The chemisorption and decomposition of NO on the (110) surface of iridium

The chemisorption of NO on Ir(110) has been studied with thermal desorption mass spectrometry (including isotopic exchange experiments), X-ray and UV-photoelectron spectroscopies, Auger electron spectroscopy,LEED and CPD measurements. Chemisorption of NO proceeds by precursor kinetics with the initial probability of adsorption equal to unity independent of surface temperature. Saturation coverage of molecular NO corresponds to 9.6 × 10¹⁴ cm^?2 below 300 K. Approximately 35% of the saturated layer desorbs as NO in two well separated features of equal integrated intensity in the thermal desorption spectra. The balance of the NO desorbs as N₂ and O₂ with desorption of N₂ beginning after the low-temperature peak of NO has desorbed almost completely. Molecular NO desorbs with activation energies of 23.4–28.9 and 32.5–40.1 kcal mole^?1, assuming the preexponential factor for both processes is between 10¹³–10¹⁶ s^?1. At low coverages of NO, N₂ desorbs with an activation energy of 36–45 kcal mole^?1, assuming the preexponential factor is between 10^?2 and 10 cm²s^?1. Levels at 13.5, 10.4 and 8.5 eV below the Fermi level are observed with HeI UPS, associated with the 4σ, 5σ and 1π orbitals of NO, respectively. Core levels of NO appear at 531.5 eV [O(1s)] and 400.2 eV [N(1s)], and do not shift in the presence of oxygen. Oxygen overlayers tend to stabilize chemisorbed NO as reflected in thermal desorption spectra and a downshift in the 1π level to 9.5 eV.     

The reduction of NO with D2 over the (110) surface of iridium

The heterogeneously catalyzed reaction between NO and D₂ to produce N₂, ND₃ and D₂O over Ir(110) was investigated under ultra-high vacuum conditions for partial pressures of the reactants between 5 × 10^?8 and 1 × 10^?6Torr, total pressures between 10^?7 and 10^?6 Torr, and surface temperatures between 300 and 1000 K. Mass spectrometry, LEED, UPS, XPS and AES measurements were used to study this reacting system. In addition, the competitive coadsorption of NO and deuterium was investigated via thermal desorption mass spectrometry and contact potential difference measurements to gain further insight into the observed steady state rates of reaction. Depending on the ratio of partial pressures ($\text{R }\text{P}{}_{\mathrm{<mtext>D</mtext>}}{}_2\text{P}{}_{\mathrm{<mtext>NO</mtext>}}$), the rate of reduction of NO to N₂ shows a pronounced enhancement when the surface is heated above a critical temperature. As the surface is cooled, the rate maintains a high value independent of temperature until a lower critical temperature is reached, where the rate drops precipitously. This hysteresis is due to a change in the structure and composition of the surface. For sufficiently large values of R and for an “activated” surface, N₂ and ND₃ are produced competitively between 470 and 630 K. Empirical models of the different regimes of the steady state reaction are presented with interpretations of these models.     

The structure of the c(2 × 2) oxygen overlayer on the unreconstructed (110) surface of iridium

An Ir(110)-c(2 × 2)O structure has been prepared by adsorbing a half-monolayer of oxygen at room temperature on an unreconstructed (1 × 1)Ir surface stabilized by a quarter-monolayer of randomly adsorbed oxygen. Results of the low energy electron diffraction structural analysis indicate that the ordered oxygen atoms are residing on the short-bridged sites on the (110) surface. The Ir-O interlayer spacing is 1.37 ± 0.05 Å, and the bond length is 1.93 ± 0.07 Å. The topmost substrate interlayer spacing is found to be 1.33 ± 0.07 Å rather than 1.26 ± 0.07 Å which is the topmost interlayer spacing of the unreconstructed (1× 1)Ir surface.     

The interaction of hydrogen and cyclopropane with the (110) surface of iridium

The interaction of cyclopropane with hydrogen and the residue resulting from the decomposition of the former on the reconstructed Ir(110)-(1×2) surface has been studied with thermal desorption mass spectrometry. Although hydrogen will not adsorb onto the saturated overlayer of dissociatively adsorbed cyclopropane, the preadsorption of hydrogen into the β₂ adstate inhibits the decomposition of cyclopropane on the surface. Desorption of the hydrogen from the saturated overlayer of the dissociatively adsorbed cyclopropane partially regenerates the reactivity of the surface.     

The chemisorption of nitrogen on the (001) surface of ruthenium

High resolution electron energy loss spectroscopy (EELS), thermal desorption mass spectrometry (TDMS) and low energy electron diffraction (LEED) have been used to investigate the molecular chemisorption of N₂ on Ru(001) at 75 K and 95 K. Adsorption at 95 K produces a single chemisorbed state, and, at saturation, a (√3x√3) R30° LEED pattern is observed. Adsorption at 75 K produces an additional chemisorbed state of lower binding energy, and the probability of adsorption increases by a factor of two from its zero coverage value when the second chemisorbed state begins to populate. EEL spectra recorded for all coverages at 75 K show only two dipolar modes — ν(RuN₂) at 280–300 cm^?1 and ν(NN) at 2200–2250 cm^?1 — indicating adsorption at on-top sites with the axis of the molecular standing perpendicular to the surface. The intensities of these loss features increase and ν(NN) decreases with increasing surface coverage of both chemisorbed states.     

The process of oxygen chemisorption on the Si(111) surface

The chemisorption of oxygen on the Si(111) surface has been studied by the ASED-MO method. Three steps of the initial oxidation process have been proposed. The first step is molecular oxygen chemisorption. The second step is that of dissociated oxygen chemisorption in which the atomic short bridge site (between the first layer and second layer silicon atoms) can be occupied only after the saturation of the dangling bonds of the surface silicon with oxygen. The third step is the diffusion of atomic oxygen from the short bridge positions into the bulk of silicon to form an SiO₂ film. For molecular chemisorption, both the peroxy vertical and peroxy bridge models are possible although the peroxy vertical model is the more stable. The dissociated atomic oxygen can chemisorb for both the on-top and the short bridge models. Our results can explain, and are consistent with, most experimental results.     

Ellipsometric studies of chemisorption on GaP(110) single crystals

Ellipsometry has been used to study the room temperature adsorption of H₂, O₂, Cl₂, Br₂, HCl, HBr, HI, H₂S, CH₃SH, H₂Se, NH₃, PH₃ and AsH₃ on GaP(110) surfaces; CO, CO₂ and CH₄ did not adsorb. Initial sticking coefficients were determined from the change in the ellipsometric angle Δ with time at a constant gas pressure and found to lie between I and 10 ^?5. The change in both Δ and ψ at monolayer coverage was measured as a function of wavelength in the visible and near UV for several gases. The observed structure was at variance with theoretical expectation for non-absorbing layers and, further, appeared similar for each adsorbate. This is attributed to small changes in the optical properties of the substrate arising from the removal of intrinsic surface states by chemisorption. Analysis of the data reveals GaP surface states with a maximum interband transition probability at a greater than band gap energy of 3.5eV. The optical constants of film-free GaP were also measured between 2 and 5.5eV, and direct interband transitions between bulk states were located at 3.76 and 5.05eV.     

Ab initio chemisorption studies of H on Fe(110)

Ab initio configuration interaction calculations are performed to study the chemisorption of atomic H on a Fe(110) surface. The lattice is modeled as an embedded three-layer, 40-atom cluster with the Fe atoms fixed at the bulk position. Fe 3d orbitals are explicitly included on five Fe atoms on the surface. Hydrogen strongly binds to the Fe(110) surface at the long-bridge, short-bridge, and quasi three-fold sites. The calculated adsorption energies are 2.76, 2.73, and 2.71 eV, respectively. H-surface bonding at the on-top Fe site is more than 0.4 eV weaker. The calculated H-surface distances are 0.89, 1.03, and 0.87 Å for H at the long-bridge, short-bridge, and quasi three-fold sites, respectively, which agrees well with the LEED value of 0.9 ± 0.1 Å. The H-surface stretching vibrational frequencies are calculated to be 1070, 1066, and 1073 cm⁻¹, at the long-bridge, short-bridge, and quasi three-fold sites, respectively. The work function of Fe(110) decreases on H adsorption. The present calculations indicate that H diffusion into the bulk through the short-bridge site will have a much higher activation barrier than via the long-bridge and quasi three-fold sites.     

Oxygen chemisorption and initial oxidation of Cr(110)

The initial stages of the interaction of oxygen with a Cr(110) surface have been investigated at 300 K by LEED, AES, electron energy loss spectroscopy (ELS), secondary electron emission spectroscopy (SES) and work-function change measurement (Δφ). In the exposure region up to 2 L, the clean-surface ELS peaks due to interband transition weakened and then disappeared, while the ～5.8 and 10 eV loss peaks attributed to the O 2p → Cr 3d transitions appeared, accompanied with a work-function increase (Δφ = +0.19 eV at2L). In the region 2–6 L the work function decreased to below the original clean-surface value (Δφ_min = ?0.24 eV at6L), and five additional ELS peaks were observed at ～2, 4, 11, 20 and 32 eV: the 2 and 4 eV peaks are ascribed to the ligand-field d → d transitions of a Cr³⁺ ion, the 11 eV peak to the O 2p → Cr 4s transition, the 20 eV peak to the Cr 3d → 4p transition of a Cr³⁺ ion and the 32 eV peak probably to the Cr 3d → 4f transition. A new SES peak at 6.1 eV, being attributed to the final state for t he 11 eV ELS peak, was observed at above 3 L and identified as due to the unfilled Cr 4s state caused by charge transfer from Cr to oxygen sites in this region. In the region 6–15 L the work function increased again (Δφ_max = +0.32 eV at15 L), the 33 and 46 eV Auger peaks due to respectively the M_2,3(Cr)L_2,3(O)L_2,3(O) cross transition and the M_2,3VV transition of the oxide appeared and the 26 eV ELS peak due to the O 2s → Cr 4s transition was also observed. Above 10 L, the ELS spectra were found to be practically the same as that of Cr₂O₃. Finally, above 15 L, the work function decreased slowly (Δφ = +0.13 eV at40L). From these results, the oxygen interaction with a Cr(110) surface can be classified into four different stages: (1) dissociative chemisorption stage up to 2 L, (2) incorporation of O adatoms into the Cr selvedge between 2–6 L, (3) rapid oxidation between 6–15 L leading to the formation of thin Cr₂O₃ film, and (4) slow thickening of Cr₂O₃ above 15 L. The change in the Cr 3p excitation spectrum during oxidation was also investigated. The oxide growth can be interpreted on the basis of a modified coupled current approach of low-temperature oxidation of metals.     

Oxygen chemisorption on Cr(110): II. Evidence for molecular O2(ads)

Oxygen chemisorption and dissociation on Cr(110) at 120 K have been studied using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Dissociative adsorption dominates although vibrational and stimulated desorption data provide evidence for a coexisting minority molecular binding state. An O₂(ads) vibrational frequency of 1020 cm⁻¹ and a six beam ESDIAD pattern are suggestive of super-oxo O₂(ads) bonding at six local sites each with the O-O molecular axis tilted away from the surface normal. These results are compared with data for chemisorbed oxygen on other transition metal surfaces.     

The re-entrant surface order-disorder transition on the BCC(110) surface

The surface order-disorder transition has been studied for the BCC(110) surface of a binary alloy by use of the Bragg-Williams approximation. In the case with a large surface segregation energy, we have found a re-entrant surface order-disorder transition above the transition temperature of the bulk crystal. The origin of this transition can be understood by considering the variation of the surface concentration with temperature.     

Oxygen chemisorption on W(110): A displacement reconstruction model

The system W(110) p(2 × 1)-O is studied. A bond stretching and bending model which predicts that the ordering of the adsorbed oxygen reconstructs the W(110) surface, is developed. The magnitude of the lateral displacement is 0.076 Å. The implications of the model in the formation of the (2 × 1) overlayer structure are also examined.     

The oxidation of the GaAs (110) surface

It is demonstrated that, contrary to previous proposals, Ga surface atoms are already involved in the oxidation process for the lowest observable oxygen coverages (0.01 monolayer). A similar involvement of As atoms could not be readily ascertained experimentally, although it is to be expected from energetic considerations. An oxidation model consisting of multiple bridge bonds to both Ga and As surface atoms is proposed, which is consistent with diverse experimental data for the GaAs(110) surface.     

Influence of surface roughness and chemisorption on magnetic hysteresis curves of a Ni(110) surface observed by spin-resolved inverse photoemission

Using the high spin asymmetry in inverse photoemission of the Ni d-band just above the Fermi level as an indicator of surface magnetization, we have measured hysteresis curves of the (110) surface of nickel. Nearly rectangular hysteresis loops indicate a well-defined behavior of the surface magnetization of the picture-frame single crystal with sides along 110 directions. The influence of geometrical order and chemisorption of O, S, and CO on the shape of the hysteresis loops has been investigated. We found a significant reduction of the coercive force (about 10%) if the clean high-quality (110) surface is disordered on an atomic scale by ion bombardment or low-coverage chemisorption.