T.E. Jackman

The pt(100) (5 × 20) ⇋ (1 × 1) phase transition: A study by Rutherford backscattering, nuclear microanalysis, LEED and thermal desorption spectroscopy

Abstract 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 × 10 15 Pt atoms cm −2 move into registry with the bulk upon adsorption of 6.4 ± 0.4 × 10 14 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 × 10 15 Pt atoms cm −2 move back into registry upon adsorption of H 2 or D 2 , 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 H 2 treatments with a residual H-coverage of ∼1 × 10 14 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 × 10 15 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.

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

Abstract The absolute coverage (θ) of deuterium adsorbed on Pt(111) in the ranges 180 T −6 P −2 Pa D 2 has been determined by nuclear microanalysis using the D( 3 He, p) 4 He 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 cm 2 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 × 10 13 D atoms cm −2 ) as P → 0, apparently invalidating a recent model of site exchange in the adsorbed layer.

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

Abstract 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.

Absolute coverages and hysteresis phenomena associated with the CO‐induced Pt(100) hex⇄(1×1) phase transition

Temperature hysteresis in the hex⇄(1×1) phase transition in the Pt(100)–CO system has been investigated using Rutherford backscattering spectroscopy, nuclear microanalysis, work function measurements, LEED, and thermal desorption spectroscopy. The onset of the hex→(1×1) transition occurs at an average surface coverage of 0.08±0.05 monolayers while the onset of the (1×1)→hex transition occurs at 0.25±0.05 monolayers. The results are consistent with a recently proposed model for the phase transition.

Intercomparison of absolute standards for RBS studies

Abstract A Harwell Series I Bi-implanted standard calibrated in Chalk River has been compared with a vacuum-deposited thin Ta standard calibrated in Paris. The standards agree to within 1–2%; the “best” value for the Bi standard is now (4.83±0.05)×1015 Bi/cm2 .

Oxygen on Ni(110): Surface phases and related absolute coverages

The adsorption of oxygen on Ni(110) was investigated by nuclear reaction analysis (NRA), XPS, Δφ, temperature programmed reaction spectroscopy (TPRS) and LEED. At 423 K, (3 × 1), (2 ×1) and (3 ×1) phases are formed in sequence with increasing O2 exposure. The coverage in the (2 ×1) phase was determined by NRA, the coverages in the other phases being determined via this calibration by XPS, TPRS and Δφ. Contrary to previous reports, the maximum in the intensity of half-order beams from the (2 × 1) phase is associated with a coverage of (5.6 ± 0.5) × 1014 O atoms cm−2 or 0.49 ± 0.05 monolayers, and not 0.25 monolayers. The two (3 ×1) phases are associated with θ = 0.33 ± 0.03 and 0.64 ± 0.06 monolayers respectively. Oxygen adsorbed at 295 K is not at thermodynamic equilibrium. Annealing to T > 400 K causes significant decreases in Δφ and the formation of the (2 ×1) phase for θ > 0.3.

How well does 4He backscattering from low‐Z nuclei obey the Rutherford formula?

A study of 4He backscattering (150°) cross sections for O, Al, and Si atoms in the energy range 0.6–2.3 MeV has been completed. We conclude that the 4He backscattering cross section for these elements follows the E−2 energy dependence of Rutherford backscattering to 1%–2% over this energy range. These results have significant implications for the use of 4He backscattering as a materials analysis tool for low‐Z elements.

Vibrational properties of Au and Pt(110) surfaces deduced from rutherford backscattering data

Abstract 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.

Ultrahigh vacuum apparatus for combined low‐energy electron diffraction, Auger spectroscopy, MeV ion scattering, and nuclear microanalysis

A bakable target manipulator/2‐axis goniometer has been developed which permits high‐precision MeV channeling studies and nuclear microanalyses to be performed over a wide temperature range (150–1000 K) under ultrahigh vacuum conditions (7×10−9 Pa). Design and performance features of the goniometer and target chamber and of the two‐stage differentially pumped beam‐line connection to a 2.5‐MV Van de Graaff accelerator are discussed. A transmission Faraday cup in conjunction with a set of optically positioned standard targets enables the various ion beam analyses to be performed with an absolute accuracy of 1%–2%.

Absolute calibration of 14N(d, α) and 14N(d, p) reactions for surface adsorption studies

Abstract The (d, α) and (d, p) reactions on 14 N have been calibrated relative to the 16 O(d, p 1 ) 17 O reaction at deuteron energies of 972 and 1200 keV, using high purity NO and N 2 O condensed on a thick Au or Pt substrate at 20 K. Six clearly resolvable nitrogen groups have been calibrated with an overall accuracy of ±3%. With the aid of two primary standards (Ta 2 O 5 and Bi-implanted Si), all relative cross-sections have been converted into absolute values. Excellent agreement with earlier measurements of the 16 O(d, p 1 ) cross-section is obtained; the nitrogen groups, however, all exhibit markedly smaller cross-sections than previously reported. One very weak 14 N group fell under the p 1 peak of the 16 O reactions; hence, the spectrum from a frozen NH 3 target was used to evaluate the necessary background correction to the observed 16 O p 1 yields.