P. Weightman

Reflection anisotropy spectroscopy

Reflection anisotropy spectroscopy (RAS) is a non-destructive optical probe of surfaces that is capable of operation within a wide range of environments. In this review we trace the development of RAS from its origins in the 1980s as a probe of semiconductor surfaces and semiconductor growth through to the present where it is emerging as a powerful addition to the wide range of existing ultra-high vacuum (UHV) surface science techniques. The principles, instrumentation and theoretical considerations of RAS are discussed. The recent progress in the application of RAS to investigate phenomena at metal surfaces is reviewed, and applications in fields including electrochemistry, molecular assembly, liquid crystal device fabrication and remote stress sensing are discussed. We show that the experimental study of relatively simple surfaces combined with continuing progress in the theoretical description of surface optics promises to unlock the full potential of RAS. This provides a firm foundation for the application of the technique to the challenging fields of ambient, high pressure and liquid environments. It is in these environments that RAS has a clear advantage over UHV-based probes for investigating surface phenomena, and its surface sensitivity, ability to monitor macroscopic areas and rapidity of response make it an ideal complement to scanning probe techniques which can also operate in such environments.

X-ray-excited Auger and photoelectron spectroscopy

This article reviews developments in the understanding of X-ray-excited Auger and photoelectron spectra in the light of theoretical developments in atom, molecular and solid-state physics. After reviewing progress in XPS and AES separately emphasis is placed on the inter-relationship between the two fields: Auger rates, for example, are the dominant contribution to core-level XPS linewidths and by combining XPS and AES it is possible to deduce information about Coster-Kronig processes which are difficult to study directly. An account is given of how the combination of measurements of environmentally dependent shifts in XPS and AES energies allows one to isolate initial- and final-state contributions which can then be related to the results of other experimental techniques. There is a brief discussion of many-electron effects which, in some instances, dominate both XPS and AES spectra. The author discusses how the combination of XPS and AES spectra involving valence levels enables one to study the effects of hole-state localisation.

Localized states in molecular auger spectra

Abstract The influence of the degree of localization of the two final holes on the Auger spectra of a simple diatomic molecule is analysed in detail and cases where such effects may be observed experimentally are discussed. The results are compared with studies of the valence band Auger spectra of solids.

The M4,5N4,5N4,5 Auger spectra of Ag, Cd, In and Sn

High-resolution M4,5N4,5N4,5 Auger and M5 and valence-band photoelectron spectra of Ag, Cd, In and Sn, measured using X-ray excitation, are combined to yield the magnitude of Ueff, the Coulomb repulsion between two holes on the same atomic site. The extra-atomic relaxation contributions to Auger and photoelectron processes in these elements are determined by comparing the experimental results with atomic structure calculations. The relationship Ueff>>2W, where W is the width of the 4d band, holds for Cd, In and Sn and the M4,5N4,5N4,5 Auger spectra of these elements are in good agreement with multiplet structure calculations. Discrepancies between the observed and the calculated M4,5N4,5N4,5 spectra of Ag are interpreted in terms of its failure to satisfy the Ueff>>2W condition and the variation of Ueff with the final-state component of the Auger spectrum. It is suggested that the 3F4 component is able to decay into the Ag 4d band and is thus substantially broader than the other components of the Auger spectrum.

Kvv Auger Spectrum Of F2 - The Importance Of Hole-Hole Correlation

The KVV Auger spectrum of F2 has been measured and analyzed in terms of theoretical models of different levels of sophistication. In contrast to the corresponding spectra of many small molecules, this spectrum cannot be described in terms of an independent‐particle model. The approach suggested by Thomas and Weightman, which allows in an approximate way for hole–hole interaction, gives noticeably improved results. A more accurate, configuration‐interaction treatment developed by Jennison gives reasonable agreement with experiment. Peak assignments based on comparison of this theory with the experimental spectrum have been made. This spectrum provides a very clear case where hole–hole correlation effects are important. The experimental spectrum is unusual in that the lines are generally narrow, in contrast to many KVV spectra, which often have quite broad lines. The ionization potentials for the 2σg and 2σu orbitals have been measured. The relative cross sections for ionization from these orbitals are equa...

Reconstructions of the GaAs(111)B surface

Abstract The (111)B surface of GaAs has been investigated using scanning tunnelling microscopy (STM) and a number of different reconstructions have been found at different surface stoichiometries. In accordance with electron diffraction studies, we find the series (2 × 2), (1 × 1) LT , (√19×√19) and (1 × 1) HT with increasing annealing temperature, corresponding to decreasing surface As concentration. We find the (1 × 1) LT to be a mixture of the local structures of the (2 × 2) and (√19×√19) phases as well as some elements of a (3 × 3) structure. This is behaviour consistent with a system, dominated by the supply of As adatoms to the surface, and may be an example of a continuous phase transition. Above the (1 × 1) LT transition, atomic resolution images of the (√19×√19) surface reveal only a threefold symmetry of the hexagonal structural units, brought about by inequivalent surface bonding. This is responsible for the disorder found in the (√19×√19) reconstruction, since the structure may form in one of two domain rotations. At lower surface As concentration, the (1 × 1) HT surface adopts a random structure containing small domains of a (√7×√7)R19.1° reconstruction.

Substrate dependence of adlayer optical response in reflectance anisotropy spectroscopy

Reflectance anisotropy spectroscopy (RAS) offers extreme surface sensitivity and can be used to study gas-solid and solid-liquid interfaces. Although the technique has already made a considerable contribution to in situ monitoring of the epitaxial growth of semiconductors, its impact has thus far been limited by difficulties in interpreting experimental results. In particular, RAS spectra are related to both Δe2′ and Δe2″, the real and imaginary parts of the surface dielectric anisotropy, with weighting factors determined by the complex dielectric function of the substrate. Here RAS calculations are performed for a variety of substrates assuming a model overlayer dielectric anisotropy. Three distinct regimes of behavior are observed: RAS spectra can resemble either Δe2′, Δe2″ or −Δe2″ (or some combination of the three) depending on the substrate and the photon energy. It is therefore crucial to properly account for these substrate effects if RAS is to be used to determine the azimuthal orientation of adso...

A rapid reflectance anisotropy spectrometer

A 16 channel reflectance anisotropy (RA) spectrometer capable of simultaneously acquiring reflectance spectra and real and imaginary RA spectra on the 0.1 s time-scale is reported. Its performance was evaluated by monitoring the electrolytic deposition/growth and removal of copper on a gold(110) surface. The slow deposition enabled very low noise spectra to be obtained, which served as the yardstick with which the spectra obtained during the rapid removal could be compared. The spectra obtained over the 15 s removal mirrored those obtained during the 500 s deposition period. The spectra are discussed in terms of the copper growth being in the form of aligned islands.

Auger vacancy satellite structure in the L3M4,5M4,5 Auger spectra of copper

High resolution L3M4,5M4,5 Auger spectra of copper are compared with multiplet structure and transition rate calculations. The comparison reveals unexplained structure to lower energies which is attributed to Auger vacancy satellite lines.

Determination of the structure of adenine monolayers adsorbed at Au(110)/electrolyte interfaces using reflection anisotropy spectroscopy

Reflection anisotropy spectroscopy (RAS) has been used to show that at saturation coverage adenine adsorbs on the Au(110)/electrolyte interface in a base-stacking configuration with the plane of the bases orientated vertically on the surface and with the long axis of the molecules parallel to the [110] direction. Changes in the RAS observed from adsorbed adenine as a result of changes in the potential applied to the Au(110) electrode could arise from slight changes in the orientation of the molecules in the vertical plane.