Chemisorption phenomena: Analytic modeling based on perturbation theory and bond-order conservation

Tremendous advances in experimental studies of chemisorption revealed that many phenomena could not be understood and projected by the current theoretical constructs. We discuss some of the experimental puzzles that prompted a development of new analytic approaches to chemisorption based on general principles such as perturbation theory (PT) and bond-order conservation (BOC). The PT results concern the periodic regularities of the heat of chemisorption, the role of the antibonding adsorbate orbitals, and universal patterns of adsorbate-induced surface polarization Some of the PT findings are further corroborated within a much broader BOC approach. The BOC model and its postulates (including the use of a Morse potential) and diverse projections are thoroughly discussed. For atomic A and diatomic AB adsorbates, it is shown how the BOC model explicitly and rigorously interrelates a variety of seemingly disparate phenomena such as preferred adsorbate sites, the activation barriers for surface migration and dissociation, relations between atomic Q_A (Q_B) and molecular Q_AB heats of chemisorption, coverage and coadsorption effects on Q_A, overlayer phase transitions and island formation, the nature of promotion and poisoning. The model also projects possible intermediates and elementary steps of surface reactions. Although some of the findings are counter to commonly held perceptions, the whole picture of chemisorption is coherent and fits experiment well. The new conceptual understanding is stressed and some comments on the theory of chemisorption are made.     

Coverage effects under atomic chemisorption: Morse-potential modeling based on bond-order conservation

Our recent analytical Morse-potential model based on bond-order conservation has been extended to treat coverage (θ) effects on the heat of atomic chemisorption Q. For highly symmetric surfaces such as fcc(111), fcc(100), and bcc(100), explicit expressions for Q versus θ have been obtained projecting regularities of Q(θ) and of the overlayer structures, in encouraging agreement with experiment. In particular, the model predicts that Q should typically decrease with θ (though at very low θ, Q can sometimes increase) and that there may be some critical coverage θ_c<1 beyond which the second-order phase transition (hollow→bridge or on-top) will occur.     

Bond making and breaking on transition-metal surfaces: Theoretical projections based on bond-order conservation

Our analytic Morse-potential model of chemisorption based on bond-order conservation [Surface Sci. 150 (1985) L115; 163 (1985) L645, L730] has been used to calculate the heats of chemisorption of various diatomic AB and polyatomic AB_x species (coordinated via A) and to estimate the activation barriers for their dissociation and transformations. Examples include adspecies such as CH_x, NH_x, OH_x, and possible intermediates and elementary steps of reactions such as CO + O → CO₂, NO + N → N₂ + O, N₂ + H₂ → NH₃, H₂ + O₂ → H₂O, and CO + H₂ → CH₄. Both the qualitative projections and numerical estimates are in good agreement with experiment. In particular, it is shown that (1) the most reactive adspecies should be the most weakly bound, and (2) the recombination activation barrier should primarily depend on (and may even be close to) the heat of chemisorption of the weaker bound partner.     

Ethylene, sulphur, and chlorine coadsorption on Pd(111): a theoretical study of poisoning and promotion

First principles calculations for the coadsorption of C₂H₄ with S and Cl on Pd(111) are presented. Sulphur poisons adsorption, decreasing the strength of the ethylene–surface interaction. While low coverages of chlorine alone do not appear to affect ethylene adsorption on the otherwise clean palladium surface, chlorine does act as a promoter on the sulphur poisoned surface, increasing the strength of the ethylene–surface interaction. The promotional effect is attributed to changes in the dative bonding of the molecule and the surface and cannot be explained solely in terms of changes in the metal workfunction.     

Hydrogen, sulphur and chlorine coadsorption on Pd(111): a theoretical study of poisoning and promotion

First principles calculations for the coadsorption of hydrogen with sulphur and chlorine on Pd(111) are presented. Individually, both sulphur and chlorine poison hydrogen adsorption, sulphur being the more efficient poison. The observed sulphur poisoning is a structural effect. The adsorption energy decreases and the diffusion barrier increases for hydrogen atoms in the vicinity of sulphur adatoms. A sulphur coverage of 0.33 ML is sufficient to completely poison hydrogen adsorption on Pd(111). The presence of chlorine adatoms on the sulphur-poisoned surface is found to stabilise localised hydrogen adsorption. The possible promotional effects of chlorine on sulphur-poisoned catalysts are discussed.     

Analytic modeling of tidal effects in the relativistic inspiral of binary neutron stars

To detect the gravitational-wave (GW) signal from binary neutron stars and extract information about the equation of state of matter at nuclear density, it is necessary to match the signal with a bank of accurate templates. We present the two longest (to date) general-relativistic simulations of equal-mass binary neutron stars with different compactnesses, C=0.12 and C=0.14, and compare them with a tidal extension of the effective-one-body (EOB) model. The typical numerical phasing errors over the ?22 GW cycles are Δ??±0.24 rad. By calibrating only one parameter (representing a higher-order amplification of tidal effects), the EOB model can reproduce, within the numerical error, the two numerical waveforms essentially up to the merger. By contrast, the third post-Newtonian Taylor-T4 approximant with leading-order tidal corrections dephases with respect to the numerical waveforms by several radians.     

Coadsorption of Ba and O on polycrystalline W

The thin film properties of the barium-oxygen-tungsten system were studied using a combination of AES, Δφ, and low-damage SIMS techniques. Due to the extreme reactivity of barium toward residual gases, all experiments not requiring oxidation (including simultaneous SIMS and barium deposition) were completed at operating pressures never exceeding the 10^-11 Torr range. The barium-oxygen-tungsten system was found to have work function-coverage properties similar to those of the cesium-oxygen-tungsten system. The order of adsorption was found to give different structures; and in the case of Ba → O/W, a patchy surface was discovered. The low-damage SIMS yields showed correlation with work function.     

Coadsorption of carbon monoxide and hydrogen on polycrystalline rhodium

The coadsorption of carbon monoxide and hydrogen on polycrystalline rhodium filament has been studied by thermal desorption mass spectrometry. From a series of thermal desorption spectra of CO and H₂ from rhodium as a function of the exposure time to the gas mixture of CO and H₂, it is indicated that there are a single broad peak for CO and three peaks designated as β₁, β₂, β₃ for hydrogen. Thermal desorption of hydrogen is complex. CO and β₁-hydrogen coadsorb on the rhodium surface with their partial pressures in the initial stage of the exposure to the gas mixture and then the β₁ -hydrogen adsorbed on the surface is replaced by CO with the further exposure time. The kinetics for the replacement of β₁-hydrogen by CO may be discussed from the standpoint of the L-H reaction process. The β₂-and β₃-hydrogen are observed with a longer exposure time after the elimination of β₁ -hydrogen. It may be suggested that the β₃-hydrogen peak is attributed to the desorption of hydrogen absorbed in the bulk. The nature of β₂ -hydrogen is also briefly mentioned in possible implications.     

Coadsorption of oxygen and sodium on Ru(001)

The interaction of oxygen with sodium predosed Ru(001) is studied by means of thermal desorption, Auger and electron loss spectroscopy and work function measurements. The initial sticking coefficient of oxygen is found to increase from 0.45 for bare Ru(001) to 1 for Ru(001) with a 0.35 monolayer sodium coverage. The adsorption capacity of the sodium predosed Ru(001) surface towards oxygen is enhanced from θ_O = 0.5 for clean Ru(001) to θ_O = 1.4 for Ru(001) with a 0.7 monolayer sodium coverage. The work function, electron loss changes and thermal desorption data give evidence that as long as θ_Na is less than 0.25, the oxygen chemisorption phase is characterized mainly by oxygen-Ru bonds and by the absence of strong sodium-oxygen interactions. At high sodium coverages (θ_Na > 0.35), the experimental data indicate the formation of a Na-O compound in the second adsorption layer at high oxygen exposures. When Ru(100) is predosed with sodium (θ_Na ? 0.25), this leads to complete suppression of oxygen penetration into the bulk during heating, the latter process being observed for the oxygen-Ru(001) system.     

Coadsorption of cyanogen and potassium on Pt(111)

The coadsorption of potassium and cyanogen on Pt(111) has been studied by thermal desorption spectroscopy. A new feature in the cyanogen desorption is identified at 363 K, 30 K above the desorption temperature for the molecular state of cyanogen on clean Pt(111). The potassium/cyanogen interaction which leads to the new desorption feature has a reasonably well defined 1 : 1 stoichiometry.     

Coadsorption of oxygen and carbon monoxide on tungsten: Desorption spectra,electron stimulated desorption and field emission microscopy

CO/W desorption spectra are characterized by an α state and multiple β states; using electron stimulated desorption (ESD) the α state was shown to comprise two sub-states, α₁ and α₂. In this paper the consecutive interactions of O₂ and CO on W are investigated using ESD, flash desorption and field emission microscopy (FEM).Desorption spectra show that the α-CO state is displaced by O₂, in two stages. The ESD probe provides an identification of the first stage with the removal of the α₁-CO state, and energy analysis of ESD ions reveals a large energy shift (～ ? 1.5 eV) during O₂ coadsorption which can be attributed to an incresae in the α₁-CO WC bond length of ～ 0.15 Å. During this O₂-induced displacement, the two β peaks converge into a single peak at the β₁ position; this is ascribed to adatom interactions in the mixed O and C adlayer. Isotope exchange experiments with ²⁸CO and ³⁶O₂ reveal (i) no exchange in the α-CO states, and (ii) complete exchange in the β-CO states, which is consistent with dissociative adsorption in the latter. The amount of coadsorbed O₂ is estimated from these results, and from FEM data: a full monolayer of O adatoms can be coadsorbed on CO-saturated W, but CO pre-adsorption inhibits the formation of W oxides. The β₁-O₂ (ESD active) state also forms on the CO-covered surface: this state is identical in population, ESD cross section and ion energy distribution to β₁-O₂ on clean W, and retains its identity in the mixed layer (it does not undergo isotopic exchange). CO₂ desorption spectra from the mixed layer were also characterised, complete isotopic scrambling being observed.Pre-exposure of tungsten to O₂ inhibits CO adsorption: a monolayer of O₂ is sufficient to prevent CO adsorption, and at low O₂ coverages, every O₂ molecule preadsorbed prevents one CO molecule from adsorbing. Isotopic exchange is again complete in the β states, and a lateral interaction model for desorption kinetics, based on dissociative adsorption in the β-CO state, quantitatively describes the CO desorption spectra.     

Numerical and physical modeling of the tomography process based on third-order nonlinear acoustic effects

The possibility of employing the nonlinear effect of generation of third-order combination waves for the purposes of medical diagnostics is analyzed. This effect can be used to reconstruct the spatial distribution of acoustic nonlinear parameters in the framework of the wave approach. Contributions of third-order nonlinear scattering itself and of the double second-order scattering are evaluated. These two competing processes evolve simultaneously and produce similar observed effects, which can nevertheless be separated. A two-dimensional experimental scheme that contains only three transmitters and one receiver, uses two primary wideband modulated waves and an introduced third monochromatic wave, is proposed. Results of the numerical and physical model experiments are provided.     

Coadsorption of oxygen and cesium on Ru(001)

The coadsorption of oxygen and Cs on Ru(001) has been studied by means of thermal desorption, Auger and electron loss spectroscopy and work function measurements. The initial sticking coefficients for oxygen adsorption and oxygen saturation coverages increase with increasing Cs coverage, θ_Cs. Irrespective of the initial θ_Cs, the Cs desorption energy always increases under the influence of the coadsorbed oxygen, the effect becoming stronger with increasing oxygen coverage. At θ_O>0.5and θ_Cs>0.14 the work function, electron loss changes and thermal desorption data give evidence of strong CsO interactions and the formation of a CsO “surface compound”.     

The effects of momentum conservation on pionization

The Mueller-Regge models of inclusive processes in the “pionization” region are constrained to obey momentum conservation. A seperate representation in the fragmentation region is found to be necessary on both theoretical and phenomenological grounds. It is also argued that the Laplace transform technique of solving the constraint equations highlights the bad low-energy behaviour of these models.     

The structure of benzene on Rh(111): Coadsorption with CO

The molecular structure of benzene coadsorbed with CO on Rh(111) has been investigated by angle resolved UV photoemission (ARUPS), LEED and thermal desorption spectroscopy. The symmetry of the benzene adsorption complex in a mixed benzene-CO (3×3) LEED structure has been determined to C_6v by ARUPS. This indicates that the benzene molecules remain essentially undistorted in the CO coadsorbate layer, as it has been found for the pure benzene layer on Rh(111).     

Coadsorption of NO and D2 on Rh(100)

The question addressed in the title was investigated experimentally for low coverage CO and H on Pt(111). The consistent result of two kinds of experiments is that the scattering cross section is in general not influenced by the surface temperature, except for H on Pt(111) in the limit of small changes of the normal component of the He wave vector, Δ k_z. The result simplifies substantially the use of He scattering as an analytical technique for surface dynamics investigations. Semi-classical arguments indicate that the lack of influence of the surface temperature at low coverages is related to two circumstances: (1) the small contribution to the specular He beam of the scattering from adsorbate induced disturbances of the repulsive potential and (2) the absence of long range modifications of the vibrational properties of Pt induced by adsorbed CO or H.     

Point defects in L1_0 FePt studied by molecular dynamics simulations based on an analytic bond-order potential

The point defects and their related physical properties in L10 FePt are investigated by molecular dynamics simulations based on an analytic bond-order potential. The calculated results agree well with the experimental value, indicating that the analytic bond-order potential is suitable to describe the structural properties and surface energies of the FePt alloy in the L10 phase. However, the calculated vacancy formation energy of an Fe atom is higher than that of a Pt atom, which disagrees with some other p...