John D. Head

Theoretical investigation of molecular water adsorption on the Al(111) surface

Abstract Cluster calculations are used to theoretically investigate the interaction of water at low surface coverages with the Al (111 ) surface. Adsorption geometries, vibrational frequencies and binding energies are computed. Molecular water is found to adsorb at an Al on-top site and the computed vibrational frequencies agree well with data from HREELS experiments. However, in contrast to the experimental interpretation which assigns a 3720 cm −1 band to a hydroxyl stretch, the calculations indicate there is little water dissociation taking place at this stage and we reassign the 3720 cm −1 band to the symmetric OH stretch in water. We also propose refinements to the previous experimental assignments for some of the low frequency modes. A novel feature of the calculations presented is that they model adsorption induced surface relaxation effects on the Al substrate. Significant relaxation effects are found although, unlike for the adsorbate geometry, the substrate geometries do not converge rapidly with cluster size.

Theoretically modelling the water bilayer on the Al(111) surface using cluster calculations

Abstract Cluster calculations involving two water molecules are used to theoretically investigate the formation of the water bilayer on the Al(111) surface. In previous calculations we found a single water to molecularly adsorb at an on-top site vertically above an Al atom. We also computed vibrational frequencies which were consistent with data obtained at low water coverage by electron energy loss spectra (EELS). The EELS experiment initially has a peak at 3720 cm −1 which becomes augmented and eventually dominated by a second peak at 3450 cm −1 as the water coverage on the Al(111) surface is increased. In the present calculations we optimized the geometry of two water molecules on Al 10 and Al 15 clusters. The resulting structures appear to be prototypes for the formation of a water bilayer on the Al(111) surface: one of the water molecules again adsorbs on-top of an Al atom, and then the second water hydrogen bonds to the first water molecule, at a height corresponding to the second layer of a water bilayer, vertically above an Al atom neighboring the Al atom bonded to the first layer water. The computed vibrational frequencies for the chemisorbed water dimer have a number of features in common with the EELS results obtained at higher water coverage. Especially striking is that we compute a ∼ 300 cm −1 blue shift in the H-bonded OH stretching mode which matches the splitting observed for the high frequency modes observed in EELS at higher water coverage. These dimer calculations add further support to our previous suggestions that the 3720 cm −1 vibration is due to a symmetric OH stretch in non H-bonded water and that water only molecularly adsorbs on Al(111) at low temperatures. Again we allow for adsorbate induced surface relaxation effects in the cluster calculations.

A localized orbitals based embedded cluster procedure for modeling chemisorption on large finite clusters and infinitely extended surfaces

A new embedded cluster procedure for modeling chemisorption on metal surfaces is developed. The procedure is similar in philosophy to the approach used by Whitten and co‐workers in that energy calculations are performed in a cluster region basis consisting of localized occupied and virtual orbitals. However, we present a new localization procedure to generate the cluster region functions which is based on orbital occupation numbers determined from the density matrix obtained in a calculation on the extended substrate. Our localization procedure avoids having to perform separate unitary transformations on the canonical occupied and virtual orbitals and as a consequence has the attractive feature of enabling the embedded cluster calculations to be applied to both large finite clusters and infinitely extended systems in essentially the same manner. We illustrate the embedded cluster procedure by performing partial SCF calculations in the cluster region basis for H adsorption at an on‐top site of a Li(100) mo...

A vibrational analysis with Fermi resonances for methoxy adsorption on Cu(111) using ab initio cluster calculations

An analysis of the vibrational frequencies observed with high-resolution electron energy loss spectroscopy (HREELS) and reflection adsorption infrared spectroscopy (RAIRS) for methoxy adsorbed at the threefold sites on the Cu(111) surface has been performed using ab initio cluster calculations. The OC stretch and the CH3 bending and rock modes can readily be assigned with the computed harmonic vibrational frequencies, but the three-peak structure experimentally observed in the CH stretching region is more challenging. The frequencies of the first overtone for the CH3 bending modes are found to be very close to the fundamental CH stretching frequencies suggesting that a Fermi resonance between the two types of modes might take place. We verify this origin for the three-peak structure in the CH stretching region by using a simple numerical differentiation procedure to estimate the energy third derivatives, which should be the principal cause of the coupling between the CH stretching fundamentals and the bending overtones. Using this procedure we compute three dipole active modes in the CH stretching region with a relative intensity pattern very similar to that observed in the HREELS and RAIRS.

A theoretical comparison of CO bonding on the Fe(100) surface

Abstract Unrestricted Hartree-Fock (UHF) cluster calculations with the Constrained Space Orbital Variation (CSOV) analysis are used to compare the bonding in vertical on-top, vertical bridging and tilted CO on the Fe(100) surface. Large charge transfers and strong back donation effects are found in the tilted CO cluster consistent with the expectation that this structure is a precursor to CO dissociation. The UHF calculations also reveal that mainly minority spin Fe d electrons are donated to the CO π ∗ orbitals. A qualitative interpretation of the recent spin polarized photoemission spectra for tilted CO on Fe(100) is presented.

A quantum chemical investigation and reassignment of the HREELS observed O vibrations on the Al(111) surface

Abstract Quantum chemical calculations on Al clusters are used to simulate the vibrations of O atoms adsorbed on the Al(111) surface. Each O atom in the different Al clusters with either a single O, O 2 or O 3 , is found to preferentially adsorb at a threefold site. We find the experimentally observed HREELS peaks in the 545–650 cm −1 range to correlate with O vibrations normal to the Al(111) surface. While the HREELS peaks in the 800–850 cm −1 range, which was assigned previously to subsurface O, correlate with surface O vibrating parallel to the Al(111) surface to give an assignment which is now more consistent with the expectations of recent STM experiments. However, in agreement with the symmetry based reassignment of the HREELS data by Frederick, Lee and Richardson, we only obtain HREELS active O vibrational modes parallel to surface after symmetric combinations of the modes from O adatoms at neighboring fcc threefold sites are able to form. Further, to obtain vibration frequency values in the ranges measured by the HREELS experiments we diagonalized a mass weighted second derivative matrix and found it necessary to avoid the common approximation of simply treating the mass of the Al surface atoms as infinite. We also used a vibration correction formula, originally developed by Black, as a check on the convergence of the frequency calculations and as a tool for exploring the range of the adsorbate to substrate interactions.

A theoretical investigation of adsorbate-induced surface relaxation effects using cluster models: Al on Si(111)

Abstract Differently sized cluster calculations are used to investigate theoretically the preferred adsorption site for an Al adatom on the Si(111) surface. By performing partial geometry optimizations at the Hartree-Fock level on AlSi n subclusters around the site of interest we find significant Al adatom-induced surface relaxation effects distorting the Si atoms from their bulk lattice positions. The largest relaxation effects take place at the T 4 site resulting in Al adsorption at the T 4 site to be 5 kcal/mol more stable than at the H 3 site and considerably more stable than adsorption at the T 1 site. However, we only have confidence in this result after performing for the T 4 site a partial geometry optimization on the AlSi 5 subcluster in the AlSi 26 H 24 cluster and by including appropriate correlation corrections.

A cluster model study of hydrogen adsorption on Al(100)

Abstract Quantum-chemical cluster calculations are used to model H adsorption on the Al(100) surface. At low H surface coverage, the bridging and on-top site vibrational frequencies are computed to be 1091 and 1767 cm −1 and are in excellent agreement with the 1125 and 1750 cm −1 vibrations observed in electron energy loss spectra (EELS) experiments. Calculations on the Al 14 H 2 cluster provide an explanation for the simultaneous growth of the 2 EELS peaks at 1750 and 750 cm −1 when the H/Al (100) surface is annealed. In the optimal Al 14 H 2 structure the two H atoms are adsorbed at neighboring on-top sites with the computed 1776 cm −1 Al—H stretch and 784 cm −1 H—H vibration. The cluster calculations which simulate H monolayer coverage indicate the H adsorption energy to be relatively insensitive to surface coverage and are consistent with the zeroth- order kinetics found in the thermal desorption spectra of H 2 from Al surfaces.

A CLUSTER MODEL STUDY OF MULTIPLE H ADSORPTION ON BE(0001)

Abstract The different results obtained from slab and cluster calculations are compared. In the clusters we model various H coverages on the Be(0001) surface using a single-layer Be 14 cluster interacting with different numbers of H atoms. With a single H atom the on-top structure is the most stable. Two H atoms are computed to preferentially adsorb at an open site above 3 Be atoms. Caution always needs to be exercised when comparing slab and cluster calculations owing to the different adsorbate coverages being modelled. However, by increasing the number of H atoms in the cluster to simulate a monolayer of H atoms on the Be(0001) surface we obtain results similar to slab calculations with the open and bridging structures close in energy and the on-top structure being least stable. These calculations with H monolayer coverage are noteworthy because they also enable the definition of a desorption energy E des corresponding to the energy of removal for a single H atom from the monolayer. E des differs in value from the average H binding energy D e often used to evaluate the stability of different adsorbate structures on a surface. By using E des , an explanation for the experimentally observed bridging structure for H on the Be(0001) surface is suggested. A similar computation of E des is not possible in a slab calculation owing to limitations on the allowed unit cell size.

An ab initio MO study of aluminium atom adsorption on the basal plane of graphite

Ab initio cluster calculation which model the adsorption of Al atoms on the basal plane of graphite are presented. One layer clusters find the Al prefers to adsorb at the center of the carbon rings. A two layer cluster determines the most stable Al adsorption site to be above the carbon atom in agreement with recent Scanning Tunneling Microscopy experiments. The charge distributions on the clusters are oscillatory and largely independent of the Al position. The form of the lowest unoccupied pi orbital for each cluster essentially determines the cluster charge distribution. From a constrained space orbital variation we find the on top site to be favored when Al and cluster wavefunctions are superimposed. However when the Al and pi orbitals interact the relaxation favors the open site.