Overlap effects on surface states

A tight-binding investigation is performed of the electronic structure of a semi-infinite monatomic chain, whose atomic orbitals are assumed to be non-orthogonal, so that the effects of overlap can be taken into account. In addition to markedly modifying the bulk band-edges, the presence of overlap also greatly influences the position and existence of the surface states. These latter effects are examined in detail.     

Chemisorption on supported-metal catalysts

A Gree-function formalism is developed to describe the electronic and chemisorption properties of a supported-metal composite substrate. Within the framework of the tight-binding approximation, the metal catalyst is represented by a finite chain of d-orbitals, while the semi-infinite semiconductor support is characterized by a linear chain of alternating s- and p-orbitals. The Anderson-Newns model is used to calculate the chemisorption energy and adatom charge transfer for hydrogen chemisorption on the Ni/ZnO composite system.     

A short story of SIN!

A brief overview is given of the theory of surface-ion neutralization (SIN) by highlighting studies from the pioneering days to the present time. Emphasis is placed on the 1-electron time-dependent Newns-Anderson (TDNA) model, which has played such a key role in studying resonance charge transfer. Recently, a many-electron (ME) version of the TDNA approach has been formulated to treat two or more electron processes, such as Auger transitions. The equation-of-motion technique is used to obtain the TD coefficients from which the required SIN probabilities can be found. After outlining the ME method, two applications are described. The first deals with the Li⁺-Cs/W(110) system, while the second addresses the laser enhancement of SIN, in the case of Li⁺-KBr, and revises some earlier results.     

A simple mathematical model to aid quantification of electrophoresis gels by image analysis

In many scientific disciplines, measurements are taken from films that have been exposed to energetic sources. Examples include radiographs where the source is an X-ray tube, autoradiography where the source is a radioactive isotope and electrophoresis gels where the source is an enhanced chemiluminescence reaction. In these situations it is of interest to quantify the darkening of the film and compute the strength of the source which in the cases of autoradiography and electrophoresis can be used to compute unknown concentrations of biochemicals. We developed a simple mathematical model of the darkening of films in radiography, autoradiography and electrophoresis bands disclosed by enhanced chemiluminescence, and present formulae to calculate the strength of the source from measurement of film blackening by image analysis. A simple model is used in two examples to predict blackening of film exposed to electromagnetic radiation. This blackening is measured by image analysis. Results show reasonable agreement between predictions of the model and blackening of film for the examples chosen. This model is proposed as an aid to quantification of electrophoresis gels.     

Surface states in electrochemisorption

The recursive Green function (RGF) method is used to obtain the surface and adatom RGFs for an electrified substate. The electrochemisorption (ECS) process is described, by invoking the Anderson–Newns (AN) model, whereby a self-consistent treatment of the ECS energy and charge transfer is provided. The role of the electric field, and the presence of surface states (SS), in governing magnetic and non-magnetic ECS is discussed in the case of H–Cr.     

Adsorption effects on resonant charge transfer in atom-surface scattering

The probability of positive ionization of an atom impinging on a solid surface is calculated by numerical solution of the equations of motion of the time-dependent molecular orbitals of the system. The model is applied to the scattering of Na atoms, in the energy range 50–1000 eV, from a W(110) surface. The presence of Na adatoms is considered and found to hinder the ionization process. This reduction in the charge-transfer probability arises because of the lowering of the surface work function brought about by increasing adatom concentration.     

The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.     

Uniform molecular analysis using femtosecond laser mass spectrometry

The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd.