Lyle D. Roelofs

Two‐dimensional chemisorbed phases

Chemisorbed atoms on single‐crystal metal surfaces frequently form commensurate diperiodic structures because of the strong, site‐specific binding. Since the adatoms are well described by the lattice gas model, these systems are valuable testing grounds for recent theories of two‐dimensional critical phenomena. We review briefly the developments in this field, emphasizing our results for the system O/Ni(111). This study includes an analysis of the multicritical low‐coverage phase diagram, Monte Carlo simulations of the overlayer, and the first measurement of critical exponents for a phase transition in a chemisorbed overlayer probed via low energy electron diffraction. We compare our results with predictions of various two‐dimensional models.

Multi-adatom interaction effects in a lattice gas model for Cu and Au adsorption on W(110)

Abstract A lattice-gas model for Cu and Au adsorption on W(110) is proposed and discussed in the light of recent cohesive energy calculations for these systems. Three types of lateral, two types of trio, and a quarto interaction were considered. It is argued that the unusual shape of the binding hollows for these systems results in anomalously large effective trio interactions. Phase diagrams were calculated via a new transfer matrix finite-size scaling method. Repulsive trio interactions were found to increase the degree of asymmetry in the T −θ phase diagram and reduce critical temperatures. Although accurate reproduction of experimental phase diagrams was not attempted, our model seems able to do so and has important advantages over previously proposed models.

Phase diagrams for H/Ni(111) based on model interactions: Effects of strong long-range attractions

Abstract We test the pairwise adatom-adatom interaction sets for H/Ni(111) computed by Muscat by using Monte Carlo techniques to determine the lattice gas phase diagrams they imply and then comparing with experiment. Although the correct ordered phase is reproduced and the maximum disordering temperature is approximately correct, there remain topological discrepancies with the experimental phase diagram. The origin of the main discrepancy is a relatively strong long-range attractive interaction. We describe the behavior this interaction causes and outline the circumstances under which similar phenomena might occur in other systems, developing insight through (re-)examination of the square-lattice Ising metamagnet problem. Finally we show how two previously studied interaction sets with relatively weak sixth-neighbor attractions lead to more accurate phase diagrams. One appendix collects some results on the metamagnet. Two others present transfer matrix scaling calculations of the critical properties of the disordering of two ordered states on a honeycomb lattice: hard hexagons (nearest neighbor exclusion) is Ising-like to high precision, while graphitic (2 × 2) is 4-state-Potts-like.

Adlayer‐induced LEED beams near order–disorder transitions

We present a Monte Carlo study of the adlayer‐induced LEED beams in a representative order–disorder transition. Calculations of the widths of fractional order beams indicate that, as seen in recent experimental work on the O/Ni(111) system, these widths rise sharply and suddenly above Tc. Furthermore the slope of this rise provides a sensitive probe of the adatom‐adatom interactions. The increase in the width results from the existence of ordered domains above Tc, and has not been previously discussed since prior calculations have concentrated on beam intensities. We further derive a functional relation between the window approach used in simulations of order–disorder transitions and the instrument response function approach of the experimentalist, to detail the effect of the LEED apparatus on the profile of the extra beams.

Extended appearance potential fine structure analysis of oxidized metal surfaces

Like EXAFS, extended appearance potential fine structure (EAPFS) probes the immediate neighborhood of a particular element and thus, unlike most other structural probes, does not require long‐range order. Furthermore, the use of low‐energy electron makes the technique surface sensitive. Because no dipole selection rules apply to electron excitation, explicit calculation must determine the angular momentum of the outgoing final‐state electron and thus the feasibility of Fourier inversion of the data. For excitation of the O 1s core, we find the outgoing electron is overwhelmingly s‐wave. Reaction of Al(100) with about 1‐1/2 equivalent monolayers of oxygen disorders the surface completely (i.e., extinguishes the LEED pattern). For this surface, analysis of the O 1s edge gives an O–Al distance of 1.98±0.05 A, the longer of the two Al–O spacings in bulk Al2O3, which indicates that the oxygen goes under the top layer of Al. For a thick oxide layer on Ni(100), diffraction effects precluded use of the O 1s edge....

Competition between direct and concerted movements in surface diffusion with application to the Au(110) surface

Abstract Recent calculations of the energetic barriers for diffusion of Au atoms on the Au(110) surface have shown that for moves perpendicular to the close-packed atomic rows, concerted, or multi-atom movements are energetically favorable over single-atom movements for the same displacement. Entropic hindrances, however are expected to limit concerted moves. In the present paper this competition is investigated via simulation techniques based on model potentials that reproduce the relevant energtic and entropic features of the Au(110) situation. Apparently non-Arrhenius behavior is found for the concerted motions. A new scheme, which avoids the necessity of long simulation runs to determine the “hop” rate at several temperatures, for determining entropic barriers directly is proposed and tested. Though this method is applied in the present context to a simplified model for the interatomic potentials, it can be straightforwardly extended to realistic treatments of those interactions.

Using LEED to study specific heat anomalies of adsorbed overlayers

Near a second order phase boundary the leading thermal dependence of the integrated intensities of “extra” LEED beams is proportional to q JTT,I’-” for T P T,, where (Y is the specific heat critical exponent. This behavior occurs when the correlation length exceeds the characteristic length of the instrument but is less than the characteristic size of defect-free regions. We illustrate these ideas using the structure factor of a ~(2x2) overlayer on a triangular lattice, simulated using Monte Carlo. Similar behavior can appear in other probes of phenomena dependent solely on finite-range correlations.

Nearest Neighbor Spacings of Clean Vanadium Surfaces from Extended Appearance Potential Fine Structure

Abstract : Fine structure in the excitation probability of the 2p core state of vanadium by electron bombardment has been found to extend for several hundred volts above the appearance potential threshold. The structure results from interference of an outgoing spherical wave of a scattered electron with backscattered components from neighboring atoms. It is therefore analogous to extended X-ray absorption fine structure (EXAFS) and it should be possible to extract geometric information using the same Fourier inversion techniques. The short mean free path for inelastic scattering of the incident electrons, however, renders the technique sensitive to the structure in just the outermost layers of the material.

Kinetic prefactors for concerted-mode diffusion: a realistic calculation — AuAu(110)

Activation energies and prefactors governing various self-diffusion processes on the Au(110) surface are calculated using the embedded atom method approximation for the surface energetics. A detailed investigation, which includes the contribution of friction, establishes that the prefactors for concerted- or exchange-type diffusion perpendicular to the close-packed rows are comparable to those governing direct, single-atom hops along the rows.

Interacting depolarizable adatoms on a triangular lattice a model for alkali metal adsorption on fcc (111) and hcp (0001) surfaces

Abstract We investigate the phase diagram of adsorbed dipoles on a triangular lattice by means of Monte Carlo simulation. Our model incorporates depolarization and multiplicity of binding sites, effects thought to be important for describing the adsorption of alkali metal atoms on metal surfaces. We successfully reproduce several important features of alkali-metal absorption systems including: the characteristic saturation in the work function decrease with coverage and the condensation behavior resulting from depolarization. We consider the T = 0 behavior for the general case and present the full condensation phase diagram for one set of parameters.