John T. Yates

Electron-Stimulated Desorption as a Tool for Studies of Chemisorption: A Review

Bombardment of solids by low-energy electrons (<500 eV) can cause various changes in the surface region, particularly if the surface contains an adsorbed layer. Electron bombardment can promote the desorption of energetic neutral and ionic fragments from the surface, can alter the bonding of surface species, and in some cases can cause decomposition of the surface region. These processes are termed electron-stimulated desorption (ESD) phenomena, and this review is primarily concerned with the use of ESD in studies of species adsorbed on surfaces. Topics covered include experimental methods, a theoretical discussion of physical mechanisms, and a detailed discussion of certain specific and well-studied adsorbate–substrate systems.

Interaction of Oxygen with Polycrystalline Tungsten. I. Sticking Probabilities and Desorption Spectra

The interaction of oxygen with a polycrystalline W filament has been studied in the temperature range 300–1200°K and for exposures between 10−7 and 1u2009torr·sec. Both sticking probabilities and desorption spectra have been measured. After low exposures (〈2×10−6u2009torr·sec) the adsorbate is all desorbed as O atoms with first order kinetics. With higher exposures, oxygen is removed as oxides: WO, WO2, WO3, W2O6. The desorption spectra for each of these species are complex, and nine distinct and reproducible oxide states are identified. Saturation coverages in each of these states are independent of adsorption temperature in the range 300–1200°K. At a total coverage of 8.5×1014u2009O2 molecules cm−2, 2.2×1014u2009O2 molecules cm−2 are desorbed as oxides; after exposures 〉 10−2u2009torr·sec at 300°K, the total uptake rises to 15×1014u2009O2 molecules cm−2. At temperatures between 500 and 1000°K and exposure up to 1u2009torr·sec, a multilayer oxide film is formed of unlimited thickness. The desorption product from this oxide film is ...

Surface chemistry at metallic step defect sites

The role of step sites on metal surfaces in promoting chemical reactions is discussed showing experimental data from the author’s laboratory. The adsorption of CO on two stepped Pt surfaces, Pt (112) and Pt (335), is compared using various methods that are sensitive to adsorption site character. The existence of one‐dimensional arrays of chemisorbed CO molecules along the step sites is demonstrated, and CO...CO repulsive steric interactions are observed to cause orthogonal tilting of the step‐bound CO species as the coverage is changed on the step sites. For Pt (335), the most active site for the oxidation of CO is found to involve a chemisorbed oxygen atom at the step site, reacting with a CO molecule chemisorbed on a terrace site. It is also shown that a new surface phenomenon, electron stimulated migration (ESM), may be spectroscopically observed for CO on Pt (335). The atomic steps act as trap sites for CO molecules that have been excited by 150 eV electrons. It is likely that this ESM phenomenon is a...

Evidence for the conformation of H2O adsorbed on Ru(001)

Abstract The electron stimulated desorption ion angular distributions (ESDIAD) method has been used to study the adsorption of H 2 O by Ru(001). The results indicate that chemisorbed, undissociated H 2 O is bonded to Ru via the oxygen atom, and that interactions between neighboring molecules occur as coverage increases. The utility of ESDIAD for structure determination in the absence of long range order is demonstrated.

Infrared spectroscopic study of activated surface processes: CO chemisorption on supported Rh

The infrared spectrum of CO chemisorbed on alumina‐supported Rh surfaces has been studied following chemisorption at cryogenic temperatures. Major differences are observed in the distribution and spectroscopic character of chemisorbed CO species produced at low temperatures (110–170 K) compared to chemisorbed CO species produced on Rh at 295 K. It has been found that the species Rh(CO)2, formed on isolated Rh sites, is produced rapidly via an activated chemisorption process above ∼200 K. On more ’’crystalline’’ Rhx sites, containing chemisorbed CO, an activated CO adsorbate‐conversion process has been detected in which ?CO decreases by ∼50 cm−1 on warming the adsorbed layer above ∼265 K. Isotopic exchange between 13CO(g) and 12CO(ads) has been shown to occur rapidly at low temperature (∼200 K) for Rh(CO)2 species, whereas Rhx(CO) species exchange rapidly only at higher temperatures (≳250 K). These results, taken together, serve to confirm a model in which isolated Rh sites coexist on the alumina support w...

ESCA study of fractional monolayer quantities of chemisorbed gases on tungsten

Abstract X-ray photoelectron spectroscopy (ESCA) has been used in a study of CO and O2 chemisorbed on a polycrystalline tungsten sample. Working under ultra-high vacuum conditions, the surface was cleaned and then covered with known monolayer and fractional monolayer quantities of adsorbed CO and O2. The O(ls) and C(ls) spectral features were detected, and the influence of an adsorbed layer on the tungsten spectral features was determined. A chemical shift of 3.4 eV in the O(ls) line from chemisorbed CO is related to the different modes of bonding of CO to tungsten. A model calculation of the photoelectron yields expected from an adsorbed monolayer is in good agreement with the experimental results.

The photochemical identification of two chemisorption states for molecular oxygen on TiO2(110)

We report the first experimental observation of two chemisorption states for molecular oxygen on a TiO2(110) surface containing anion vacancy sites. The first molecular species can be photoactivated to oxidize coadsorbed CO to CO2 (α channel) and undergoes slow photodesorption. The second molecular oxygen species only undergoes fast photodesorption (β channel). Conversion from α‐O2, to β‐O2 occurs upon heating the surface to above 200 K.

ESCA study of carbon monoxide and oxygen adsorption on tungsten

Abstract The chemisorption of both CO and O 2 on a clean tungsten ribbon has been studied using an ultrahigh vacuum X-ray photoelectron spectrometer. For CO, the energy and intensity of photoemission from O(1s) and C(1s) core levels have been studied for various adsorption temperatures. At adsorption temperatures of ∼100 K., the “virgin”-CO state was the dominant adsorbed species. Conversion of this state to more strongly-bound β-CO is observed upon heating the adsorbed layer to ∼320K. Thermal desorption of CO at 300⩽ T ⩽640 K causes sequential loss of α 1 -CO and α 2 -CO as judged by the disappearance of O(1s) and C(1s) photoelectron peaks characteristic of these states. Oxygen adsorption at 300K gives a single main O(ls) peak at all coverages, although at high oxygen coverages there exist small auxiliary peaks at ∼2eV lower kinetic energy. The photoelectron C(1s) and O(1s) binding energies observed for these adsorbed species are all lower than for gaseous molecules containing C and O atoms. For CO adsorption states there is a systematic decrease in photoelectron binding energy as the strength of adsorption increases. These observations are in general accord with expectations based on electronic relaxation effects in condensed materials.

Ion angular distributions in electron stimulated desorption: Adsorption of O2 and H2 on W (100)

The angular distributions of ions liberated by electron stimulated desorption (ESD) of H+ and O+ from adsorbed layers of oxygen and hydrogen on the (100) surface of tungsten have been examined. The resultant patterns of ion emission are observed visually in a display-type apparatus. Two types of ESD angular distribution patterns are observed: (1) hazy patterns with ill-defined boundaries, and (2) patterns in which the ions are liberated in well-defined cones of emission whose directions are correlated with the symmetry of the underlying substrate. These latter patterns are postulated to arise from ESD from a directionally-bonded adsorbate in which the direction of the ground state adsorbate-substrate bond may be preserved in ion desorption.

The adsorption of xenon by W(111), and its interaction with preadsorbed oxygen

Abstract The physisorption of Xe on W(111) and of Xe on partial layers of oxygen chemisorbed on W(111) has been studied using flash desorption and work function methods. It has been found that xenon adsorbs up to monolayer coverages at 104K. Xenon desorbs from W(111) as a single binding state following first order kinetics. At low coverages (θ Xe Xe > 0.07 the desorption data fit a first order rate expression with a desorption energy of 9.3 kcal/mol and preexponential ν = 10 15 s −1 . The observed work function change of −1.1 ± 0.1 eV is consistent with monolayer estimates reported in field emission studies of physisorbed xenon on tungsten. The effect of preadsorbed oxygen layers on the physisorption of xenon on this surface is very striking. The energy of desorption shifts as much as 50% higher for a moderate exposure of oxygen. Several physisorption models are explored along with estimates of dispersion and electrostatic interaction contributions.