D. R. Jennison

Copper wetting of α-Al2O3(0001): theory and experiment

XPS studies have been carried out on sputter deposited copper on a substantially hydroxylated {alpha}-Al{sub 2}O{sub 3}(0001) (sapphire) surface under ultra-high vacuum (UHV) conditions. XPS-derived Cu uptake curves show a sharp change in slope at a coverage of 0.35 monolayer (on a Cu/O atomic basis), indicative of initial layer-by-layer growth. CU(LMM) lineshape data indicate that, prior to the first break in the curve, Cu is oxidized to Cu(I). At higher coverages, metallic CU(0) is. observed. These data agree with first principles theoretical calculations, indicating that the presence of ad-hydroxyl groups greatly enhances the binding of Cu to bulk sapphire surfaces, stabilizing Cu(I) adatoms over two-dimensional metallic islands. In the absence of hydroxylation, calculations indicate significantly weaker Cu binding to the bulk sapphire substrate and non-wetting. Calculations also predict that at Cu coverages above 1/3 monolayer (ML), Cu-Cu interactions predominate, leading to Cu(0) formation. These results are in excellent agreement with experiment. The ability of surface hydroxyl groups to enhance binding to alumina substrates suggests a reason for contradictory experimental results reported in the literature for Cu wetting of alumina.

Role of surface vacancies and water products in metal nucleation: Pt/MgO(100)

Abstract Density-functional calculations reveal that isolated F s and V s surface vacancies on MgO(100) ionize single Pt atoms, roughly tripling their adsorption energy. This trapping inhibits Pt atoms from binding to other Pt atoms, resulting in a negative addimer binding energy. Hence, isolated surface vacancies demote nucleation, contrary to popular belief. A defect that does promote nucleation is the F s V s divacancy, which increases the addimer binding energy by 20%. Amongst water and its dissociation products, only adsorbed hydroxyls are found to have a similar (but stronger) effect.

Multidimensional dynamics in the electron stimulated desorption of ammonia from Pt(111)

We characterize the electron stimulated desorption of neutral ammonia (NH3 and ND3) from Pt(111) with vibrational and rotational quantum resolution by using (2+1) resonance enhanced multiphoton ionization detection. Two significant isotope effects appear: (1) the desorption yield of NH3 is three times that of ND3 and (2) NH3 desorbs with considerably more ‘‘spinning’’ rotational energy than does ND3. We find virtually identical translational energy distributions for each desorbate and roughly equal vibrational energy distributions. Vibrational excitation is found exclusively in the ν2 symmetric deformation or ‘‘umbrella’’ mode, independent of isotope. These effects cannot be explained by desorption induced by vibrational energy transfer. Instead, desorption is the result of excitation of a 3a1 electron principally on the N atom, which causes the pyramidal NH3 adsorbate to rapidly invert. Ab initio calculations of two‐dimensional potential energy surfaces (intramolecular bond angle and surface bond length)...

Ab initio calculations of Ru, Pd, and Ag cluster structure with 55, 135, and 140 atoms

A massively parallel ab initio computer code, which uses Gaussian bases, pseudopotentials, and the local density approximation, permits the study of transition-metal systems with literally hundreds of atoms. We present total energies and relaxed geometries for Ru, Pd, and Ag clusters with N=55, 135, and 140 atoms. The N=55 and 135 clusters were chosen because of simultaneous cubo-octahedral (fcc) and icosahedral (icos) subshell closings, and we find icos geometries are preferred. Remarkably large compressions of the central atoms are observed for the icos structures (up to 6% compared with bulk interatomic spacings), while small core compressions (∼1%) are found for the fcc geometry. In contrast, large surface compressive relaxations are found for the fcc clusters (∼2%–3% in average nearest neighbor spacing), while the icos surface displays small compressions (∼1%). Energy differences between icos and fcc are smallest for Pd, and for all systems the single-particle densities of states closely resembles bu...

Ultrathin aluminum oxide films: Al-sublattice structure and the effect of substrate on ad-metal adhesion

First principles density-functional slab calculations are used to study 5 {angstrom} (two O-layer) Al{sub 2}O{sub 3} films on Ru(0001) and Al(111). Using larger unit cells than in a recent study, it is found that the lowest energy stable film has an even mix of tetrahedral (t) and octahedral (o) site Al ions, and thus most closely resembles the {kappa}-phase of bulk alumina. Here, alternating zig-zag rows of t and o occur within the surface plane, resulting in a greater average lateral separation of the Al-ions than with pure t or o. A second structure with an even mix of t and o has also been found, consisting of alternating stripes. These patterns mix easily, can exist in three equivalent directions on basal substrates, and can also be displaced laterally, suggesting a mechanism for a loss of long-range order in the Al-sublattice. While the latter would cause the film to appear amorphous in diffraction experiments, local coordination and film density are little affected. On a film supported by rigid Ru(0001), overlayers of Cu, Pd, and Pt bind similarly as on bulk truncated {alpha}-Al{sub 2}O{sub 3}(0001). However, when the film is supported by soft Al(111), the adhesion of Cu, Pd, and Pt metal overlayers is significantly increased: Oxide-surface Al atoms rise so only they contact the overlayer, while substrate Al metal atoms migrate into the oxide film. Thus the binding energy of metal overlayers is strongly substrate dependent, and these numbers for the above Pd-overlayer systems bracket a recent experimentally derived value for a film on NiAl(110).

Ab initio calculations of adsorbate hydrogen-bond strength: ammonia on Pt(111)

Seven-layer slab results for 14 monolayer of NH3 on Pt(111) (so called α-NH3) are compared with NH3 on a 91-atom Pt cluster; we find that the latter closely mimics the extended surface. The calculations predict atop site occupancy for α-NH3 with N-down. The H-bond between α-NH3 and an additional N-down molecule (β-NH3) approaching from the gas-phase is then compared with that of two molecules in the gas phase; we discover the H-bond on the surface is almost three times stronger and the bond length appreciably shorter. Geometry relaxation then results in a 65 ± 5 degree tilt of the β-NH3 axis. Finally, slab calculations with 14ML each of α- and β-NH3 support this geometry over symmetrically coordinated β-NH3 and predict an adsorption energy in good agreement with experiment.

Chemisorbed-molecule potential energy surfaces and DIET processes

Abstract We report the use of the local-density approximation, with and without gradient corrections, for the calculation of ground-state potential energy surfaces (PESs) for chemisorbed molecules. We focus on chemisorbed NO and ammonia on Pd(1 1 1) and compare our results with the latest experimental information. We then turn to two aspects of excited-state PESs. First, we compare first-principles calculations of the forces on an ammonia ion as a function of distance from the surface. We find that the image-charge model fails significantly at distances which are the most relevant for dynamics, closer than ∼3 A, and discuss reasons for the failure. We then summarize a purely electronic adiabatic model of the moleuule-surface bond and use empirical parameters to estimate hot carrier-produced excited states of chemisorbed NO. We find multiple PESs and a novel interpretation of the π ∗ resonance, seen in inverse photoemission.

Quantum-resolved angular distributions of neutral products in electron-stimulated processes: NO desorption from and NO2 dissociation on Pt(111)

Abstract We present the first quantum-resolved angular distributions of ground-state neutral molecules which are products of electron stimulated desorption (ESD) and electron stimulated dissociation. Laser resonance-enhanced multiphoton ionization (REMPI) and two-dimensional imaging have been used to obtain angular distributions of NO desorbed by 350 eV electrons from O-precovered Pt(111). In a similar fashion, we have measured angular distributions for the NO product of NO 2 dissociation on clean and O-precovered Pt(111). In all cases, we observe narrow widths which are roughly the same as ion distributions determined by ESDIAD (ESD ion angular distributions). The angular distribution for NO ESD is sharply peaked (7° half-width at half maximum) along the surface normal for an O coverage (θ o ) of 0.25 monolayer (ML). The angular distribution of the NO product from dissociation of side-bonded NO 2 on clean Pt(111) is unexpectedly peaked about the surface normal, and thus does not reflect dissociative forces parallel to the surface or the ∼ 25° off-normal ground-state bond direction. On O-precovered Pt(111), where NO 2 is N-bonded, ∼ 6° off-normal beams are observed. When the substrate is precovered with θ o > 0.5 ML, local disorder creates asymmetric site geometries which result in multiple peaked angular distributions with both normal and off-normal (∼9–10°) components; similar effects for NO ESD are observed. In all these studies, the NO angular distributions are invariant to rotational or vibrational state. This implies that the lateral translational degrees of freedom are essentially de-coupled from the internal modes of the molecule. The results are discussed in terms of desorption mechanisms, dissociative forces, site geometries, and disordered coadsorbate layers.

Localized Auger final states in linear alkanes and polyethylene

The energies and Auger lineshapes of the linear alkanes and polyethylene strongly suggest the presence of highly correlated (localized) two‐hole excitations in these materials. This conclusion is supported by a comparison of the polyethylene spectrum to the predictions of independent particle theory. A semiempirical configuration–interaction formalism which explicitly considers final‐state hole screening in the static limit is applied to the alkane spectra. The calculations show that while the ethane Auger spectrum is independent, particle‐like correlation effects are present in longer alkane chains and become more pronounced with increasing chain length. The dominant central peak in the alkane spectra is assigned to transitions to localized states while the higher valence region is assigned to delocalized states.

Electron-stimulated production of NO2(g) from O2 coadsorbed with NO on Pt(111)

Using laser resonance‐enhanced ionization spectroscopy, we have detected O(3PJ=2,1,0) and NO X 2Π3/2,1/2 (ν=5) above a 6–350 eV electron beam‐irradiated Pt(111) surface containing coadsorbed O2 and NO at 90 K. Both product yields have the same chemisorbed NO coverage dependence at saturation O2 precoverage as well as the same ≊10 eV excitation threshold. We conclude that both O(3PJ) and NO(ν=5) are laser‐induced photodissociation fragments of NO2(g). This is established by the observation of identical O(3P2) and NO(ν=5) time‐of‐flight distributions that correspond to NO2 desorption from the surface. The NO2(g) is probably the reaction product of a collision between an O atom (created by electron‐stimulated dissociation of adsorbed O2) and NO(a). We correlate the 10 eV NO2 production threshold with the dissociative ionization of the 3σg molecular bonding orbital of O2(a).