The adsorption of cyclic hydrocarbons on Ru(001): I. Cyclopropane

The adsorption of cyclopropane on Ru(001) at 90 K has been investigated by high-resolution electron energy loss spectroscopy, He I photoelectron spectroscopy, low-energy electron diffraction and thermal desorption mass spectrometry. The results indicate that the molecule adsorbs nondissociatively without long-range order and that no opening of the carbon ring occurs. The adsorption bond is weak, and multilayers form at 90 K only in the presence of a relatively high c-C₃H₆ equilibrium pressure. The vibrational spectrum is characterized by strong, dipole-active ring deformation modes and an additional mode at 570 cm^?1, which is due to a frustrated translation of the admolecule perpendicular to the surface with some ring deformation character. The photoelectron spectrum is characterized by a Jahn-Teller splitting of the 3e' molecular orbital, which is observed also in the gas phase. With the exception of a uniform relaxation shift, no other shifts could be observed in comparison with the gas phase. The results are discussed in relation to possible bonding mechanisms.     

The adsorption of cyclic hydrocarbons on Ru(001): II. Cyclohexane

The adsorption of cyclohexane on Ru(001) at 90 K has been investigated by thermal desorption mass spectrometry, EELS, UV photoemission and LEED. Thermal desorption indicates the adsorption of the undissociated molecule first in a chemisorbed monolayer (T_d = 200 K) with subsequent formation of multilayers (T_d = 165 K) at higher exposures. The vibrational spectrum obtained by EELS is characterized by a frequency shift of the C-H stretching mode from 2920 cm^?1 (multilayer) to 2560 cm^?1 for the chemisorbed monolayer. Off-specular EELS data indicate two different electron scattering mechanisms for the C-H stretching mode. Whereas for the C-H stretching mode of the multilayer, large angle electron impact scattering is observed, the C-H soft-mode of the monolayer is largely due to small angle dipolar scattering. The He I photoelectron spectra of cyclohexane multilayers are characteristic of the undissociated molecule. A new assignment of C(2s) and the lowest C(2p) level, based on a comparison with benzene, shows that the chemisorbed monolayer is characterized by the absence of emission or broadening of the 2a_1u level. This is attributed to C_3v symmetry of the chemisorbed layer and to a possible interaction of the 2a_Iu orbital with the metal surface.     

An IR study of the adsorption of water on Ru(001)

The strong OH stretch at 3400 cm^?1 (fwhm ～ 230 cm^?1) in the IR reflection absorption spectra of the system $\text{H}{}_2\text{O}\text{Ru(001)}$ at 85 K indicates the presence of hydrogen-bonded clusters. These clusters appear to form even at the lowest coverages. On the clean surface there is a linear relationship between integrated absorption intensity and coverage. The presence of small quantities of preadsorbed oxygen delays, however, the onset of absorption. It is thought that the oxygen atoms “bind” the water molecules, thus preventing cluster formation and in turn eliminating the intensity enhancement due to hydrogen bonding. Flash desorption spectra also indicate a second binding state when oxygen is coadsorbed. The relevance of these results to models of the electric double layer at the metal-electrolyte interface is discussed.     

Photoelectron spectroscopic studies of adsorption of CO and oxygen on Ru(001)

XPS and UPS have been used for a detailed study of the adsorption and coadsorption of CO and oxygen on a clean Ru(001) single crystal. The measured substrate and adsorbate core level binding energies and valence levels are discussed. The O 1s XPS peak intensity has been used for kinetic studies of adsorption and coadsorption. Some studies of the angular dependence of adsorbate and substrate peak intensity ratios are presented. We also present data on the shifts of XPS peaks and changes in UPS spectra as a function of adsorbate coverage. The data are correlated with the results of earlier measurements with other methods.     

The structure and mobility of carbon on Ru(001)

Interaction of ethylene with Ru(001) at temperatures above 300 K causes carbon to be deposited on the surface. Thermal desorption spectroscopy (TDS) of CO adsorbed on the precarburized surface shows that at carbon coverages below 0.5 ML the CO bond to the surface is weakened, but the adsorption capacity is only slightly diminished. At carbon coverages above 0.5 ML, CO adsorption sites are blocked. Temperature programmed oxidation (TPO) of the carbon layer shows a first-order peak at 570 K which indicates carbon-oxygen neighbors and a peak at ≈ 650K that requires mobility in the surface layer. CO TDS, TPO, Auger electron spectroscopy (AES), and SIMS all show that annealing the carbon layer causes growth of graphitic islands that starts at ≈ 600 K and is complete at ≈ 900 K. Temperatures above 1145 K cause dissolution of carbon into the bulk. The island formation, which requires a minimum carbon coverage, is indicated by increased CO uptake, a high-temperature TPO peak, loss of the AES carbidic feature, and restricted isotope mixing in the C₂⁻ ions in SIMS of ¹³C deposited on an annealed ¹²C surface. The SIMS experiments demonstrate the use of this technique for the study of proximity and structure in surface layers.     

Coadsorption of oxygen and cesium on Ru(001)

The coadsorption of oxygen and Cs on Ru(001) has been studied by means of thermal desorption, Auger and electron loss spectroscopy and work function measurements. The initial sticking coefficients for oxygen adsorption and oxygen saturation coverages increase with increasing Cs coverage, θ_Cs. Irrespective of the initial θ_Cs, the Cs desorption energy always increases under the influence of the coadsorbed oxygen, the effect becoming stronger with increasing oxygen coverage. At θ_O>0.5and θ_Cs>0.14 the work function, electron loss changes and thermal desorption data give evidence of strong CsO interactions and the formation of a CsO “surface compound”.     

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.     

The interaction of oxygen and potassium on the Ru(001) surface

The interaction of oxygen and different coverages of potassium on Ru(001) has been investigated by thermal desorption spectroscopy (TDS), metastable quenching spectroscopy (MQS), electron stimulated desorption spectroscopy (ESD), and work-function change measurements. The results show that this is a complex surface system with several different oxides forming, depending on the surface stoichiometry and temperature. While we cannot uniquely identify all the surface species, our interpretation of the present data combined with previous information is as follows. For potassium coverages up to about three monolayers (θ_K ≈ 1), exposure to oxygen initially gives oxygen atoms on the surface. Further exposure produces some surface monoxide ions O²⁻, which are converted with additional exposure to Superoxide ions O⁻₂ and possibly peroxide ions O²⁻₂. Thermal annealing causes strong changes in the surface oxide composition, and with potassium multilayers (θ_K ≈ 10) all the oxides diffuse beneath the K surface layer with annealing to only 300 K. K₂O and K₂O₂ are found to desorb together in the 600–700 K region.     

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.     

Chemisorption and rotational epitaxy of lithium on Ru(001)

Rotational epitaxy of an overlayer along a non-symmetry direction of the substrate has been observed as a mechanism for the accommodation of lattice mismatch in rigid thin films. We have examined the adsorption and rotational epitaxy of Li on the Ru(001) surface using low energy electron diffraction and thermal desorption spectroscopy. At 80 K, commensurate (2 × 2) and (√3 × √3)R30° phases were observed at coverages below 0.33 monolayer. At higher coverages, the Li layer orientation rotates relative to the substrate as the interatomic spacing of the layer is compressed. This behavior is qualitatively similar to that observed for Na on Ru(001), but differences were observed which may suggest that Li overlayers are less rigid than Na overlayers.     

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).     

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.     

The reconstruction of Rh(001) upon oxygen adsorption

We report on a first-principles study of the structure of O/Rh(001) at half a monolayer of oxygen coverage, performed using density functional theory. We find that oxygen atoms sit at the center of the black squares in a chess-board c(2×2) pattern. This structure is unstable against a rhomboid distortion of the black squares, which shortens the distance between an O atom and two of the four neighboring Rh atoms, while lengthening the distance with respect to the other two. We actually find that the surface energy is further lowered by allowing the O atom to get off the short diagonal of the rhombus thus formed. We predict that the latter distortion is associated with an order–disorder transition, occurring below room temperature. The above rhomboid distortion of the square lattice may be seen as a rotation of the empty white squares. Our findings are at variance with recent claims based on STM images, according to which it is instead the black squares which would rotate. We argue that these images are indeed compatible with our predicted reconstruction pattern.     

Alkali adsorption on the Al(001) surface

The chemisorption of Na on the Al(001) surface has been studied by 2D bandstructure calculations on slab models using a density functional STO-LCAO method. Two slab models of three and five layers of substrate atoms have been used. Overlayers of the structuresp(2×2),c(2×2) andp(1×1), representing coverages of a quarter, a half and a full atomic monolayer of sodium atoms, respectively, have been investigated. The electronic structure of the adatoms and the charge transfer to the substrate are discussed. Satisfactory agreement with experiment is obtained for the adsorption induced change of the work function, correctly reproducing its monotonic character.     

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.     

The behaviour of carbon species produced by CO disproportionation on Ru(1, 1, 10) and Ru(001) surfaces

The behaviour of adsorbed CO on Ru(001) flat and Ru(l,1,10) stepped surfaces in the CO pressure range between 10^?6 and 10¹ Pa has been investigated by TDS, AES, LEED and UPS. The disproportionation of CO proceeds rapidly on the stepped surface and its apparent activation energy was obtained to be 20 kJ mol^?1 at nearly zero coverage. The carbon species produced by CO disproportionation show non-uniform reactivity with ¹⁸O₂ and provide four CO desorption peaks in TPR spectra, which are assigned to α-C¹⁸O,ß-C¹⁸O and those derived from carbidic and graphitic carbons. At smaller carbon coverage, only α-CO and β-CO were observed, but with increasing coverage the amount of ß-CO reaches a maximum and carbidic carbon is newly formed. Further increase of carbon deposition gives graphitic carbon. The conversion from carbidic to graphitic carbon and the dissolution into the bulk took place upon heating to 1000 K. It is remarkable that very active carbon species are converted to molecular CO through the reaction with O₂ even at low temperature such as 200 K. It was also confirmed that active carbon species are formed on Ru surface during COH₂ reaction.