Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation U. Memmert, J‐W. He, K. Griffiths, W. N. Lennard, P. R. Norton, N. V. Richardson, T. E. Jackman, W. N. Unertl; D2 on Pd(110): Surface and subsurface phases, absolute coverages, and interconversion. Journal of Vacuum Science & Technology A 1 May 1989; 7 (3): 2152–2154. https://doi.org/10.1116/1.575947 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAVS: Science & Technology of Materials Interfaces and ProcessingJournal of Vacuum Science & Technology A Search Advanced Search |Citation Search

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)17O∗ 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.

The interaction of O2 with Pt(100)–hex and (1×1) surfaces at 123 K was studied by thermal desorption spectroscopy, x-ray photoemission, and work function techniques. On the hex surface, oxygen is adsorbed entirely as the molecular state with a sticking coefficient of 0.13 and exhibits an activation energy for desorption ∼37 kJ mol−1. The saturation coverage is 0.24×1015 O2 molecules cm−2 and desorption does not lead to any dissociation. On the (1×1) surface, oxygen dissociates, even at 123 K. The sticking coefficient is 0.4, and the saturation coverage is 0.48×1015 O atoms cm−2. On warming to 240 K, a (2×1) LEED pattern is observed that is believed to originate from an oxygen overlayer on an unreconstructed substrate. At ∼360 K, the surface transforms irreversibly to the oxygen stabilized (3×1) reconstructed phase.

The potential usefulness of work function measurements made simultaneously with Rutherford backscattering has been illustrated through the study of the temporal oscillations observed in the catalytic oxidation of CO over Pt(100).

The kinetics and energetics of the interaction of O2 with Pt(100)-hex and (1 × 1) surfaces were studied by thermal desorption spectroscopy, work function techniques and X-ray photoemission. Three states of adsorbed oxygen are formed in roughly equal amounts at saturation (saturation coverage = 0.81 ± 0.04 × 1015O atoms cm−2). The state desorbing at the lowest temperature (β1) which exhibits a very narrow desorption peak is associated with a phase transition (complex to (3 × 1)) involving Pt atom displacements. The next state to desorb (β2), is best modeled by second order kinetics, a constant activation energy for desorption Ed0 (≈ 10−3 cm2 atom−1 s−1, Ed ≈ 160 kJ mol−1) and is associated with a further phase transition, (3 × 1) → hex. The high temperature state (β3) which populates first with an initial sticking coefficient of ≈ 4 × 10−3 at 573 K is believed to be associated with surface defects in the hex-overlayer. The rate of adsorption of the β2 state at T ≳ 573 K increases with increasing coverage at 0.1 ≲θ ≲ 0.3, probably because of nucleation and growth of (3 × 1) islands which act as traps for further adsorption. The maximum sticking coefficient into either the β1 or β2 states is ≈ 10−3. The initial sticking coefficient of O2 on Pt(100)−(1 × 1) is ≈ 0.1.

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.

The absolute deuterium coverages associated with the (2×1) and (1×2) phases on Ni(110) at 175 K have been measured by nuclear reaction analysis to be 1.0 and 1.5 monolayers, respectively. Our measured values are consistent with the previous models but are substantially larger than those reported recently by Stensgård and Feidenhans'l.

The structure and composition of a Pt(100) surface have been monitored by Rutherford backscattering (RBS), nuclear microanalysis (NMA), LEED, and work function (Δφ) techniques during temporal oscillations in the rate of CO oxidation. For a Δφ oscillation amplitude of 60 mV, a constant 0.46±0.06×1015 Pt atoms are out of registry with the bulk throughout an oscillation cycle. The maximum fraction of the surface that could be oscillating between the hex and (1×1) phases is thus 8%. The average CO and O coverages during the oscillations are 0.19±0.04 and 0.13±0.03 monolayers, respectively. These results are discussed in terms of a recent model for the temporal oscillations. No oscillations in Δφ have been detected in similar experiments on Pt(111).

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 × 10 15 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.

The detection of Auger electrons under MeV ion bombardment has been investigated as a surface analytical probe of particular utility for low- and middle-Z elements. Using UHV chambers at Bell Laboratories and Chalk River, we have observed Auger electron emission with cylindrical mirror analyzers operated in both the energy-integral and energy-differential modes. Single crystal targets of Si, Ni and Au were bombarded with H+ and 4He+ beams in the 1–2 MeV range under channeled and random directions of incidence. A large reduction in Auger yield occurs under channeling conditions and the observed channeled-to-random ratio, YCAYRA, correlates well with a simple model based on the shadow cone radius, the lattice vibrational amplitude, the adiabatic electron excitation distance and the Auger electron escape length. An even stronger channeling effect isseen in the total electron yield. This is attributed to the enhancement of YR by inelastically scattered electrons of higher energy, originating deep within the target.

Rutherford backscattering measurements (RBS) of Pt(110) and Au(l10) surfaces, both of which exhibit a (1×2) reconstruction, have been made over the temperature range 160–600 K (Pt) and 150–1000 K (Au). Angular scans about the [110] and [100] directions have been measured for the (1×2) phase on both materials. Detailed Monte Carlo simulation studies, including the effects of equal-time displacement correlations, have been used to interpret the vibrational properties of this surface phase. The Pt [110] surface peak data provide clear evidence that correlations of the atom vibrations exist and are consistent with those expected from bulk inelastic neutron scattering measurements. This was not the case for the Au [110] surface peak which exhibited higher yields than predicted.

The reconstruction exhibited by clean Pt(100) surfaces [(5 × 20) LEED pattern] is removed by the adsorption of CO. Rutherford backscattering (RBS) indicates that 1.65 ± 0.05 × 1015 Pt atoms cm−2 move into registry with the bulk upon adsorption of 6.4 ± 0.4 × 1014 CO molecules cm−2 (θ = 0.50 ± 0.03 monolayers). The data indicate that some atoms in the second and perhaps even subsequent layers must be displaced by ≳0.01 nm in the reconstructed surface. By contrast, only 1.3 ± 0.1 × 1015 Pt atoms cm−2 move back into registry upon adsorption of H2 or D2, and the LEED pattern also indicates that residual reconstruction remains. The stability of the CO-covered, H-covered and “almost clean” (1 × 1) surfaces (the latter prepared by NO and H2 treatments with a residual H-coverage of ∼1 × 1014 H atoms cm−2) was investigated by RBS. The CO-covered surface starts to reconstruct only when the CO coverage drops below 0.5 monolayers (T ≳ 450 K) while the H-covered surface (produced by adsorption on the (5 × 20) surface) reconstructs rapidly at T ≳ 350 K, by which temperature the adsorbed hydrogen coverage drops below ∼0.2 monolayers. The “almost clean” surface reconstructs at T ≳ 390 K and the data indicate that the process exhibits an activation energy of 88 ± 17 kJ mol−1. The absolute coverages of CO and D were determined by nuclear microanalysis (NMA) and excellent agreement was achieved between the LEED and NMA data. The saturation CO coverage was found to be 0.77 ± 0.03 monolayers, consistent with the observed c(4 × 2) LEED pattern. Deuterium (and hence hydrogen) coverages of 1.54 ± 0.1 × 1015 D (H) atoms cm−2 (θ = 1.20 ± 0.08) were found at saturation at ∼150 K and the hydrogen adsorbed on the (1 × 1) surface was more strongly bound than that resulting from adsorption on the (5 × 20) surface.

Nuclear microanalysis (NMA) has been used to determine the absolute coverages of oxygen and CO absorbed 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 × 1014COcm-2 (θ = 0.49±0.02). The low temperature saturation CO coverages on Pt(100), (110) and (111) surfaces are compared.

An analytic expression is derived for the magnitude of the “surface peak” in the energy spectra of ions backscattering from an atomic string in the regime where more than two string atoms contribute to the backscattering. The thermal amplitudes of the atoms comprising the model string can be varied, thus facilitating calculations relevant to the investigation of the vibrational properties of surfaces by RBS. This aspect of the model is illustrated by comparison wit experimental measurements for the He/Pt system.

The adsorption of CO (or NO) at temperatures below 250K onto the clean reconstructed (1*2) surface of Pt(110) produces a new metastable c(8*4) phase. Rutherford backscattering measurements have shown that the equivalent of an entire monolayer of Pt atoms is displaced during the adsorption.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation W. N. Unertl, T. E. Jackman, P. R. Norton, D. P. Jackson, J. A. Davies; Summary Abstract: Surface phases of clean, CO and NO covered Pt(110). J. Vac. Sci. Technol. 1 March 1982; 20 (3): 607–608. https://doi.org/10.1116/1.571404 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAVS: Science & Technology of Materials Interfaces and ProcessingJournal of Vacuum Science and Technology Search Advanced Search |Citation Search

Backscattering of MeV 4He+ ions in conjunction with nuclear microanalysis has been used to study the effect of adsorbed gases on the interplanar spacing between the outermost Pt layers of the Pt(100)−(1 × 1) surface at 175 K. For the “almost clean” surface, the spacing is found to be within ~0.2% of the bulk spacing: i.e. the surface is unrelaxed. Similar measurements on CO- or H2-covered surfaces, however, show that monolayer adsorption of either gas produces a small outward relaxation of ~0.015 nm (i.e. 0.8%). The results also suggest that the surface Debye temperature is considerably lower than that of the bulk.

High resolution (0.09 eV) UPS spectra have been obtained of condensed films of N 2 and CO. All spectral features are broadened by ≳ 0.6 eV upon condensation. The origin of this broadening is discussed. The difference in linewidths for all equivalent levels, Δε CON 2 ∼ 0.1 to 0.2 eV can be understood in terms of a hole-dipole multiphonon excitation mechanism. Photoemission from what is believed o be the a 3 Π excited neutral state of CO has been detected in the solid phase for the first time.

High-energy (Rutherford) ion-scattering, in conjuncion with channeling techniques, provides a simple and direct method of measuring surface relaxation. Its sensitivity is comparable to LEED, but it avoids many of the interpretative difficulties. We report an experimental study of the relaxation of a (111) platinum surface using MeV He+ backscattering. An outward relaxation of (0.31 ± 0.06) Å was obtained for surfaces with monolayer coverages of adsorbed oxygen.