The adsorption of CO on the (100) plane of tungsten; thermal and work function measurements

Thermal desorption and work function measurements indicate that a largely molecular layer, with some dissociation, is formed at 80–100 K, with an increase in work function of 0.55 eV. The coverage in this layer is 11.5 × 10¹⁴ molecules/cm², or CO/W = 1.15. On heating, equal amounts of a β precursor, possibly dissociated, and a molecular α species are formed at ≈300 K, with abundances of 5 × 10¹⁴ molecules/cm² each. The α desorption is complete at 360 K. The β precursor evolves on heating without desorption in the range 400–700 K as indicated by work function decreases, to β-CO, which is almost certainly dissociated. This change occurs at lower temperatures for low coverages. Thermal desorption shows 3 peaks, which have been traditionally labelled β_{1, β2}, and β₃ at 930, 1070, and 1375 K. Of these only β₃ corresponds to a well defined state. Readsorption after heating to 950 or 1150 K results in a doubly peaked spectrum at 1070 and 1375 K. The β₁ and β₂ peaks obey complex desorption kinetics, probably corresponding to desorption and rearrangement. The coverage of β₃ is 2.5 × 10¹⁴ molecules/cm², suggesting that the c(2 × 2) LEED pattern corresponds to occupany of every other unit cell by a C or an O atom. For coverages ? 1.5 × 10¹⁴ molecules/cm² β₃ desorption obeys second order kinetics with an activation energy of 83 ± 3 kcal/mole. For β₃ the work function decreases from the clean W value by 0.1 eV, suggesting adsorption of C and O in the center of the W unit mesh, below the surface layer of W atoms. Readsorption on β and β precursor layers leads to formation of electropositive α-CO, with a multiply peaked thermal desorption spectrum, indicating the existence of different binding sites. Adsorption-heatingreadsorption, -heating-readsorption sequences indicate that additional changes in the α desorption spectrum occur, suggesting reconstruction in the β layer.     

Absolute coverage measurements: CO and oxygen on the (110) plane of tungsten

An effusion source, calibrated with a vibrating quartz microbalance, has been used to determine absolute coverages of CO and of oxygen adsorbed on a tungsten (110) plane by an extension of the field emitter detector method: The amount of gas reflected from the substrate is measured as a function of the absolute amount impinged per unit area; maximum coverage in the chemisorbed layer can be obtained very directly from this information. Work function increments vs. absolute coverages were measured in the same apparatus by the vibrating condenser method. Results were as follows: For virgin CO, adsorbed at 100 K, the maximum coverage obtained was CO/W = 0.71 ; this leads to a maximum coverage for beta or beta-precursor CO of CO/W = 0.28. The maximum work function increment for virgin CO was 0.8 eV. For oxygen, adsorption at 100 or 300 K a decrease in sticking coefficient by several orders of magnitude occurs when the coverage is O/W = 0.5. Adsorption at 20 K leads to the molecular precursor noted by Leung and Gomer, which converts to atomic oxygen at <50 K. By adsorbing at 20 K the O atom coverage can be increased to O/W = 0.62. Work function versus coverage data for this system are also presented.     

Adsorption of CO on the (110) plane of tungsten electron impact,thermal desorption,and work function measurements

The adsorption of CO on the (110) plane of tungsten has been studied using electron impact desorption, thermal desorption, and work function measurements in a single apparatus combining these various techniques. It is concluded that a single molecular adsorption state exists at 20–250 K (virgin-CO). At 300–400 K, 60% of the low temperature layer desorbs, the remainder converting principally to a beta-1 state, which has very small electron impact cross section; in addition to beta-1 an O⁺ yielding state, which we call beta-precursor is formed. The beta-1 state is stable to 900 K, where some desorption and conversion of the remaineder to a beta-2 state occurs. The O⁺ yielding state decays with increasing T and is gone at 800 K. Readsorption on beta-1 leads to two types of adsorption states called alpha and gamma, which seem to be site specific. Electron impact desorption yields mostly CO⁺ and CO for virgin, O⁺ for beta-precursor, and CO⁺ and CO for the readsorption states. There is no isotopic mixing in virgin or in readsorbed CO, nor does readsorbed CO exchange with beta-1 or beta precursor. There is complete isotopic mixing in beta desorption. In addition, massive EID creates another state, characterized by a large dipole moment, also yielding O⁺ in EID. This state can be converted to beta-1 by heating to 400 K. The total disappearance cross sections for the various states are virgin-CO5 × 10^?17cm²; γ-CO 1.6 × 10^?16cm²; α-CO 5 × 10^?17cm²; β-precursor 6 × 10^?18cm²and 1.2 × 10^?19cm²; EID induced state 8 × 10^?18cm². In addition, cross sections for ion production are determined and found to be several orders of magnitude less than total disappearance cross sections. These results, and Leed and coverage data obtained in parallel investigations are used to formulate models of the various adsorption states. It is concluded that virgin and readsorbed CO are molecular and beta-precursor and beta dissociated, although strong interactions between C and O remain. The electron impact desorption of physisorbed CO was investigated and found to yield C⁺, O⁺, and neutral CO, but very little CO⁺. These results suggest primary dissociation of CO by electron impact, and desorption of neutral physisorbed CO by the energetic fragments. Physisorbed CO⁺, although undoubtedly created, lies on the attractive part of its potential curve relative to the surface, and thus does not desorb as CO⁺.     

Adsorption of oxygen on the tungsten (110) plane at low temperatures; Spectroscopic measurements

The adsorption of oxygen on the W(110) plane was carried out at 26 K and investigated by means of ultraviolet and X-ray photoelectron spectroscopy. It was found that atomic oxygen is adsorbed first to essentially saturation coverage (O/W = 0.6) before adsorption of molecular oxygen occurs. The spectrum of the latter is very similar to that of gas phase O₂ but the shift to weaker binding energies is greater for the 1 s level than for the valence orbitals.     

Adsorption of krypton on the basal plane of graphite: LEED and Auger measurements

The adsorption of krypton on (0001) graphite has been studied by LEED and Auger. Stepwise isotherms are observed and thermodynamic quantities such as the latent heat of two dimensional adsorption and the binding energy of a krypton atom are determined. The mean free path of electrons in krypton is measured. The LEED pattern of the krypton layer shows a √3 × √3 superstructure. Some vibrational properties are examined by LEED and possible implications of the experimental findings are discussed. The potentials and limitations of the techniques used in the present work are critically examined with respect to other techniques.     

LEED,Auger and work function study of iodine adsorbed on W(110)

The adsorption of iodine on a W(110) surface has been studied by LEED, Auger and work function changes. LEED has revealed several phases which desorb in different temperature regimes and are accordingly designated γ, α, β₁, β₂, and β₃. The γ and α phases exhibited p(2 × 2) and p(2 × 1) surface nets respectively with coherently positioned antiphase boundaries which produced a splitting of selected LEED beams. The separation between the antiphase boundaries of the α phase increased with decreasing coverage. Both the γ and α phases were associated with molecularly absorbed iodine. The three β phases were associated with dissociatively adsorbed iodine which formed chain structures on the surface with the arrangement of iodine atoms within each chain being unique to the particular phase. Continuous changes in coverage then occurred by sheets of these chains shearing to produce packing faults at the resulting shear lines. This shearing process occurred coherently in the β₁ and β₃ phases and incoherently in the β₂ phases. In the former cases, the effect was seen by the continuous movement of coherent LEED beams with changing coverage. A phase diagram was constructed to describe the relative coverages and thermal stability of the phases. The characteristic Auger electron emission of iodine was observed at 495 eV and used to estimate the surface coverage. The work function was found to decrease by 0.4 eV with the adsorption of the first half monolayer and remained unchanged with further adsorption.     

The adsorption of sulfur on the tungsten (110) surface

The adsorption and desorption of S on a W(110) surface is studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), work function change (δφ) measurements and thermal desorption spectroscopy (TDS). The evolution of the structure with coverage θ is quite different from that reported for S on Mo(110) and — at low coverages — from that of Te on W(110). At low coverages the structure indicates complex lateral interactions. The bonding changes with coverage similar to S on W(100). Evidence for more complex desorption kinetics than assumed in the past is presented.     

A UPS and LEED/Auger study of adsorbates on Fe(110)

Ultraviolet photoemission spectroscopy (UPS) and LEED/Auger were used to study adsorbed species of C, N, O, S, CO, NO, and C₂H₂ on Fe(110). The complicated “carbon ring” LEED patterns were shown to be due to atomic carbon and/or nitrogen. Molecular nitrogen does not stick at or above room temperature on Fe(110). The optical excitation probability of the 3p electrons of segregated sulphur is found to have a Cooper minimum aroundhω=40.8 eV. Carbon monoxide chemisorbs molecularly at room temperature and then dissociates slowly. Only dissociative CO adsorption was observed atT=385 K. Acetylene also adsorbs molecularly but does not dissociate at room temperature. By contrast, nitric oxide chemisorption is completely dissociative at room temperature.     

A LEED investigation of Xe adsorption on W(110)

Four ordered LEED patterns are observed for Xe adsorption on W(110) for temperatures between 77 and 90 K. A (2 × 2) structure with an area per Xe atom of 28.3 Ų is transformed into two coincidence structures which correspond to a disordered (100) Xe layer. The area per Xe atom in these structures is 17.6 and 20.2 Ų. Xe adsorption on oxygen covered W(110) leads to one-dimensional disorder in the structures observed on clean W(110) without the formation of new structures.     

The adsorption of CO on Cu(110)

The adsorption of CO on Cu(110) has been studied by LEED, surface potentials and infrared spectroscopy. With increasing surface coverage the s.p. passes through a maximum value of 0.29 V and than falls to 0.17 V at saturation. The heat of adsorption is nearly constant (~55 kJ mol^?1) up to the maximum s.p. but then falls rapidly. A ( 2× 1) structure is formed near the s.p. maximum, followed by a structure which is compressed in the [11?0] direction and poorly ordered in the [001] direction but tending towards c(1.3 × 2). At low coverage two infrared bands appear at 2088 and 2104 cm^?1; their relative intensity is similar at 77, 195 and 295 K. As the coverage increases, the bands shift in frequency and merge into a single band at 2094 cm^?1. The origin of the two bands is discussed in relation to the overlayer structure. Strong interaction between CO molecules is shown by the spectra of mixtures of ¹³CO and ¹²CO.     

Water adsorption on the (001) plane of Fe2O3: An XPS,UPS, Auger,and TPD study

Thermal programmed desorption, ultraviolet photoelectron spectroscopy, and X-ray photoelectron spectroscopy have been used to study the reaction of H₂O with stoichiometric and partially reduced single crystals of α-Fe₂O₃. On the stoichiometric surface only ice condensation below 200 K was observed. Oxygen deficient surfaces were prepared by Ar bombardment giving rise to a decrease in the work function of the crystal of up to 1 eV. On these surfaces OH species were formed as detected by UPS that were stable up to 320 K. Annealing the defective surfaces between 475 and 700 K increased the work function by values between 0.5 and 0.7 eV respectively. These surfaces contained reduced Fe²⁺ species in subsurface layers as shown by UPS and XPS, but were inactive towards H₂O chemisorption. The Fe²⁺ species were stable for long periods of time at temperatures of up to 775 K. Potassium deposited on the surface forms a strongly bound monolayer compound. With H₂O it produced a complex that resulted in H₂ evolution upon annealing.     

Oxygen adsorption on the tungsten (110) plane. Electron impact and thermal desorption at high temperatures

When a layer of oxygen on the (110) plane of tungsten at coverages O/W≦0.5 is heated from 100 K, O⁺ evolution under electron impact becomes almost negligible at 600 K. On further heating, however, a slow, temperature-dependent evolution of O⁺ current is observed atT≳1500 K. For O/W>0.3 there is also desorption under massive bombardment. Once an equilibrium value of O⁺ current has been established, there is rapid adjustment to the appropriate equilibrium value when the temperature changes in the range 1500–1700 K. On cooling toT<1000 K, O⁺ decreases rapidly; on reheating toT>1500 K, O⁺ increases slowly again. Above 1700 K there is thermal desorption which is also reflected in the O⁺ signal. These facts indicate that there is a slow activated evolution of an electron sensitive state above 1500 K, from a reconstructed state formed by heating the low temperature layer toT≧1000 K. The latter state seems to be reformed on cooling below 1500 K.     

Adsorption of CO on the (110) plane of tungsten; temperature dependence of the sticking coefficient and absolute surface coverages

Absolute coverages and sticking coefficients as functions of both gas and surface temperatures are presented for CO adsorption on the (110) plane of tungsten. For a low temperature layer the CO/W ratio at saturation is 1.1, while that for alpha or beta adsorption is 0.5, indicating some crowding in the low temperature phase at high coverage. Initial sticking coefficients drop with increasing gas or surface temperature but eventually level off at ~ 0.5. Qualitative reasons for this behavior are given.     

An XPS study of the water adsorption on Cu(110)

The adsorption of H₂O on clean and oxygen precovered Cu(110) is studied at temperatures between 90 and 300 K by XPS. On the oxygen precovered surface three O(1s) emission lines are detected at lower temperature. They originate from adsorbed atomic oxygen, from OH groups, and from H₂O molecules. For an oxygen coverage of half a monolayer, XPS indicates that during H₂O decomposition the preadsorbed oxygen does not directly participate in the OH formation. After water adsorption on the clean surface three O(1s) emission lines are found, which are due to H₂O molecules, “disturbed” H₂O molecules, and OH groups. The XPS results are directly correlated with information about the adsorbates obtained by UPS. Coverages are determined from the XPS spectra for the detected species.     

Diffusion and compressibility of sodium on the (110) plane of tungsten

The surface diffusion parameters and the compressibility of sodium on the (110) plane of tungsten have been measured using the field emission fluctuation method for sodium coverages from 0.2 to 3 × 10¹⁴ atoms cm^?2 and for temperatures from 170 to 500 K. Two temperature regimes can be defined. In the high temperature regime (? 300 K) the diffusion is essentially normal with an activation energy ranging from 0.28 to 0.58 eV and a preexponential coefficient D₀ from 10^?8.1 to 10^?2.7 cm² s^?1. In this regime the compressibility increases with temperature indicating an effective repulsive adatoms interaction. In the low temperature regime (? 300 K) the diffusion coefficient decreases with temperature at high coverage and slowly increases with temperature at lower coverage. The transition between both regimes appears on the compressibility versus temperature curve as an inflection point. The comparison of the present results with slow electron diffraction results furnishes strong evidence that the observed transition corresponds to a continuous short-range order-disorder transition.     

Adsorption of oxygen on Rh(110): a LEED, Auger electron spectroscopy and thermal desorption study

The adsorption of oxygen on the Rh(110) surface has been studied by a variety of techniques. Low-energy electron diffraction shows the following patterns: (2 × 1)p2mg at 1 ML coverage and temperatures between 125 and 300 K; (2 × 2)p2mg at 0.5 ML coverage after heating to above 470 K; c(2 × 8) and complex streaked c(2 × 2n) patterns at coverages above 0.5 after heating to 470 K. These results are in partial agreement with previous work. Models for the first two structures are suggested. In the (2 × 2) structure, the oxygen is found to be much less reactive with CO at room temperature than in the (2 × 1) structure, suggesting that it is subsurface. A metastable (1 × 2) structure was produced from the (2 × 2) by reduction of the oxygen by CO at 450 K, and is interpreted as a surface reconstruction.     

Mobility of CO on the (110) plane of Tungsten

The measurement of time autocorrelation functions of field emission current fluctuations has been applied to the adsorption of CO on the (110) plane of tungsten. For molecularly adsorbed virgin-CO, no evidence for diffusion was found at any coverage, although a weak, exponentially decaying correlation function, suggesting flip-flop between differing ad-sites, was seen. A hysteresis in the mean square fluctuation could be clearly identified with conversion to β-CO. For the latter state a flip-flop signal was seen for 280 ? T ? 650 K, and for T$\text{\$}\text{?}$650 K a correlation function corresponding to diffusion was found. The activation energy of diffusion was 23 kcal, independent of coverage. This value agrees closely with that found for oxygen diffusion at $\text{O}\text{W}$ = 0.5. For CO readsorbed on a β-layer (α-CO) neither diffusion nor flip-flop was seen.     

Isotope effects in electron impact desorption of CO and O2 adsorbed on the (110) plane of tungsten

Results on the isotope effect for total and ionic desorption cross sections in the electron impact desorption of various binding states of CO on the (110) plane of tungsten, and of oxygen on this plane are presented and discussed. It is shown that the observations allow a dissection of cross sections into excitation cross sections and escape probabilities, and that the latter can be used to estimate lifetimes of excited or ionic states. It is found that excitation cross sections for total desorption are of the order of 10^?16–10^?17 cm², but seem to be significantly smaller in some cases for excitation to ionic states, suggesting that different excitations are involved. In all cases examined here the isotope effect for total desorption is much smaller than for ion production. This can be explained by the fact that ion lifetimes are somewhat shorter than those of excited neutrals. Lifetimes are estimated, in the cases examined, to be of the order of 10^?14s.