C. Benndorf

Adsorption and orientation of NH3 on Ru(001)

Abstract The interaction of NH3 with clean Ru(001) surfaces has been studied using LEED (low energy electron diffraction), ESDIAD (electron stimulated desorption ion angular distribution), TDS (thermal desorption spectroscopy), and work function changes (Δφ). Four different binding states (denoted as α1, α2, β and γ) were detected with TDS. At low coverages, NH3 desorbs from the α1 state with a TDS peak maximum at ~ 310 K. The broadening of the TDS peaks and their shift to lower temperature with increasing NH3 coverage are related to repulsive lateral interactions between neighboring NH3 molecules. At higher NH3 coverages (θNH3≳ 0.15), a second desorption peak (α2) develops at T = 180 K, accompanied by a (2 × 2) LEED structure. With further increase of NH3 exposure a sharp desorption peak (β state) is found at T = 140 K, and is interpreted as due to NH3 species desorbing from a second adsorption layer. Finally a desorption peak due to multilayer adsorption (γ state) is found at 115 K. At low NH3 coverages (α1 state), a “halo”-like H+ ESDIAD pattern gives evidence of randomly oriented or freely rotating NH3, monomers, bounded via the N atoms to the surface with the H atoms pointing away from the surface. This orientation of NH3 is supported by work function measurements showing a linear decrease of Δφ in the α1 state. Structural information concerning the adsorption geometry of NH3 in the β state has been obtained from LEED and ESDIAD. During the formation of the second NH3 layer (β) a (2√3 × 2√3)R30° LEED pattern is observed and is accompanied by an ESDIAD pattern with a hexagonal outline. A structural model of the β-state bonding, in which second layer NH3 molecules are bonded via threefold hydrogen bonds to the first layer NH3, is proposed.

Photocorrosion of cadmium sulfide: analysis by photoelectron spectroscopy

Abstract Photocorrosion of cadmium sulfide electrodes in aqueous electrolytes leads to sulfate formation in the presence and to sulfur formation in the absence of oxygen. All three sulfur species involved (S2−, S0 and SO2−4) can be detected on CdS electrodes after treatment in photoelectrochemical cells using AES or XPS. Both, the S2p/3p XPS and the S Auger peaks are broadened on photoelectrochemical oxidized CdS due to deposition of elemental sulfur. The XPS binding energy of the S electrons is shifted by about 1 eV per sulfur oxidation step from around 161.5 eV for S2− to around 163.5 eV for S0 and around 168 eV for SO2−4. The results obtained by photoelectron spectroscopy of electrodes transferred from the electrolyte directly into the UHV system without any cleaning and special precautions are in excellent agreement with the predictions based on photoelectrochemical experiments. For example, it could be proved by XPS that sulfur was transformed into sulfate in a reductive oxidation step in the presence of oxygen, leading to the CdS surface being cleaned of S0.

H2O interaction with clean and oxygen precovered Ni(110)

Abstract The interaction of water vapour with clean as well as with oxygen precovered Ni(110) surfaces was studied at 150 and 273 K, using UPS, ΔΦ, TDS, and ELS. The He(I) (He(II)) excited UPS indicate a molecular adsorption of H 2 O on Ni(110) at 150 K, showing three water-induced peaks at 6.5, 9.5 and 12.2 eV below E F (6.8, 9.4 and 12.7 eV below E F ). The dramatic decrease of the Ni d-band intensity at higher exposures, as well as the course of the work function change, demonstrates the formation of H 2 O multilayers (ice). The observed energy shift of all water-induced UPS peaks relative to the Fermi level (Δ E max = 1.5 eV at 200 L) with increasing coverage is related to extra-atomic relaxation effects. The activation energies of desorption were estimated as 14.9 and 17.3 kcal/mole. From the ELS measurements we conclude a great sensitivity of H 2 O for electron beam induced dissociation. At 273 K water adsorbs on Ni(110) only in the presence of oxygen, with two peaks at 5.7 and 9.3 eV below E F (He(II)), being interpreted as due to hydroxyl species (OH) δ- on the surface. A kinetic model for the H 2 O adsorption on oxygen precovered Ni(110) surfaces is proposed, and verified by a simple Monte Carlo calculation leading to the same dependence of the maximum amount of adsorbed H 2 O on the oxygen precoverage as revealed by work function measurements. On heating, some of the (OH) δ- recombines and desorbs as H 2 O at ≅ 320 K, leaving behind an oxygen covered Ni surface.

Chemisorption and initial oxidation of Ni(110): AES, ELS and work function measurements

Abstract The interaction of oxygen with Ni(110) surfaces was investigated from 85 to 800 K by AES. ELS, TDS and work function changes (Δo). At T = 300 K three different phases of oxygen bonding are observed: (1) Chemisorption of oxygen up to 0.6–0.8 L accompanied by an increase of work function and a kinetic energy shift of the O(KLL) Auger spectra ( ΔE = 1.1 eV ). (2) A rearrangement of the chemisorption layer and incorporation of oxygen into the Ni sublayer associated with further oxygen uptake, characterized by a decrease of Δo per adsorbed oxygen atom. (3) Nucleation and island growth of NiO indicated by a decrease of Δo, shifts of the oxygen and Ni Auger transitions to lower kinetic energy and drastic changes in ELS and AES line shape. Low temperature adsorption experiments indicate the existence of a molecular oxygen species — marked in TDS by a partial desorption of oxygen at 250 K — and characterized by the electron spectroscopic investigations and Δo measurements.

Adsorption of H2O on clean and oxygen-preposed Ni(110)

Abstract The adsorption of H2O on both clean and modified Ni(110) surfaces has been studied using a variety of methods: electron stimulated desorption ion angular distribution (ESDIAD), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). Fractional monolayers, θ(H2O) 0.5–1 larger hydrogen bonded clusters with long range c(2 × 2) symmetry are formed. Upon heating to ⩾ 200 K, a fraction of the H2O dissociates, forming OH(ad). TDS of H2O from clean Ni(110) reveals four binding states having peak temperatures of 155, 210, 245 and 370 K. They are related to multilayer desorption (155 K), desorption from larger bilayer clusters (210 K), desorption from H2O dimer clusters which might be stabilized by OH (245 K), and recombination of OH to yield H2O(g) (360 K). Dissociation of H2O is promoted by surface oxygen. For the adsorption of H2O on oxygen-dosed Ni(110) at θ(O) > 0.08, a mixture of molecular and dissociative adsortion occurs immediately at 80 K, producing inclined OH. Isotropic exchange of H216O with 18O(ad) is observed even for binding states in which dissociation is believed not to to occur and is related to a proton exchange involving H2O(ad) hydrogen bonded to O(ad).

Interactions of CO + K on Ru(001): Structure and bonding

Abstract Recent studies of CO + K on Ru(001) revealed an anomalously low CO streaching frequency of 1460 cm −1 for low CO and K coverages. Hoffmann and de Paola proposed a side-on bound molecule with the CO molecular axis parallel to the metal surface: Weimer and Umbach suggested that CO is bound perpendicular to the surface, as on clean Ru(001), but that the CO is sp 2 hybridized in the presence of K(ads). The main objectives of the present work were to search for configurational changes of adsorbed CO on K + Ru(001) and to compare these results with CO + O(ad). We used the ESDIAD (electron stimulated desorption ion angular distribution) method in combination with LEED (low energy electron diffraction) and TDS (thermal desorption spectroscopy). The ESDIAD data for low coverages ( θ K × 0.1) of CO + K on Ru(001) show that the O + ESD signal from CO is suppressed by coadsorption with K; this can arise from a reorientation of the CO molecular axis, from perpendicular to inclined, or from increased reneutralization rates due to coadsorption with K. TDS provides evidence for strong interaction between CO and K, and a combination of LEED and ESD measurements suggest the formation of ordered species having K x (CO) 2 x stoichiometry.

Surface science lettersUnusually low stretching frequency for CO adsorbed on Fe(100)

For CO adsorption on Fe(100) different adsorption species are detected with high resolution EELS (electron energy loss spectroscopy) which sequentially fill in with increasing coverage. Up to ∼ 350 K and low CO exposure (≦1 L), a predominant molecular species with an unusually low stretching frequency, 1180–1245 cm−1, is detected. This unusual CO bond weakening is consistent with a “lying down” binding configuration of CO. For higher CO coverages at 110 K, further CO adsorption states with vibrational frequencies of 1900–2055 cm−1 are populated which are due to CO bound with the molecular axis perpendicular to the surface.

Identification of Cu(I) and Cu(II) oxides by electron spectroscopic methods: AES, ELS and UPS investigations

Abstract The externally prepared black-coloured copper oxide ( T ⋍ 700 K, P O 2 ⋍ 100 torr) on a Cu(100) surface is identified by electron spectroscopy as CuO. Compared to the red-coloured Cu(I) oxide (in situ oxidation at T ⩾ 400 K, P O 2 ⩽ 0.5 torr, ∼ 10 9 L), the He(I)- excited photoemisson from CuO reveals characteristic shake-up satellites 10–12 eV below E F and a broadened emission from overlapping oxygen-induced 2 p and Cu 3 d states. From the AES and ELS results, in correlation with the data from core electron spectroscopy, chemical shifts of Cu 2 p , Cu 3 s and Cu 3 p in CuO to higher binding energy and decreases in binding energy of the oxygen-induced states were deduced. The unoccupied electron states of Cu at 5 and 7.5 eV above E F — postulated from the ELS results — are preserved in Cu 2 O and CuO compounds. Annealing of the Cu(II) oxide at 670 K is accompanied by decomposition into Cu 2 O due to the solid-state reaction following the scheme: 2CuO → 1/2 O 2 + Cu 2 O.

Oxygen adsorption on Ag(111) in the temperature range from 100–500 K: UPS, XPS and EELS investigations

At 300 K oxygen adsorbs dissociatively on Ag(111) with a sticking coefficient S0 − 10−5. The uptake was followed with XPS using the O 1s intensity calibrated via the O(2×1) symmetries on Ni(110) and Ag(110). The maximum coverage amounts to ϑ − 0.6 (2×109 L, −1 Torr). This atomic oxygen state is characterized in the UPS by two induced maxima from bonding and antibonding 2p orbitals 8.8 and 3.4 eV below EF. The O 1s BE is found to be 530.2 eV; in the EELS one loss is measured at 27 meV (220 cm−1). At 100 K, the interpretation of the photoelectron as well as of the EEL spectra is hindered due to H2O adsorption from the residual gas.

H2O adsorption on Ni(100): Evidence for oriented water dimers

Abstract H 2 O adsorption on clean Ni(110) surfaces at T ≦ 150 K leads at coverages below θ ≊ 0.5 to the formation of chemisorbed water dimers, bound to the Ni substrate via both oxygen atoms. The linear hydrogen bond axis is oriented parallel to the [001] surface directions. With increasing H 2 O coverage (θ ≧ 0.5) , the accumulation of further hydrogen bonded water molecules induces some modification of the dimer configuration, producing at θ ≊ 1 a two-dimensional hydrogen bonded network with a slightly distorted ice lattice structure and long range order.