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 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 adsorption of H2O on clean and oxygen-dosed silver single crystal surfaces

The adsorption of H₂O on the surface of a single-crystal sphere of silver with exposed (111), (100) and (112) facets has been examined using ESDIAD (electron stimulated desorption ion angular distribution), LEED (low energy electron diffraction) and TDS (thermal desorption spectroscopy). The purpose of the study was (a) to examine the influence of substrate geometry for adsorption of H₂O on a metal surface for which the adsorbate-substrate interaction is weak, and (b) to study the influence of a surface impurity, oxygen, on the surface chemistry and local bonding structure of H₂O on Ag. We have found no evidence for either long-range or short-range local bonding order for adsorbed H₂O at 80 K on any of the surfaces studied. This appears to be a consequence, in part, of the lattice mismatch between the Ag crystal structure and the two-dimensional H₂O ice crystal structure. Adsorbed H₂O reacts with preadsorbed oxygen to form OH species which are bonded with the molecular axis perpendicular to Ag(111) and (100) but “inclined” on (112) surfaces, as identified using ESDIAD. The “inclined” OH species are associated with atomic steps on the (112) surface.     

The adsorption of CO,O2, and H2 on Pt(100)-(5×20)

The adsorption/desorption characteristics of CO, O₂, and H₂ on the Pt(100)-(5 × 20) surface were examined using flash desorption spectroscopy. Subsequent to adsorption at 300 K, CO desorbed from the (5×20) surface in three peaks with binding energies of 28, 31.6 and 33 kcal gmol^?1. These states formed differently from those following adsorption on the Pt(100)-(1 × 1) surface, suggesting structural effects on adsorption. Oxygen could be readily adsorbed on the (5×20) surface at temperatures above 500 K and high O₂ fluxes up to coverages of $\text{2}\text{3}$ of a monolayer with a net sticking probability to ssaturation of ? 10^?3. Oxygen adsorption reconstructed the (5 × 20) surface, and several ordered LEED patterns were observed. Upon heating, oxygen desorbed from the surface in two peaks at 676 and 709 K; the lower temperature peak exhibited atrractive lateral interactions evidenced by autocatalytic desorption kinetics. Hydrogen was also found to reconstruct the (5 × 20) surface to the (1 × 1) structure, provided adsorption was performed at 200 K. For all three species, CO, O₂, and H₂, the surface returned to the (5 × 20) structure only after the adsorbates were completely desorbed from the surface.     

The interaction of Cs and O2 on the basal plane of MoS2

The interaction of Cs and O₂ on MoS₂(0001) has been studied both in the alternate adsorption and the codeposition mode by LEED, AES, TDS and WF measurements at 170 and 300 K. Oxygen does not interact with Cs when θ_Cs?0.04 at 300 K or θ_Cs?0.08 at 170 K, where Cs is known to adsorb as strongly ionized, individual adatoms. The interaction at higher θ_Cs, where Cs is known to form clusters on MoS₂(0001), leads to clusters of a Cs/O complex characterized by a Cs(563 eV)/O(512 eV) Auger peak ratio of 1.1–1.3. The minimum WF is 2.1 eV at 300 and 170 K upon alternate adsorption, and 1.7 eV at both T upon codeposition. Upon heating, oxygen and Cs desorb independently, as no oxide desorption is observed. The Cs TDS spectrum is shifted to lower T in the presence of oxygen and a new desorption peak appears at ～ 880 K. The differences in the Cs/O interaction between MoS₂(0001) and other semiconductors and metals are attributed to the Cs clustering and the inertness of MoS₂(0001) to O₂ adsorption.     

The chemisorption of CO,CO2, C2H2, C2H4, H2 and NH3 on the clean Fe(100) and (111) crystal surfaces

The chemisorption of small molecules (CO, CO₂, C₂H₂, C₂H₄, H₂ and NH₃) has been studied on the clean Fe(110) and (111) crystal faces by low-energy electron diffraction (LEED) and thermal desorption. C₂H₄ and C₂H₂ yield the same sequence of surface structures that change with temperature and crystal orientation. CO and CO₂ chemisorption similarly results in the formation of the same types of surface structures that change with surface temperature and crystal orientation. Ammonia forms several ordered surface structures on both iron crystal faces. All of the molecules decompose as a function of temperature on the iron surfaces as indicated by the Auger and thermal desorption spectra.     

Adsorption of H2O on Al(111)

The adsorption of H₂O on Al(111) has been studied by ESDIAD (electron stimulated desorption ion angular distributions), LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and thermal desorption in the temperature range 80–700 K. At 80 K, H₂O is adsorbed predominantly in molecular form, and the ESDIAD patterns indicate that bonding occurs through the O atom, with the molecular axis tilted away from the surface normal. Some of the H₂O adsorbed at 80 K on clean Al(111) can be desorbed in molecular form, but a considerable fraction dissociates upon heating into OH_ads and hydrogen, which leaves the surface as H₂. Following adsorption of H₂O onto oxygen-precovered Al(111), additional OH_ads is formed upon heating (perhaps via a hydrogen abstraction reaction), and H₂ desorbs at temperatures considerably higher than that seen for H₂O on clean Al(111). The general behavior of H₂O adsorption on clean and oxygen-precovered Al(111) (θ_O ? monolayer) is rather similar at low temperature, but much higher reactivity for dissociative adsorption of H₂O to form OH _adsis noted on the oxygen-dosed surface around room temperature.     

Chlorine K X-ray absorption-edge structures of CIS-[Pt(NH3)2Cl2], trans-[Pt(NH3)2Cl2], trans-[Pd(NH3)2Cl2] and (NH4)2PdCl4

The K absorption-edge spectra of the ligand chlorine ion in square-planar complex compounds cis- and trans-[Pt(NH₃)₂Cl₂], trans-[Pd(NH₃)₂Cl₂], and (NH₄)₂PdCl₄ are reported and discussed in connection with the chlorine K absorption spectra of K₂PtCl₄ and K₂PdCl₄, reported previously. The observed chemical shift of a white line at the absorption threshold is interpreted in terms of the difference of the ligand-field splitting of electronic states for metal ions. The white line is attributed to the electronic transition from the Cl^? ls level to the lowest unoccupied antibonding molecular orbital (MO), which is specified by a $\text{MO}\text{b}{}_{\mathrm{1g}}\text{(σ}{}^1\text{)}$ in the square-planar complex with D_4h symmetry. The other absorption structures are regarded as continuum “shape resonances” of the outgoing electron trapped by the cage of the surrounding atoms. The effect of geometrical isomerism is found in the chlorine K absorption spectra of cis- and trans-[Pt(NH₃)₂Cl₂].     

ESDIAD and LEED studies of the clean TiO2(100) surface

Electron-stimulated desorption ion angular distribution (ESDIAD) and LEED were used to investigate the structure changes at the TiO₂(100) surface. The angular distribution of O⁺ ions from the (1 × 3) reconstructed surface is consistent with the microfacet model proposed from X-ray diffraction and STM studies. The (1 × 3) reconstructed surface can be transferred back to the (1 × 1) surface after annealing at 950 K in oxygen, through a stage where the surface consists of (1 × 1) and (1 × 3) domains which are smaller than the coherent width of the LEED electron beam. Evidence for surface reconstruction on the (1 × 1) surface is also found.     

Oxygen and carbon monoxide adsorption on W(111): An investigation with angular resolved ESD,AES and LEED

Oxygen and carbon monoxide adsorption on clean W(111) surfaces have been studied by angular resolved ESD emission (ESDIAD). In addition, the specimen could be characterized in situ with AES and LEED. Adsorption was performed at room temperature. The electron stimulated desorption yielded O⁺ ions from the two investigated adsorption layers. Upon oxygen adsorption followed by subsequent annealing at least eight different ESDIAD patterns have been obtained. However, a convincing interpretation on the basis of the surface geometry can only be presented for three patterns produced without annealing as well as for one pattern at a very high annealing temperature. The difficulties are a consequence of complex structure changes which the surface undergoes in the intermediate annealing temperature range. This may influence the little known neutralisation probability of the desorbing ions. In this special case ESDIAD probably reflects in contrast to LEED a picture of some specific adsorption sites (minority species) and therefore, no clear correlation of the two techniques can be seen. ESDIAD from carbon monoxide shows four different patterns and supports the model of linear bonded CO molecules at room temperature with oxygen in the “standing up” position. At T > 900 K, CO starts to dissociate and results in similar ESDIAD patterns as obtained from O₂ adsorption.     

Interaction of PH3 coadsorbed with H2, D2, O2 and H2O on Rh(100)

The coadsorption of PH₃ with H₂, D₂, O₂ and H₂O on Rh(100) has been studied using temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The adsorption and molecular desorption of PH₃ is not affected by preadsorbed H₂, D₂ and O₂. Preadsorbed PH₃ blocks H₂ desorption sites while postdosed PH₃ displaces H₂ (D₂₁) from the Rh(100). When D₂ and PH₃ are coadsorbed, no D appears in desorbed phosphine. Preadsorbed O₂ reduces the amount of H₂ desorption (from PH₃ decomposition) and increases the H₂ desorption temperature. There is also some reaction between O(a) and H(a) to form water. Preexposure to H₂O decreases the extent of PH₃ adsorption and of PH₃ decomposition.     

NMR spectral densities and two dimensional diffusion in TiS2(NH3)1.0 and TaS2(NH3)x

NMR measurements of proton spin-lattice relaxation times T₁ and T_1? in the layered intercalation compounds TiS₂(NH₃)_1.0 and TaS₂(NH₃)_x (x = 0.8, 0.9, 1.0) are reported as functions of frequency and temperature (100 K – 300 K). These observations probe the spectral density of magnetic fluctuations due to motions of the intercalated molecules at frequencies accessible to the T₁ (4–90 MHz) and T_1? (1–100 kHz) measurements. Since the average molecular hopping time (τ) can be changed by varying temperature, different regions of the spectral density can be examined. For T > 200 K, both T^?1₁ and T^?1_1? vary logarithmically with frequency, reflecting the two dimensional character of the molecular diffusion. The temperature dependence of T₁ suggests that a more accurate picture of the short time dynamics is required. No dependence of relaxation rate on vacancy concentration is found.     

The adsorption of no on Ni(111); an esdiad/leed study

The ESDIAD method (electron stimulated desorption ion angular distributions) has been combined with LEED (low energy electron diffraction) in a study of the adsorption of NO on Ni(111). For adsorption at 80 K, NO appears to be bonded with its molecular axis perpendicular to the Ni(111) surface at all coverages. Heating the 80 K layer leads to a striking structural change which we interpret as the formation of inclined or bent NO in the range 120 ? T ? 250 K. Upon adsorption at 150 K, only the bent form of NO is present at low coverages; at higher coverages at 150 K, the perpendicular form appears, in agreement with recent electron energy loss spectroscopy (EELS) data of Lehwald, Yates, and Ibach. When NO is coadsorbed with p(2 × 2) oxygen, the perpendicular form of NO dominates at all coverages and temperatures studied. Dissociated NO adsorbed at steps and defect sites on Ni(111) produces a welldefined hexagonal ESDIAD pattern.     

Magnetic susceptibility studies of (CH2)3(NH3)2FeCl2Br2 and (CH2)6(NH3)2FeCl2Br2

The magnetic susceptibility of the layered compounds (CH₂)₃(NH₃)₂FeCl₂Br₂ and (CH₂)₆(NH₃)₂FeCl₂Br₂ has been measured in the range 80 < T < 300 K. The results follow a Curie-Weiss behavior in the range 120 < T < 300 K but are field dependent for T < 120 K. The results are interpreted in terms of a two-dimensional antiferromagnetic interaction which is canted. A comparison with the corresponding pure chloride compounds is given.     

X-ray induced electron stimulated desorption versus photon stimulated desorption: NH3 on Ni(110)

The X-ray induced desorption of H⁺ ions from NH₃ layers adsorbed at T = 90 K on Ni(110) has been compared to the corresponding total electron yield (TY) in the photon energy range 390 to 900 eV. The H⁺ yield exhibits a jump at the N K-edge and the Ni L-edge which inversely varies with the NH₃ layer thickness. The H⁺ Ni L-edge jump is closely correlated to the TY jump. Both vanish for the saturated NH₃ multilayer, indicating that the observed Ni L-edge jump in the H⁺ yield is exclusively due to X-ray induced electron stimulated desorption (XESD). At the N K-edge, the near edge absorption fine structure of the H⁺ yield and TY of the saturated NH₃ multilayer are distinctly different. This is interpreted as the H ⁺ yield being the superposition of direct photon stimulated ion desorption (PSID) and XESD. Based on the observed variation of the H⁺ yield near edge fine structure with varying NH₃ layer thickness, a deconvolution of the PSID and XESD contributions is used to derive the relative contribution of PSID versus XESD to be 40% versus 60%, respectively. The relevance of this result for future PSID-SEXAFS studies is discussed. For monolayer NH₃ on Ni(110) the polarization dependence of the N K-edge fine structure in the N(KVV) Auger yield indicates that the symmetry axis of NH₃, is perpendicular to the surface.     

LEED and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 and C) chemisorbed on the stepped rhodium (755) and (331) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₂, C₂H₄ and C has been studied on the clean stepped Rh(755) and (331) surfaces. Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) were used to determine the size and orientation of the unit cells, desorption temperatures and decomposition characteristics for each adsorbate. All of the molecules studied readily chemisorbed on both stepped surfaces and several ordered surface structures were observed. The LEED patterns seen on the (755) surface were due to the formation of surface structures on the (111) terraces, while on the (331) surface the step periodicity played an important role in the determination of the unit cells of the observed structures. When heated in O₂ or C₂H₄ the (331) surface was more stable than the (755) surface which readily formed (111) and (100) facets. In the CO and CO₂ TDS spectra a peak due to dissociated CO was observed on both surfaces. NO adsorption was dissociative at low exposures and associative at high exposures. C₂H₄ and C₂H₂ had similar adsorption and desorption properties and it is likely that the same adsorbed species was formed by both molecules.     

The adsorption of sulphur on Pd(111) I. A LEED analysis of the (2 × 3)R30° S adsorbate structure

The adsorption of sulphur on the Pd(111) surface is studied by low energy electron diffraction (LEED). Four different adsorbate structures are identified. LEED intensity analyses are performed for the clean surface and for the ordered initial adlayer, i.e. the $\text{(}\text{3}\text{×}\text{3}\text{)}\text{R}\text{30°}\text{S}$ adsorption phase. It is found that the sulphur atoms occupy threefold-symmetric hollow sites, with a SPd chemisorption bond length of 0.222±0.003 nm.     

Adsorption of Cs and O2 on MoS2

Adsorption of Cs on basal planes of MoS₂ has been studied with LEED, Auger and work function measurements. LEED observations show that in the 200–300 K range Cs is adsorbed as amorphous layers on MoS₂. Correlation of Auger and work function measurements indicates that the work function, sticking coefficient and the maximum density of Cs that can be deposited on the MoS₂ surface depend strongly on substrate temperature. Cesium is deposited on MoS₂ in two adsorption states. Although MoS₂ is extremely inert to O₂ adsorption, the presence of Cs causes a drastic increase in the adsorption of oxygen which in turn increases the amount of Cs that can be deposited on the surface. Lastly, it has been found that part of the Cs adatoms are diffused into the bulk of MoS₂.     

Interaction of CO,CO2 and O2 with nonpolar,stepped and polar Zn surfaces of ZnO

The nonpolar (10$\text{1}$0), stepped (40$\text{4}$1) and (50$\text{5}$1), and the polar (0001) surfaces of ZnO were prepared. Stable unreconstructed nonpolar and stepped surfaces were obtained. LEED analyses showed that the step height and the step width of the stepped surfaces were similar to the theoretical values. The polar surface showed a 1 × 1 LEED pattern of six-fold symmetry after annealing at 500°C, and evidence of a more complicated pattern at 300–400°C. Temperature programmed desorption of CO resulted in the desorption of CO from the stepped and the polar surfaces. However, desorption of CO₂ was observed from the stoichiometric nonpolar surface, and no desorption from the reduced nonpolar surface. CO₂ was also observed by interacting CO with all surfaces at elevated temperatures. A total of four temperature programmed desorption peaks of CO₂, α, β, γ, and δ were observed. The α and β peaks were observed on the nonpolar and the stepped surfaces, and the γ peak was observed on the polar surface. The α peak was assigned to adsorption on a surface ZnO pair, and the β peak was assigned to adsorption on an anion vacancy or a step. While adsorbed water enhanced the β, preadsorbed methanol reduced it. O₂ adsorption was similar on the nonpolar and the stepped surfaces, but was weak on the polar surface.     

Intermediate product identification for ammonia decomposition at Ni (110)

Ammonia decomposition at Ni(110) has been identified to proceed via NH₃(ad) → NH₂(ad) → NH(ad) → N + H. The decomposition activation of NH is determined to be 47 kcal/mol, suggesting an amazing stability of the NiNH bond. Decomposition of NH₂ occurs up from about 350 K; no kinetic data can be given yet. NH₃ decomposition is found to proceed slower than NH₃ desorption at least below 300 K.