Core level binding energy shifts between free and condensed atoms

Assuming a fully screened final state (in the metallic case) and using the (Z + 1) approximation and a Born—Haber cycle we calculate the shift in the core level binding energy between the free atom and its metallic state. The agreement with known experimental shifts is shown to be very good. Utilization of the present method can for example give accurate core level binding energies for those free atoms where such data only exist for the metallic phase.     

Core level binding energy shifts in Ni on Au and Au on Ni overlayers

Core level binding energy shifts of the Ni-2p and Au-4? lines have been measured for Ni on Au and Au on Ni overlayers down to mean coverages of less than 0.1 monolayers. When normalized to the maximum shift at submonolayer coverage the Ni on Au and Au on Ni shifts show the same dependence as function of the monolayer coverage. Using the thermodynamical approach for the calculation of binding energy shifts in metals, recently developed by Mårtensson and Johansson, the submonolayer shifts together with experimental results of core level binding energy shifts in dilute NiAu and AuNi alloys are used to calculate surface segregation energies in these alloys. They are compared with semiempirical determinations of these energies by Seah.     

Characteristic energy loss measurements of slow electrons on clean and adsorbate-covered Ru(001)

Characteristic energy losses of low energy electrons backscattered from Ru(001) have been measured under conditions of very low primary electron currents for the clean and the CO- or oxygen-covered surface. The main losses found for the clean and the CO-covered surface are similar to those observed as XPS core satellites which may mean that the influences of the core hole on the initial and the final states of the valence shake-up are about the same. A peak in the secondary electron spectrum of the clean surface is found at 11 eV which is changed by adsorption. The results are discussed in terms of the excitations of the metal and the adsorbates.     

Direct observation of vibrational energy delocalization on surfaces: CO on Ru(001)

We report the experimental observation of the gradual transition from a local oscillator to a two-dimensional delocalized phonon, observed for the CO-stretch vibration of carbon monoxide adsorbed on a Ru(001) surface by means of broadband-infrared saturation sum-frequency spectroscopy. The data are theoretically reproduced by an exchange model with residence times of the excitation down to 2.5 ps.     

Electron energy loss spectroscopy of the decomposition of formic acid on Ru(001)

Electron energy loss spectroscopy has demonstrated the existence of both a monodentate and a symmetric bidentate bridging formate as stable intermediates in the decomposition of formic acid on the Ru(001) surface. The monodentate formate converts upon heating to the bidentate formate which decomposes via two pathways: CH bond cleavage to yield CO₂ and adsorbed hydrogen; and CO bond cleavage to yield adsorbed hydrogen, oxygen and CO. Thermal desorption spectra demonstrate the evolution of H₂,H₂O, CO and CO₂ as gaseous products of the decomposition reaction. The observation of this product distribution from Ru(100), Ni(100) and Ni(110) had prompted the proposal of a formic anhydride intermediate, the existence of which is rendered questionable by the spectroscopic results reported here.     

The properties of K and coadsorbed CO + K on Ru (001): II. Electronic structure

The electronic properties of K and CO + K mixed layers on Ru(001) have been examined in detail with XPS, polarization and angle dependent UPS, and work function changes. The adsorption of K is accompanied by a gradual decrease of the K 2p binding energies and a normal work function behaviour which are discussed in detail. Adsorption of CO on K predosed surfaces also causes a K 2p binding energy decrease at all K coverages which can be understood as repulsion of substrate charge back into the K atom induced by CO orbitals overlapping with the substrate valence band. The complex change in work function caused by CO adsorption is explained by the combination of three effects, CO addition, charge exchange, and K displacement. All results in this and the first paper, in particular the additional peaks in the He I spectra and the HREELS results, are only compatible with the model of a sp²-rehybridized CO molecule in the vicinity of coadsorbed K.     

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.     

Observation of opposite 4f-surface core level binding energy shifts for W(111) and Ta(111)

Surface 4f core level binding energy shifts have been measured in photoemission from W(111) and Ta(111). The surface shift was found to change sign across the row of 5d-metals: for the topmost layer of Ta(111) a +0.40 eV shift toward higher binding energy is found, whereas for W(111) the shift is -0.43 eV toward lower binding energy. The shifts are shown to be dependent on surface crystallography. Chemical shifts are determined for saturation coverage of hydrogen.     

Final state core level binding energy shifts in binary alloys

The final state or relaxation contribution to core level binding energy shift is calculated for some noble metal/transition metal alloy systems, using the pseudopotential linear response method developed previously for pure metals. The core hole perturbation is described by an ab-initio pseudopotential. A basic parameter of the method is the homogeneous electron gas density for which, in the present case, a mean value between those corresponding to each pure metal is adopted. The final state contribution is generally found to be important and sometimes dominant. From the experimental core level shifts and the calculated final state shifts, some considerations about initial state contribution are given.     

X-ray-induced polymerization of furan overlayers on Ru(001)

The effects of Mg K X-rays on furan overlayers on the Ru(001) surface have been investigated using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). It was found that X-ray beams can polymerize furan multilayers condensed at 80 K, resulting in the appearance of new emission features at 532.8 eV in the O 1s XPS spectra and at 3 eV in the UPS spectra. In contrast, monolayer furan on Ru(001) at 80 K shows no signs of polymerization under the same conditions.     

The adsorption and dehydrogenation of cyclopentane on Ru(001)

The adsorption of cyclopentane on Ru(001) has been studied using Electron Energy Loss Spectroscopy (EELS) and Thermal Desorption Mass Spectroscopy (TDMS). Thermal desorption shows with increasing coverage a chemisorbed first layer desorbing at 180 K with subsequent multilayer formation. The vibrational spectrum of the first chemisorbed layer is characterized by a C-H soft mode at 2610 cm^?1. This mode is ascribed to a C-H-metal interaction, which is also responsible for the dehydrogenation to cyclopentene upon annealing to 200 K. It appears that a $\text{close}$ geometrical fit between the $\text{entire}$ molecule and the metal substrate is not necessary for this type of interaction. Coadsorbed oxygen suppresses the C-H-metal interaction. This is believed to be due to site-blocking or ligand effects of oxygen on the three-fold hollow sites of Ru(001).     

Adsorption of N2O on Ru(001)

The adsorption of N₂O on Ru(001) at ~ 100K has been studied using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and thermal desorption spectroscopy (TDS). At low exposures, N₂O partly dissociates leaving atomic oxygen on the surface and desorbing N₂. With increasing N₂O exposures, molecular adsorption becomes dominant. He II UPS of the gas phase, solid and monolayer adsorbed molecular N₂O are compared. To within experimental error, the peak spacings in all three are the same. The distributions of intensities in the gas and solid phase spectra are the same. In the monolayer spectra, the 7~σ (terminal nitrogen lone pair) orbital intensity is decreased significantly indicating that it is more strongly coupled to the surface than the other valence orbitals. No molecular N₂O remains after heating to above 180 K and no detectable amount of dissociated nitrogen appears. Molecularly adsorbed N₂O is easily dissociated by an electron beam to give N₂(g), NO(g) and O(a).     

Surface vibrations of Na(001) and K(001) surfaces

We present the results of a theoretical study of the dynamics of the atom motion of Na(001) and K(001) surfaces. The total electronic energy is calculated using a pseudopotential approach with a confined electron gas as unperturbed system. With this theory the dynamical matrix can he derived without resorting to empirical parametrizations. Surface phonon dispersion curves are reported for the high symmetry directions of the two-dimensional Brillouin zone for ideal and relaxed configurations. The calculated spectra are compared with the results of semi-empirical force constant calculations. The effects of single and multilayer relaxations on the location and the nature of the main surface bands are examined.     

The properties of K and coadsorbed CO + K on Ru(001): I. Adsorption,desorption, and structure

An extensive photoemission and LEED study of K and CO+K on Ru(001) has been carried out. In this paper the LEED and some XPS results together with TPD and HREELS data are presented in terms of adsorption, desorption. and structural properties, and their compatibility is discussed. Potassium forms (2 × 2) and $\text{(}\text{3}\text{×}\text{3}\text{)R30°}$ overlayers below and near monolayer coverage, and multilayer bonding and desorption is similar to that of bulk K. The initial sticking coefficients for CO adsorption on K predosed surfaces are correlated with the initial K structure, and s₀ and CO saturation coverages decrease with increasing K coverage. Two well-characterized mixed CO+K layers have been found which are correlated with predosed (2 × 2) K and $\text{(}\text{3}\text{×}\text{3}\text{)R30°}\text{K}$. They have CO to K ratios of 3:2 and 1:1, and lead to LEED patterns with (2 × 2) and (3 × 3) symmetry, respectively. The molecule is believed to be sp² rehybridized under the influence of coadsorbed K, leading to stronger CO-Ru and weaker C-O bonds as indicated by the TPD and HREELS results, and to stand upright in essentially twofold bridges.     

Water dissociation on Ru(001): an activated process

It is shown using x-ray photoelectron spectroscopy that water is adsorbed either nondissociatively or partially dissociatively on Ru(001) under ultrahigh vacuum conditions. We found an activated dissociation process with a barrier slightly larger than that of desorption. A difference in dissociation barriers is found between H2O and D2O that explains the anomalous isotope effects in the thermal desorption. Previous theoretical and experimental disagreements can be rationalized based on electron or x-ray beam-induced dissociation of the water overlayer and an earlier underestimation of the dissociation barrier.     

Interaction between CO and NH3 coadsorbed on Ru(001): its effects on the ordering in mixed adlayers and the ammonia dissociation

The coadsorption of CO and ammonia on Ru(001) has been investigated by low-energy electron diffraction (LEED), temperature-programmed desorption (TPD) and high-resolution electron energy-loss spectroscopy (HREELS). The main focus has been on the interaction between different admolecules on the surface and its important role in surface reaction. Exposing CO-precovered Ru(001) to ammonia at 100 K leads to the formation of mixed ordered layers with a (2 × 2) periodicity. It was found that two types of (2 × 2) structures are formed depending on the CO precoverage. One of the (2 × 2) structures (-phase) contains one CO and two ammonia molecules per (2 × 2) unit cell and the other (β-phase) contains two CO and one ammonia. Structure models for the two phases are proposed based on vibrational spectra measured for the coadsorbed phases of CO and ammonia (¹⁵NH₃ or ND₃). TPD results suggest that the ammonia dissociation takes place on clean and CO-precovered Ru(001). The amount of dissociated ammonia decreased initially with increasing CO precoverage, passed a minimum at θ_CO = 0.25, increased with a further increase of CO coverage, and eventually reached a saturation value above θ_CO = 0.5. The dissociation of ammonia in the β−(2 × 2) structure was found to be enhanced by a factor of 4–6 as compared with the dissociation in the −(2 × 2) structure. The HREEL spectra indicated that the C_3v molecular axis of ammonia is tilted in the coadsorbed layers, the tilting being most pronounced in the β−(2 × 2) phase with a high CO partial coverage. This observation suggests that the tilting of ammonia due to the interaction with CO facilitates electron donation from Ru 4d to LUMO of ammonia, leading to the N-H bond dissociation. The microscopic model for the CO-NH₃ interaction on metal surfaces is presented.     

The GaP(001) surface and the adsorption of Cs

GaP(001) cleaned by argon-ion bombardment and annealed at 500°C showed the Ga-stabilized GaP(001)(4 × 2) structure. Only treatment in 10^?5 Torr PH₃ at 500°C gave the P-stabilized GaP(001)(1 × 2) structure. The AES peak ratio $\text{P}\text{Ga}$ is 2 for the (4 × 2) and 3.5 for the (1 × 2) structure. Cs adsorbs with a sticking probability of unity up to 5 × 10¹⁴ Cs atoms cm^?2 and a lower one at higher coverages. The photoemission measured with uv light of 3660 Å showed a maximum at the coverage of 5 × 10¹⁴ atoms cm^?2. Cs adsorbs amorphously at room temperature, but heat treatment gives ordered structures, which are thought to be reconstructed GaP(001) structures induced by Cs. The LEED patterns showed the GaP(001)(1 × 2) Cs structure formed at 180°C for 10 h with a Cs coverage of 5 × 10¹⁴ atoms cm^?2, the GaP(001)(1 × 4) Cs formed at 210°C for 10 hours with a Cs coverage of 2.7 × 10¹⁴ atoms cm^?2, the GaP(001)(7 × 1) and the high temperature GaP(001)(1 × 4), the latter two with very low Cs content. Desorption measurements show three stability regions: (a) between 25–150°C for coverages greater than 5 × 10¹⁴ atoms cm^?2, and an activation energy of 1.2 eV; (b) between 180–200°C with a coverage of 5 × 10¹⁴ atoms cm^?2, and an activation energy of 1.8 eV; (c) between 210–400°C with a coverage of 2.7 × 10¹⁴ atoms cm^?2, and an activation energy of 2.5 eV.