Absolute coverages for the various surface phases of CO and O adsorbed on Pd(110)

The absolute surface coverages of CO and O on Pd(110) have been measured by nuclear reaction analysis (NRA) using the ¹²C(d, p)¹³C and ¹⁶O(d, p₁)¹⁷O¹ reactions. The CO coverages of the (2 × 1) and (4 × 2) phases of CO on Pd(110) are 1.00 ±0.05 and 0.73 ±0.05 ML (1 ML = 1 monolayer = 9.4 × 10¹⁴ CO molecules cm⁻²) respectively. The oxygen coverage in the c(4 × 2) phase of O on Pd(110) is 0.50 ±0.05 ML.     

Absolute coverage measurements of deuterium on Ni (110)

The absolute coverage of deuterium adsorbed on Ni(110) at temperatures below 170 K to the formation of a (1 × 2) LEED pattern has been determined by nuclear microanalysis (NMA). The result, θ_D = 0.96 ± 0.08, is consistent with a saturation coverage of one full monolayer. Heating the crystal above ～ 190 K is shown to result in a gradual loss of deuterium from the system, accompanied by streaking of the LEED pattern, with complete desorption above ～ 340 K. The low-temperature (2 × 1)-D phase was found to correspond to θ_D = 0.64 ± 0.05 monolayers. The results are expected to be valid also for the equivalent phases obtained by hydrogen adsorption.     

Interaction of O2 with Pt(100): I. Equilibrium measurements

Two newly discovered phases on the Pt(100) surface produced by the adsorption of oxygen have been investigated using Rutherford baekscattering (RBS), nuclear microanalysis (NMA), work function changes (Δφ) and LEED. One phase is associated with the oxygensaturated surface (0.63 ± 0.03 monolayers0.81 × 10¹⁵ O atoms cm^?2), where a very complex LEED pattern is observed; the other is observed at an average coverage of 0.44 ± 0.05 monolayers and gives rise to a (3 × 1) LEED pattern (when observed at room temperature). For both surfaces, RBS measurements indicate large (? 0.025 nm) Pt atom displacements. Also discussed is a new method for preparing the “clean” (1 × 1)-Pt(100) surface without the need for NO adsorption/decomposition.     

Displacive surface phases formed by hydrogen chemisorption on W {001}

A detailed LEED study is reported of the surface phases stabilised by hydrogen chemisorption on W {001}, over the temperature range 170 to 400 K, correlated with absolute determinations of surface coverages and sticking probabilities. The saturation coverage at 300 K is 19(± 3) × 10¹⁴ atoms cm^?2, corresponding to a surface stoichiometry of WH₂, and the initial sticking probability for both H₂ and D₂ is 0.60 ± 0.03, independent of substrate temperature down to 170 K. Over the range 170 to 300 K six coverage-dependent temperature-independent phases are identified, and the transition coverages determined. As with the clean surface $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$ displacive phase, the c(2 × 2)-H phase is inhibited by the presence of steps and impurities over large distances (～20 Å), again strongly indicative of CDW-PLD mechanisms for the formation of the H-stabilised phases. These phases are significantly more temperature stable than the clean $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$, the most stable being a c(2 × 2)-H split half-order phase which is formed at domain stoichiometries between WH_0.3 and WH_0.5. LEED symmetry analysis, the dependence of half-order intensity and half-width on coverage, and I-V spectra indicate that the c(2 × 2)-H phase is a different displacive structure from that determined by Debe and King for the clean $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$. LEED I-V spectra are consistent with an expansion of the surface-bulk interlayer spacing from 1.48 to 1.51 Å as the hydrogen coverage increases to ～4 × 10¹⁴ atoms cm^?2. The transition from the split half-order to a streaked half-order phase is found to be correlated with changes in a range of other physical properties previously reported for this system. As the surface stoichiometry increases from WH to WH₂ a gradual transition occurs between a phase devoid of long-range order to well-ordered (1 × 1)-H. Displacive structures are proposed for the various phases formed, based on the hypothesis that at any coverage the most stable phase is determined by the gain in stability produced by a combination of chemical bonding to form a local surface complex and electron-phonon coupling to produce a periodic lattice distortion. The sequence of commensurate, incommensurate and disordered structures are consistent with the wealth of data now available for this system. Finally, a simple structural model is suggested for the peak-splitting observed in desorption spectra.     

Absolute coverage and isostemc heat of adsorption of deuterium on Pt(111) studied by nuclear microanalysis

The absolute coverage (θ) of deuterium adsorbed on Pt(111) in the ranges 180< T<440 K and 5 × 10^?6 < P < 5 × 10^?2 Pa D₂ has been determined by nuclear microanalysis using the D(³He, p)⁴He reaction. From these data, the isosteric heat of adsorption (E_a) has been determined to be 67 ± 7 kJ mol^?1 at θ ? 0.3. This heat of adsorption yields values of the pre-exponential for desorption (10^?5 to 10^?2 cm² atom^?1 s^?1) that lie much closer to the normal range for a second order process than those determined from previous isosteric heat measurements. The E_a versus θ relationship indicates that the adsorbed D atoms are mobile and that there is a repulsive interaction of 6–8 kJ mol^?1 at nearest neighbour distances. At 300 K the coverage decreases to ? 0.05 monolayer (? 8 × 10¹³ D atoms cm^?2) as P→ 0, apparently invalidating a recent model of site exchange in the adsorbed layer.     

COMBINED EFFECTS OF SUBCOOLING AND SURFACE ORIENTATION ON POOL BOILING OF HFE-7100 FROM A SIMULATED ELECTRONIC CHIP

The effects of orientation and subcooling on pool boiling of the HFE-7100 dielectric liquid near atmospheric pressure (0.085 MPa) from a 10 × 10 mm smooth copper surface are investigated experimentally. Results are obtained for inclination angles θ = 0° (upward-facing), 30°, 60°, 90°, 120°, 150°, and 180° (downward-facing) and liquid subcoolings ΔT_sub = 0, 10, 20, and 30 K. Increasing θ decreases the saturation nucleate boiling heat flux at high surface superheats (ΔT_sat > 20 K), but increases it only slightly at lower surface superheats. The critical heat flux (CHF) decreases slowly with increasing θ from 0° to 90°, and then deceases faster with increasing θ to 180°. CHF increases linearly with increased subcooling, but the rate increases from 0.016 K^?1 at 0° to 0.048 K^?1 at 180°. At θ = 0° and ΔT_sub = 30 K, CHF is ～ 36 W/cm² and 24.45 W/cm² for saturation boiling, while at θ = 180° CHF = 10.85 W/cm² at ΔT_sub = 30 K and only 4.30 W/cm² at saturation. The developed correlation for CHF of HFE-7100, as a function of θ and ΔT_sub, is within ±10% of the present data. The recorded still photographs of the boiling surface in the experiments illustrate the effects of liquid subcooling and surface orientation at different nucleate boiling heat fluxes and surface superheats on vapor bubble accumulation and/or induced mixing at the surface.     

A study of the adsorption of oxygen on Ni(111) using auger electron spectroscopy: Chemical shifts and valence spectra

Auger electron spectra have been recorded when oxygen is adsorbed on a Ni(111) single crystal surface. For the coverage range θ < 1, an analysis of the plot of the peak to peak height (H) of the oxygen KVV (516 eV) transition versus the total number of molecules cm^2? impinging on the surface (molecular beam dosing) shows agreement with the kinetic mechanism proposed by Morgan and King [Surface Sci. 23 (1970) 259] for the adsorption of oxygen on polycrystalline nickel films. In this coverage range, no energy shifts of the nickel or oxygen Auger peaks were recorded.At coverages θ > 1 (standard dosing procedure) shifts in the valence spectra M_{2, 3}VV (61 eV) and L₃M_{2, 3}V (782 eV) of ?2.3 eV and ?1.8eV respectively are recorded at 1.4 × 10^?2 torr-sec. Up to these coverages no shift of the L₃VV transition (849 eV) is observed. A chemical shift of ?2.1 eV is recorded in the L₃M_{2, 3}M_{2, 3} Auger transition (716 eV) at 1.4 × 10^?2 torr-sec.In the coverage range θ > 1, shifts in the energy of the oxygen Auger peaks are observed. At 5.8 × 10^?3 torr-sec. the KVV (516 eV) and KL₁V (495.2 ± 0.3 eV) transitions show shifts of ?1.5 eV and ?(1.0 ±0.3) eV respectively. No shift up to this coverage is recorded in the KL₁L₁ (480.6 ± 0.3 eV) transition.     

The Pt(110) phase transitions: A study by rutherford backscattering,nuclear microanalysis,LEED and thermal desorption spectroscopy

The adsorbate induced (1×2) (1×1) (2×1)p1g1 phase transitions on Pt(110) have been studied by Rutherford backscattering (RBS), nuclear microanalysis (NMA), LEED and thermal desorption spectroscopy. RBS data indicate that any displacement of the surface atoms from their expected bulk-like lattice sites in the (1×2) phase is ? 0.002 nm laterally and ? 0.007 nm vertically. This contraint eliminates models for the reconstruction which involve significant lateral displacements (e.g., the paired-atom or hexagonal overlayer models). The RBS data are consistent with both the rumpled model with up/down displacements not exceeding ~0.007 nm and the missing row model with an unrelaxed surface in which the out-of-plane vibrational amplitude is slightly enhanced. A c(8×4) phase, produced by CO (or NO) exposure at T?250 K, has also been characterized by RBS which demonstrated that 0.92×10¹⁵ Pt cm^?2 move on average by ~0.017 nm laterally out-of-registry with the bulk upon formation of this phase. The values of the saturation adsorbate coverages at T?200K were determined by NMA to be 0.92 ± 0.05×10¹⁵, 1.0 ± 0.06×10¹⁵ and 1.07 ± 0.10×10¹⁵ CO molecules, NO molecules and D atoms, respectively, per cm². The value of the saturation coverage by CO (θ = 1.0) supports recent models of the (2×1)p1g1 overlayer. The isosteric heat of adsorption of CO is 160 ± 15 kJ mol^?1 in the range 0.2?θ?0.5.     

Hydrogen chemisorption on Pd(100) studied with he scattering

He scattering from the clean Pd(100) surface yields extremely weak diffraction beams relative to the specular, corresponding to a very small maximum corrugation amplitude of ~ 0.04 Å. Hydrogen adsorption at a temperature of 110 K leads to the formation of a c(2 × 2) ordered phase at a coverage of 0.5 monolayers and a (1 × 1) phase at saturation coverage. The maximum corrugation amplitude of the c(2 × 2)H is ~0.13 Å; surface charge density calculations using overlapping atomic charge densities indicate a normal distance of the hydrogens to the topmost Pd layer d_n ? 0.65–0.70 Å corresponding to a H-Pd bonding distance of ~ 2.05 Å in the fourfold hollow sites. The result that the maximum corrugation amplitude of the (1 × 1) hydrogen phase, with ~ 0.025 Å, even smaller than that of the clean surface may indicate a movement of the hydrogens closer to the topmost metal layer, when the coverage is increased from 0.5 monolayers to saturation.     

Absolute coverages of CO and O on Pt(111); Comparison of saturation CO coverages on Pt(100), (110) and (111) surfaces

Nuclear microanalysis (NMA) has been used to determine the absolute coverages of oxygen and CO adsorbed on Pt(111). The saturation oxygen coverage at 300 K is 3.9 ± 0.4 × 10¹⁴ O atoms cm^?2 (θ = 0.26 ± 0.03), confirming the assignment of the LEED pattern as p(2 × 2). The saturation CO coverage at 300 K is 7.4 ± 0.3 × 10¹⁴ CO cm^?2 (θ = 0.49 ± 0.02). The low temperature saturation CO coverages on Pt(100), (110) and (111) surfaces are compared.     

Author index volumes 111–120

Adsorption of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe hole field emission microscopy and electron spectroscopy. Sub-monolayer bismuth forms a relatively strongly bound layer of bismuth-tungsten dipoles with dipole moment μ = (0.18 ± 0.02) × 10^?30C m and polarizability α = (6.3 ± 1.3) × 10^?40J V^?2m². Changes in work function and their dependence on temperature closely parallel those produced by adsorbed lead which was shown by LEED to form c(2 × 2), p(2 × 2) and (1 × 1) structures. Bismuth is thought to behave in a similar way, but, unlike lead, forms a second monolayer which replicates the first. Electron spectroscopy shows that sub-monolayer bismuth removes the surface state (Swanson) peak and at monolayer coverage a new peak emerges and shifts with increasing coverage. Using Gadzuk's theory, this peak is tentatively attributed to the ²P level in bismuth adatoms which form a p(2 × 2) structure in the first and second monolayers. Its shift with coverage is ascribed to an increase in the local surface field. There remains the difficulty of reconciling the proposed occupation of the ²P level with the observed small positive charge on the bismuth adatom.     

Surface phases of Se deposited on Ni(001)

Reflection electron diffraction and Auger electron spectroscopy have been used to study the temperature versus coverage (T versus θ) phase diagram for Se submonolayers on Ni(001). Disordered, p(2×2) and c(2×2) phases are observed for θ < 0.5. (2×1) and c(4×2) phases were nerver observed. High Se exposures (θ > 0.5) lead to bulk compound formation for substrate temperatures T>350 K with the only distinguishable diffraction pattern being c(4×4). The p(2×2)-disorder and c(2×2)-disorder transitions appear to be continuous (second-order) over the range observed. The character of the p(2×2)−c(2×2) transition may also be continuous but this cannot be unambiguously determined from the data. Symmetry arguments predict the Se/Ni(001) system to be a realization of the Ashkin-Teller model and thus make specific predictions which need to be confirmed in future studies. Good agreement with the phase diagram is obtained using a transfer matrix scaling calculation with up to fourth-nearest-neighbor pairwise interactions.     

Fractional-order desorption of D2 from a Pd(110) surface

The desorption of deuterium from the low temperature (α₂) state on a Pd(110) surface was studied by TDS, LEED and Δφ. Simultaneous measurements of the isothermal desorption of the α₂ state, intensity of a half-order beam of the (1 × 2) phase and work function change show clearly that the desorption of this low temperature state is associated with the phase transition (1 × 2) to (2 × 1). This transition correlation with a work function change of ∼ 70 mV. The desorption follows nearly zero-order kinetics with an activation energy of ∼ 71 kJ mol⁻¹. The desorption kinetics are thus closely analogous to those on Ni(110).     

High-resolution electron energy-loss spectroscopy of CO on an Ni(110) surface

High-resolution vibrational electron energy-loss spectra of CO on an Ni(110) surface were studied at 300 K with the in-situ combination of LEED, Auger electron spectroscopy and work-function change measurement. The observed peaks are at 436 cm^?1, 1855 cm^?1 (shifting to 1944 cm^?1 with increasing coverage) and at 1960 cm^?1 (shifting to 2016 cm^?1 with increasing coverage). The experimental results indicate that CO is adsorbed non-dissociatively at all coverages. Three adsorbed states of CO have been found. At fractional CO coverages less than θ ~ 0.9 where the disordered adsorbed structure dominates, CO is adsorbed in two inequivalent sites (short- and long-bridge sites) at random with its axis oriented perpendicular to the surface. At high coverages (θ > 0.9) where the (2 × 1) structure develops, our results indicate that the adsorbed CO molecules may occupy the distorted long-bridge sites forming zig-zag chains which lie essentially in the troughs of the (110) surface.     

Low energy electron diffraction observations on Au-enriched Ni-0.8%Au alloy(110) surfaces prepared by surface segregation

Au-enriched (110) surfaces with Au coverages θ up to 1 monolayer (ML) were prepared by heating a Ni-0.8%Au crystal at temperatures T > 400°C. Low energy electron diffraction (LEED) indicated the following domains at T ≈ 20°C: 5× 1 (prominent for θ ≈ 0.7 ML), c(2 × 2) (0.7−0.9 ML). 7 × 7 (0.8−1.0 ML). The 7 × 7 domain was shown to contain c(2 × 4) subunits. The observations are shown to be consistent with a nearly close-packed 7 × 7 overlayer structure in which each c(2 × 4) subunit is associated with a hexagonal formation of six Au atoms, with a seventh Au atom at the center. Ni(210)-1 × 1 facets and Au(321)-3 × 1 facets due to Au crystallites were present on θ ≈ 1 ML surfaces. The effect of increasing T on θ and on LEED intensities was observed for θ ≈ 1 ML surfaces. θ was found to decrease by 15% on increasing T from 300 to 800°C. A first-order 7 × 7 → 2 × 7 phase transition was observed at T = 340 ± 10°C. Specific nearly close-packed overlayer models with AuAu equilibrium interatomic distance 2.7 Å were proposed for both the 7 × 7 and 2 × 7 surfaces. The models were shown to explain the observed LEED patterns qualitatively. Other LEED observations were interpreted tentatively to indicate a transition to a disordered overlayer structure for T > 550°C, disappearance of facets for T > 600°C, and one-dimensional melting of Au overlayers for T ≈ 800°C.     

A LEED study of adsorbed neon on graphite

Neon monolayers on graphite have been investigated by high resolution LEED in the range 14.5 < T < 7.5 K and 10^?8 < p < 10^?4 Torr. The fluid-solid transition line has the form ln$\text{(}\text{p}\text{Torr}\text{)= ?}\text{A}\text{T + B}\text{,}\text{with}\text{A = 740 ± 45}\text{K and}\text{B = 31.0 ± 2.8}$. The solid is an incommensurate rotated phase whose lattice parameter decreases and rotation angle increases away from the transition line. Comparison is made with other thermodynamic and diffraction studies, and a preliminary phase diagram is constructed. Extrapolation of these data to higher temperatures and pressure suggests that this rotated solid monolayer is stable up to 23–25 K (P = 3?10 Torr) at coverage x = 1, and is stable over the range 0.88 < x < 1.0 at T = 16 K (2 × 10^?7 < p 1.5 × 10^?2 Torr). Extrapolation to lower temperatures gives the 2D triple point pressure in the range p = (0.3?3) × 10^?10 Torr. The steep slope of the fluid-solid transition line is consistent with the fluid phase having a high density (x?0.80) in the temperature range studied.     

Plasmons in monolayer Na,K and Rb films adsorbed on Ni(100)

Electronic excitations in denser monolayer Na, K and Rb films and Na duolayer films adsorbed on a Ni(100) surface have been investigated using Electron Energy Loss Spectroscopy (EELS). Lateral adatom distributions were monitored by LEED. Angular integrated EEL spectra from the ordered c(2 × 2)Na, coverage θ = 0.5, and the ordered hexagonal structures of K and Rb, θ = 0.29, show prominent losses at 3.1, 1.9 and 1.7 eV, of presumably collective nature. The loss energies shift with coverage as ∝ θ^0.4 and as ∝ θ^0.8 for the Na and K, Rb respectively. Angular resolved EEL spectra indicate an only weak dependence of the loss energies on the momentum transfer, Q. In particular the K and Rb losses seem to pass through shallow energy minima, which is predicted by the “box model”. Low energy losses observed at ?1.3 and ?1.0 eV for the c(2 × 2)Na and the hexagonal K and Rb, respectively, are tentatively identified with interband excitations. The observed interband energies yield, when introduced in the “box model”, 3.1., 2.3 and 2.4 eV for the Na, K and Rb, Q = 0 plasmon energies, which is in fair agreement with the observed plasmon loss energies.     

Chemisorption studies on cobalt single crystal surfaces: I. Carbon monoxide on Co(0001)

The chemisorption of CO on Co(0001) and on a polycrystalline specimen has been studied by LEED, Auger spectroscopy, and thermal desorption measurements. Annealing of the polycrystal was found to result in a surface dominated by crystallites of (0001) orientation in the surface plane, along with a few (101&#x0304;2) oriented crystallites. CO adsorbs on the clean surface at 300 K with an initial sticking probability of 0.9 and the system follows precursor state kinetics. The saturation coverage under UHV conditions corresponds to a well-ordered (√3 × √3)R30° structure; with P_CO>5 × 10^-9 a uniform compression of the adlayer takes place and a (√7 × √7)R19.2° structure begins to form. Models are proposed for these two ordered phases which are in agreement with the observed relative coverage data and the appearance of the corresponding desorption spectra. The desorption enthalpy of CO at low coverages is 103 ± 8 kJmol^-1, and a fairly sharp fall in this enthalpy occurs for coverages $\text{>}\text{1}\text{3}$. In many respects, the system's behaviour closely resembles that of Ni(111)-CO. Oxygen contamination leads to the appearance of a strongly adsorbed CO state with a desorption enthalpy of ~170 kJmol^-1. This is reminiscent of a strongly adsorbed non-dissociated state of CO on Ru(101&#x0304;1) which occurs under similar conditions.     

The adsorption sites of CO on Ni(111) as determined by infrared reflection-absorption spectroscopy

The adsorption of CO on Ni(111) has been studied using infrared reflection-absorption spectroscopy combined with LEED, Auger electron spectroscopy, thermal desorption spectroscopy and work function measurements. At low CO coverage (θ = 0.05) CO adsorbs on threefold sites with a strecthing frequency given by ω_CO = 1817 cm^?1. At θ = 0.30 all molecules have shifted to two-fold sites, and θ = 0.50, where a c(4 × 2) structure is observed, ω_CO = 1910 cm^?1. At θ = 0.57, with a (√7/2) × √7/2)R19.1° structure, one quarter of the molecules are adsorbed on top of the nickel atoms with the others in two-fold sites. Molecules bonded on the top sites give rise to a band at 2045 cm^?1. The frequency shift due to dipole-dipole interactions is small compared with the shift resulting from bonding to different crystallographic sites.