E.M. Williams

The orientation of acetate on a TiO2(110) surface

The adsorption of acetic acid on a TiO2(110) surface has been studied using electron stimulated desorption ion angular distribution (ESDIAD) and low energy electron diffraction (LEED). The acetate intermediates arising from the dissociative adsorption of acetic acid form an ordered (2×1) overlayer at saturation coverage. The H+ ESD ion angular distributions can be resolved into two contributions: Those ions desorbing from hydrogen atoms bonded at the oxide substrate, and those ions desorbed via the rupture of the C–H bonds of the acetate. The geometry of the ESDIAD pattern led us to propose that the acetates are bridge bonded with the five-fold coordinated Ti 4+ ions, with their molecular plane perpendicular to the surface. Decomposition of acetate at room temperature occurs under electron beam irradiation, resulting in the desorption of CH2CO and CH3/CH4.

The adsorption of benzoic acid on a TiO2(110) surface studied using STM, ESDIAD and LEED

The adsorption of benzoic acid on a TiO2(110) surface at room temperature has been studied using scanning tunneling microscopy (STM), electron stimulated desorption ion angular distribution (ESDIAD), and low energy electron diffraction (LEED). The adsorption is dissociative, forming benzoate and surface hydroxyl. The adsorbed benzoate is bonded through the car☐yl group with the five-fold coordinated Ti4+ cations, and forms an ordered pseudo-(2 × 1) overlayer at a saturation coverage of 0.5 ML. This (2 × 1) structure is mainly determined by the relatively strong absorbate-substrate interaction. Attractive interactions between aromatic rings of the benzoates lead to the formation of dimerised benzoate rows along the [001] direction.

The effect of adsorbate–adsorbate interaction on the structure of chemisorbed overlayers on TiO2(110)

Abstract The adsorption of acetic and benzoic acid on a TiO 2 (110) surface has been studied using scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). Both acids dissociate upon adsorption at room temperature and with saturation coverage for both species, LEED identifies a (2×1) pattern. Under the same conditions, STM shows a (2×1) form for acetate, but a predominance of (2×2) with benzoate. It is proposed that rotation of the phenyl ring takes place, allowing the formation of dimers of benzoate at the surface.

The interaction of electron beams with water admitted at aluminium single crystal (100) studied by ESD and AES

Processes of adsorption and of electron beam induced desorption of water at aluminium single crystal (100) have been investigated simultaneously using the techniques of Electron Stimulated Desorption (ESD) and of Auger Electron Spectroscopy (AES). The predominant ion production with ESD is H+ and OH+ ions with, respectively, kinetic energies of around 4.9 and 1.8 eV, and with desorption efficiencies (maxima) of 1×10−5 and 2×10−7 ion/electron. The behavior of these ion signals in the initial region of gas exposure (<10 L) at the clean surface is complex - the yield of OH+ ions, for example, passes through a maximum at an exposure of around 4 L. Both ions are identified as arising from OH groups at the surface, and the findings, corroborated by the observations with AES, are identified with the transformation of an initial chemisorbed phase to an oxide surface. The kinetics of adsorption, both in the initial chemisorbed phase and with the evolution of oxide, are shown to support an island growth model. The desorption induced by electron impact has similarly been monitored by the two techniques. The technique of ESD is far more responsive to the action of electron beams, and this is attributed to its greater sensitivity to the topmost surface layers. Both techniques contribute to the finding of a process of oxidation induced by electron bombardment.

ESDIAD studies of the structure of TiO2(110)(1 × 1) and (1 × 2) surfaces and interfaces in conjunction with LEED and STM

The TiO2(110)-(1 × 1) surface and its reconstruction as a (1 × 2) form have been studied with low energy electron diffraction (LEED), electron stimulated desorption ion angular distribution (ESDIAD) and scanning tunnelling microscopy (STM). Oxygen ion desorption occurs within a lobe perpendicular to the (1 × 1) surface, changing to two off-normal lobes for the (1 × 2) reconstruction. This transformation in the ESDIAD pattern is consistent with the added Ti2O3 row model of the (1 × 2) reconstruction proposed by Onishi and Iwasawa. STM studies of the stoichiometric and electron irradiated surfaces reinforce the association of the O+ ESD contribution with majority sites at the surface. Adsorption of acetic acid on the (1 × 1) surface produces a (2 × 1) overlayed and induces a reconstruction of the underlying substrate. ESDIAD reveals H+ ions emitted off-normally from dissociatively adsorbed acetate, and along the surface normal from surface hydroxyls. Adsorption of acetic acid on the (1 × 2) surface does not modify the LEED pattern, but ESDIAD reveals H+ desorption with a weaker off-normal contribution consistent with the Ti2O3 model of the reconstruction.

The structure of TiO2(110) (1 × 1) and (1 × 2) surfaces with acetic acid adsorption — a PES study

Abstract The adsorption of acetic acid on the TiO 2 (110) surface has been studied with PES, using synchrotron radiation ∼47 eV, both at the stoichiometric (1 × 1) surface and the (1 × 2)-reconstructed surface. The results indicate a similar bonding configuration for each surface, and that the saturation acetate coverage at room temperature on the (1 × 2)-reconstructed surface is approximately half that of the (1 × 1) surface. The results are interpreted in favour of the added Ti 2 O 3 -row model of the (1 × 2) reconstruction at the clean surface.

The adsorption and bonding of chlorine at silicon (100) investigated using ESD/ESDIAD with Cl+ and Cl− ions

Abstract The adsorption of chlorine on Si(100) 2 × 1 surface has been studied using electron stimulated desorption (ESD) and electron stimulated desorption ion angular distribution (ESDIAD) in conjunction with AES and gas uptake techniques. ESDIAD and ESD measurements were performed on negative as well as positive atomic chlorine species, and the responses with the different polarity of charged species are not seen as complementary. Gas uptake at the surface proceeds initially with a high sticking probability with the atomic chlorine resulting from dissociation not being limited to single dimer sites. ESDIAD studies with positive Cl + ions reveal normal and off-normal beams associated with symmetric and asymmetric dimers, with relative contributions depending on surface coverage and temperature. Transformations between bonding configurations seen in positive ion ESDIAD are linked with lateral interactions in the adsorbate layer, and their influence is also evident in the forms of the ion yields of both polarity of species with changing coverage. Negative chlorine ions exhibit a predominance of emission around the surface normal, and are produced via a dipolar dissociation process. Missing atom defect sites with an associated high electron density are postulated as playing a central role in their production. The desorption of positive chlorine ions follows mainly from a two-hole, one-electron (2h1e) repulsive state initiated by the ionisation of the Cl 3s level.

Electron stimulated desorption of ions from surfaces: techniques, methodology and some recent findings with water at metals and semiconductors☆

Abstract A review is presented of the basic principles, theory and instrumentation underlying the observation of ions with the technique of electron stimulated desorption (ESD) and the results of some recent investigations with water adsorbed at metal and semiconductor surfaces are employed to illustrate the state of research in the field. The evolution of theoretical models of desorption from the pioneering work of Redhead, Menzel and Gomer to the present state of theory with considerations of hole-hole interactions are summarised, with particular emphasis on the physical actions involved in the desorption processes. Methods of study of desorbed ions using mass spectrometers, retarding field analysers and time-of-flight techniques are examined, as well as methods for determining the angular distribution of desorbed ions. The response with ESD at the interface formed with water is particularly striking in view of the variety and the range of desorption efficiency of the ions produced. Experimental factors contributing to the possible enhancement of ion production are examined and the results of measurements of desorption parameters are discussed and compared with the predictions of theory.

Electron desorption cross sections with oxygen at tungsten (100)

The electron induced desorption of oxygen at tungsten (100) single crystal has been studied simultaneously with the techniques of electron stimulated desorption (ESD) and Auger electron spectroscopy (AES). Measurements of desorption cross section are reported for both the β1 and β2 phase of adsorbed oxygen, identified with the technique of ESD, which are compared with the depletion of the total of adsorbed gas as inferred with AES.

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

Abstract Electron-stimulated desorption ion angular distribution (ESDIAD) and LEED were used to investigate the structure changes at the TiO 2 (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.