Adsorption of hydrogen and carbon monoxide on platinum

TDS spectra and ESD ion energy distributions have been measured for coadsorption of H₂ and CO on recrystallized platinum. Sample exposure to a 90% H₂?10% CO gas mixture results in appearance of structurein both TDS spectra of H₂ and CO. Coadsorption also results in an O⁺ ion energy distribution which is much narrower compared to the distribution resulting from pure CO adsorption. These results are interpreted as evidence for the formation of an HCO surface complex.     

Adsorption and coadsorption of carbon monoxide and hydrogen on Pd(111)

The adsorption and coadsorption of CO and H₂ have been studied by means of thermal desorption (TD) and electron stimulated desorption (ESD) at temperatures ranging from 250 to 400 K. Three CO TD states, labelled as β₂, β₁, and β₀ were detected after adsorption at 250 K. The population of β₂ and β₁ states which are the only ones observed upon adsorption at temperatures higher than 300 K was found to depend on adsorption temperature. The correlation between the binding states in the TD spectra and the ESD O⁺ and CO⁺ ions observed was discussed. Hydrogen is dissociatively adsorbed on Pd(111) and no ESD H⁺ signal was recorded following H₂ adsorption on a clean Pd surface. The presence of CO was found to cause an appearance of a H⁺ ESD signal, a decrease of hydrogen surface population and an arisement of a broad H₂ TD peak at about 450 K. An apparent influence of hydrogen on CO adsorption was detected at high hydrogen precoverages alone, leading to a decrease in the CO sticking coefficient and the relative population of CO β₂ state. The coadsorption results were interpreted assuming mutual interaction between CO and H at low and medium CO coverages, the “cooperative” species being responsible for the H⁺ ESD signal. Besides, the presence of CO was proved to favour hydrogen penetration into the bulk even at high CO coverage when H atoms were completely displaced from the surface.     

Evidence for multiple states of hydrogen on palladium

Adsorption of hydrogen on polycrystalline palladium has been investigated using ESD techniques. Although a single thermal desorption peak is observed for H₂ and Pd, total ESD cross section measurements suggest the existence of four distinct sources for desorbing H⁺ and H^? species. The very large cross section associated with the H^? signal along with its behavior during sample heating suggests the possible existence of a molecular precursor state.     

On the origin of electron stimulated desorption of H+ from metal surface

The emission of hydrogen ESD ions from polycrystalline Nb is studied experimentally. The results demonstrate clearly that no ESD emission of H⁺ occurs from a pure H adsorbate. However, adsorption of H₂O, present as an impurity in H₂, and H₂ adsorption on Nb preceded by O adsorption, produce strong H⁺ ESD emission. It is shown that Auger ionization of O can account for the increased H⁺ yield and it is postulated that similar effects also occur on other metal surfaces.     

Reactions and anion desorption induced by low-energy electron exposure of condensed acetonitrile

Thermal desorption spectrometry (TDS) and electron stimulated desorption (ESD) are employed to investigate mechanisms responsible for the formation of C₂H₆ in electron irradiated multilayer films of acetonitrile (CH₃CN) at 30 K. Using a high sensitivity time-of-flight mass spectrometer, we observe the ESD of anionic fragments H⁻, CH₂ ⁻, CH₃ ⁻ and CN⁻. Desorption occurs following dissociative electron attachment (DEA) via several negative ion resonances in the 6 to 14 eV energy range and correlates well with a “resonant” structure seen in the TDS yield of C₂H₆ (i.e., at mass 30 amu). It is proposed that C₂H₆ is formed by the reactions of CH₃ radicals generated following DEA to CH₃CN which also yields CN⁻. Between 2 and 5 eV, a second resonant feature is seen in the C₂H₆ signal. While DEA is observed in the gas phase at these energies, no anion desorption occurs since anionic fragments likely have insufficient kinetic energy to desorb. Since the CH₂ ⁻ ion has not been observed in gas-phase measurements, we propose that it is formed, along with HCN (that is detected in TDS) when dissociation into CH₃ ⁻ and CN is hindered by adjacent molecules.     

Chemisorption of CO on tungsten (100): Combined flash desorption and electron stimulated desorption study. II

The chemisorption of CO on W(100) at ~ 100K has been studied using a combination of flash desorption and electron stimulated desorption (ESD) techniques. This is an extension of a similar study made for CO adsorption on W(100) at temperatures in the range 200–300K. As in the 200–300 K CO layer, both α₁-CO and α₂-CO are formed in addition to more strongly bound CO species upon adsorption at ~ 100K; the α-CO states yield CO⁺ and O⁺ respectively upon ESD. At low CO coverages, the α₁ and α₂-CO states are postulated to convert to β-CO or other strongly bound CO species upon heating. At higher CO coverages, α₁-CO converts to α₂-CO upon thermal desorption or electron stimulated desorption. There is evidence for the presence of other weakly-bound states in the low temperature CO layer having low surface concentration at saturation. The ESD behavior of the CO layer coadsorbed with hydrogen on W(100) is reported, and it is found that H(ads) suppresses either the concentration or the ionic cross section for α₁ and α₂-CO states.     

SIMS and TDS study of the reaction of water and oxygen on Pt(111)

The reaction of H₂¹⁶O with ¹⁸O on Pt(111) was investigated using temperature programmed static secondary ion mass spectrometry (TPSSIMS), SSIMS and thermal desorption spectroscopy (TDS). The TPSSIMS behavior of the H₃O⁺ and H₃¹⁸O⁺ signals, after coadsorption of the reactants at 95 K, indicates that the isotopic exchange rate increases sharply at temperatures above 130 K. At 150 K, hydroxyl formation is indicated by a sharp drop in the H₃O⁺ and a similar sharp increase in the H₃¹⁸O⁺ signals. Using TDS, the overall stoichiometry of the reaction was determined to be 2H₂O(a)+O(a)→ 3 OH(a)+H(a). SSIMS data suggest that at least two kinds of hydroxyls are formed and that they do not interconvert at 160 K.     

Ion angular distributions in electron stimulated desorption: Oxygen and CO on W(111)

The ion angular distributions resulting from electron stimulated desorption (ESD) of oxygen and carbon monoxide chemisorbed on a tungsten (111) crystal have been determined. The O⁺ ions released during ESD of adsorbed oxygen exhibit three-fold symmetric angular distributions in orientational registry with the W(111) substrate. The CO⁺ and O⁺ ions released during ESD of a monolayer of CO are desorbed normal to the (111) surface. Models for both oxygen and CO adsorption are discussed. The data for CO are consistent with adsorption of CO in “standing up” carbonyl structures in the virgin and α-CO binding states.     

Chemisorption of H2 on W(100): Absolute sticking probability,coverage, and electron stimulated desorption

Measurements of both the absolute sticking probability near normal incidence and the coverage of H₂ adsorbed on W(100) at ~ 300K have been made using a precision gas dosing system; a known fraction of the molecules entering the vacuum chamber struck the sample crystal before reaching a mass spectrometer detector. The initial sticking probability S₀ for H₂/W(100) is 0.51 ± 0.03; the hydrogen coverage extrapolated to S = 0 is 2.0 × 10¹⁵ atoms cm^?2. The initial sticking probability S₀ for D₂/W(100) is 0.57 ± 0.03; the isotope effect for sticking probability is smaller than previously reported. Electron stimulated desorption (ESD) studies reveal that the low coverage β₂ hydrogen state on W(100) yields H⁺ ions upon bombardment by 100 eV electrons; the ion desorption cross section is ~ 1.8 × 10^?23 cm². The H⁺ ion cross section at saturation hydrogen coverage when the β₁ state is fully populated is ? 10^?25 cm². An isotope effect in electron stimulated desorption of H⁺ and D⁺ has been found. The H⁺ ion yield is ? 100 × greater than the D⁺ ion yield, in agreement with theory.     

Hydrogen on W(100): temperature dependence of LEED and angle-resolved ESD

For hydrogen adsorption on W(100), the evolution of the c(2 × 2) LEED intensities and of the H⁺ ESD signal with H coverage have been investigated for various adsorption and annealing temperatures. Striking changes have been found for the half-order LEED intensities in the temperature range 140–360 K, in agreement with other workers, where the H⁺ signal showed only minor differences. The maxima of the LEED and the ESD intensities, however, occurred at the same exposure throughout this range (≈25% of saturation coverage). A temperature dependent variation of the height of the H⁺ maximum was observed which was reversible up to the desorption temperature of the β₂ hydrogen phase. The H⁺ ESDIAD lobe was found to have a polar FWHM of about 21°, independent of temperature between 140 and 450 K, and without any azimuthal dependence. These results provide evidence for the assumption that the observable H⁺ ions desorb from reconstructed sites. The number of these sites depends on temperature and hydrogen coverage, as shown by the change of the H⁺ current with these parameters. The transition from H on reconstructed to H on unreconstructed sites is of the order-order type; the energy difference between the two different adsorbate situations is about 135 meV/site at the quarter coverage. The consistency of the results and conclusions with a bridge-site model for H adsorption is shown. Elastic interactions lead to agglomeration of adsorbed H. The azimuthal isotropy of the ESDIAD lobes is interpreted by a superposition of emission from various types of bridge-sites which smear out the anisotropy expected for individual bridge-sites.     

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.     

Adsorption of hydrogen and carbon monoxide on nickel

Ion induced desorption was used to examine the emission of secondary ion clusters during coadsorption of H₂ and CO on nickel. In addition to the expected CO bearing species, a significant NiOH⁺ signal was observed. It is suggested that the source of this cluster ion may be interaction of the parent adsorbed H₂ and CO. Evidence was also found for emission of a negative species of mass 26, presumably C₂H^?₂.     

Adsorption-desorption studies using ESD of CCl2F2, C2H2F2 and C2F6 on tungsten

The behavior of the desorbing F⁺ ion current from electron bombarded CCl₂F₂, C₂H₂F₂ and C₂F₆ adsorbed on tungsten has been used to investigate the processes of adsorption and desorption of these gases. For tungsten near room temperature, measurements of the F⁺ ion current as a function of electron bombardment time indicated very similar or even identical F⁺-yielding adsorbed species resulting from adsorption of either CCl₂F₂ or C₂H₂F₂ and widely different species from C₂F₆. Cl⁺ ions were also observed to desorb from CCl₂F₂ ad-layers. The behavior of the Cl⁺ ion current with time during electron bombardment indicated electronic conversion between adsorbed binding modes. Complementary investigations on the interaction of CCl₂F₂, C₂H₂F₂ and C₂F₆ with tungsten were carried out by thermal desorption experiments in which the F⁺ ion signal was used to observe the coverage decrease of the F⁺-yielding species. The experiments were performed at tungsten temperatures in the 1200–1600 K range. It was concluded that the F⁺-yielding adsorbed species from CCl₂F₂ and C₂H₂F₂ were strongly bound to the tungsten surface. The F⁺-yielding species from C₂F₆ were found to be weakly bound. From a comparison of the ESD and thermal desorption results, the possibility of dissociative adsorption as well as the nature of the adsorbed species is discussed.     

Second virial coefficient for a mixture of nonspherical molecules of arbitrary symmetry

The mixture and interaction second virial coefficients of a binary gas mixture of nonspherical molecules of arbitrary symmetry have been calculated for a set of unlike force parameters which is obtained from the force parameters for like interactions by using empirical combination rules. In the calculation molecular anisotropy of very general type has been accounted. The relative contribution of each branch of interactions has been evaluated as a function of temperature. The theoretical values of interaction second virial coefficient have been compared with the experimental data of N₂+A, N₂+He, N₂+H₂, N₂+O₂, N₂+C₂H₄, N₂+C₂H₆, CO₂+A, CO₂+H₂, CO₂+N₂, CO₂+O₂, CO₂+CO, CO₂+C₂H₄, CO₂+C₂H₆, O₂+A and H₂+CO. The agreement between theory and experiment is satisfactory for all the systems. Numerical estimations of the mixture second virial coefficients as a function of temperature and composition are given for the systems CO₂+A, CO₂+H₂, CO₂+N₂, CO₂+O₂ and CO₂+CO.     

Study of lithium adsorption on the TiO2 surface by electron-stimulated desorption

This paper reports on a study of electron-stimulated desorption (ESD) of O⁺ and Li⁺ ions from titanium dioxide as a function of the preheating temperature T and of the concentration of lithium adsorbed at 300 K, which was carried out with a static magnetic mass spectrometer combined with a retarding-field energy analyzer. For T>1500 K, the TiO₂ surface undergoes irreversible rearrangement. At temperatures from 300 to 900 K and at lithium coverages Θ<1, the ESD cross sections of the O⁺ and Li⁺ ions vary in a reversible manner with temperature, while for lithium coverages Θ>1, the changes in the Li⁺ and O⁺ ESD cross sections become irreversible. For θ<1, the appearance threshold of the Li⁺ and O⁺ ions is 25 eV, whereas for θ>1, the ESD threshold of Li⁺ ions shifts to 37 eV.     

Femtosecond electron dynamics on solid surfaces probed by low energy ion scattering and stimulated desorption of secondary ions

The cations emission from condensed matter surfaces has been investigated on the basis of localization and delocalization of valence hole(s) in the femtosecond timescale. The yield of scattered H⁺ (E₀=100 eV), though negligibly small from the Pt(1 1 1) substrate, increases markedly when Ar is adsorbed on it, indicating the localization of a valence (H⁺ 1s) hole on the physisorbed Ar layer. However, the yield of H⁺ scattered from a thick H₂O layer is considerably small relative to that from Ar and CO layers. The delocalized nature of a valence hole in water ice is caused by some covalency in hydrogen bonds. Hydrated protons, H⁺(H₂O)_n, n=1,2,…,10, are emitted efficiently in electron stimulated desorption from water molecules adsorbed on the Ar layer; the ion yields are highest at the initial adsorption stage and decay steeply with increasing coverage. Coulombic repulsion between the hydrated protons confined in physisorbed nanoclusters is responsible for the explosive ion emission.     

Investigations on field desorption products from silver surfaces by mass spectrometry

Field evaporation of silver and field desorption of silver surface compounds were investigated by analysing positive ions with a mass spectrometer. In particular, the well known adsorption states of oxygen, and further the interactions of H₂O, NH₃, H₂, CO and CH₄ were measured in the field ion mass spectrometer under steady state fields of > 0.1 V/Å with a sensitivity of < 0.1 ions s^?1 and at temperatures between 80 °K and 425 °K. Although oxygen is usually chemisorbed at Ag surfaces, no AgO⁺, AgO⁺₂ or other Ag-O compounds could be detected as positive ions, Ag⁺ and O₂⁺ are the only observed ions at best image fields in oxygen up to fields of field evaporation of Ag⁺(≈ 2.2 V/Å). Even after the actual adsorption of oxygen with zero-field (6 × 10⁵ Langmuir at 10^?3 Torr) at 323 °K and 473 °K and subsequent application of the desorption field at 210°K no silver-oxygen compounds were found in positive ionic form. Small quantities of AgO⁺ and AgO⁺₂ were only formed — besides Ag(H₂O)_x⁺ complexes — if atomic oxygen was supplied by the field induced dissociation of water.Gases which do not adsorb on silver under zero-field conditions (H₂, CO, CH₄, N₂) yield the ions Ag(H₂)_n, Ag(CO)_n⁺, n=1, 2; AgCH₄⁺, AgN₂⁺. The situation with H₂O and NH₃ is more complicated: Molecular ions [Ag(H₂O)_n]⁺·mH₂O, n=1,…, 4, m=1,…, 8 and [Ag(NH₃)_n]⁺·mNH₃, n=1, 2, m=1,…, 6 are found besides Ag⁺.From the temperature and field dependence conclusions are drawn about the mechanisms of evaporation and formation of ionic surface complexes. The activation energies of evaporation of Ag⁺ are found to depend on the square root of the field strength. In general, the generation of surface compounds can be described by field induced reactions rather than usual gas adsorption.     

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 geometry of CO on Ru(001): Evidence for bending vibrations in adsorbed molecules

As a test of the utility of the ESDIAD method (Electron Stimulated Desorption Ion Angular Distributions) in studies of the geometry of adsorbed molecules, the chemisorption of CO on Ru(001) has been examined. Data previously reported using UPS (ultraviolet photoemission spectroscopy) and EELS (electron energy loss spectroscopy) have indicated that CO is terminally bonded to the Ru surface through the C atom, with the CO axis perpendicular to the surface. The ESDIAD results for CO confirm this orientation; for all CO coverages in the temperature range 90 K to ~ 350 K, the angular distributions of O⁺ and CO⁺ ESD ions are centered about the surface normal. The widths of the ion beams are temperature dependent; for both O⁺ and CO⁺, the half widths at half maximum, α, of the ion cones are ~16° at 300 K, and ~12° at 90 K. This temperature dependence, coupled with a simple model calculation, indicates that the dominant factors contributing to the width of the ESD ion beams are the CO surface bending vibrations, i.e., initial state effects. Thus, the data suggest that both the directions and widths of ESDIAD beams are determined largely by the structure and dynamics of the initial adsorbed state.     

Electron stimulated desorption of condensed,branched alkanes

Electron stimulated desorption of H⁺, CH₃⁺, H₂⁺ and D⁺ has been measured as a function of excitation energy for condensed phase neopentane, tetramethylsilane, 2-methylpropane-1-d₁, and 2-methylpropane-2-d₁. The data show that the processes which result in both H⁺ and CH₃⁺ production are initiated by the same excitation. The excitations which produce H⁺, CH₃⁺ and H₂⁺ appear to be largely localized on the methyl groups. The results indicate that multi-electron (hole) final states are responsible for H⁺ and CH₃⁺ desorption.