Baerends

1998

“…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.…”

Section: Introductionmentioning

confidence: 99%

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

Somers

McCormack

et al. 2002

Six-dimensional quantum dynamical calculations are presented for the reaction of (v, j) H 2 on Cu͑100͒, at normal incidence, for vϭ0 -1 and jϭ0 -5. The dynamical calculations employed a potential energy surface computed with density functional theory, using the generalized gradient approximation and a slab representation for the adsorbate-substrate system. The aim of the calculations was to establish signatures from which experiments could determine the dominant reaction site of H 2 on the surface and the dependence of the reaction site on the initial rovibrational state of H 2 . Two types of signatures were found. First, we predict that, at energies near threshold, the reaction of (vϭ1) H 2 is rotationally enhanced, because it takes place at the top site, which has an especially late barrier and a reaction path with a high curvature. On the other hand, we predict the reaction to be almost independent of j for (vϭ0) H 2 , which reacts at the bridge site. Second, we predict that, at collision energies slightly above threshold for which the reaction probabilities of the (vϭ0) and (vϭ1) states are comparable, the rotational quadrupole alignment of (vϭ1) reacting molecules should be larger than that of (vϭ0) reacting molecules, for jϭ1, 4, and 5. For ( j ϭ2) H 2 , the opposite should be true, and for ( jϭ3) H 2 , the rotational quadrupole alignment should be approximately equal for (vϭ1) and (vϭ0) H 2 . These differences can all be explained by the difference in the predicted reaction site for (vϭ1) and (vϭ0) H 2 ͑top and bridge͒ and by the differences in the anisotropy of the potential at the reaction barrier geometries associated with these sites. Our predictions can be tested in associative desorption experiments, using currently available experimental techniques.

“…Recent experimental observations of the rotational distributions of D 2 , which associatively desorbs from Cu͑111͒, indicate that dissociation probabilities of molecules with helicopter and cartwheel rotational motion may be comparable, 58 and that the preference for dissociation of molecules with helicopter-type rotation is not as high as suggested by previous model calculations. 25,27,28,30 The strong preference for helicopter dissociation found in these studies was either due to the use of a flat surface model 25,27 or due to the use of a potential that did not incorporate the dependence as rigorously as our potential.…”

Section: Resultsmentioning

confidence: 84%

Dissociative adsorption of H2 on Cu(100): A four-dimensional study of the effect of rotational motion on the reaction dynamics

Mowrey

Wiesenekker

et al. 1997

The reaction of H 2 on Cu͑100͒ is investigated using a four-dimensional ͑4D͒ quantum dynamical fixed-site model to assess the influence of molecular rotation on dissociation over the most reactive ͑the bridge͒ site. The potential energy surface ͑PES͒ is a fit to the results of density functional calculations performed using a generalized gradient approximation treating a Cu slab with a periodic overlayer of H 2 . Dissociation probabilities for molecules with ''helicoptering'' (m j ϭ j) and ''cartwheeling'' ͑m j ϭ0͒ rotational motions are here found to be comparable because of the strong corrugation in the azimuthal coordinate. The calculations indicate that reaction is accompanied by significant rotationally inelastic scattering. Surprisingly, vibrational excitation is also found to be an efficient process in collisions with the reactive bridge site. In these collisions, the molecular axis is tilted away from the orientation parallel from the surface. Considering the approximate nature of the 4D model used, the calculated reaction probabilities are in good agreement with experiment, indicating that the PES that was used is accurate.