R. J. Cole

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.

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...

Separation of optical anisotropies by angular dependent reflection anisotropy spectroscopy

Double rubbing of polymer films, which finds applications in modern liquid crystal devices, has been investigated using reflection anisotropy spectroscopy (RAS) and atomic force microscopy. A method of gauging the relative strength of rubbing in two different directions, based upon the dependence of observed RAS signals on the sample azimuthal angle, has been found. Angular dependent measurements are shown to be a useful extension of the standard technique by allowing angular dependent sources of optical anisotropy to be separated experimentally.

Disorder broadening of core level photoemission spectra in CuxPt1−x alloys

Abstract High resolution photoelectron spectroscopy has shown the presence of disorder broadening of the Cu 2p 3/2 photoelectron line in CuPt alloys. The broadening is greatest for the Cu 50 Pt 50 composition as predicted by the charge correlated model. The broadening is less in Cu 75 Pt 25 than in Cu 25 Pt 75 probably due to the tendency to order in Cu 75 Pt 25 .

Relative core level shifts in XPS: a theoretical study

Abstract The commonly used assumption that all core levels exhibit the same chemical shift is examined using Dirac–Fock electronic structure calculations for a wide range of free atoms and ions. In contrast to experimental results for molecules and solids, the calculated core level shifts are found to be almost uniform. The role of the Madelung potential in exaggerating relative core level shifts in solids and molecules is discussed. Calculated results are found to agree well with the most reliable experimental data.

Contributions from surface-modified bulk electronic bands to the reflection anisotropy of Au(110)-(1 × 2)

We investigate the temperature dependence of the optical reflectance anisotropy (RA) of the Au(110)-(1 × 2) surface and find that transitions involving surface-modified bulk bands contribute to the RA spectrum. The RA peaks observed at room temperature at photon energies of 3.52 and 4.50 eV are assigned to the transitions and , respectively. The assignments are based upon a comparison between temperature-induced shifts in the energy of these RAS peaks and thermovariation optical spectroscopy results of the temperature dependence of transition energies between bands at the L symmetry point. The application of RAS to Au(110) can be seen as a model system for exploring surfaces in a range of environments including ultra-high vacuum, high pressures and at the solid/liquid interface. The results reported here further the understanding of the RA spectrum of the clean Au(110) surface.

The physical and electronic structure of the Cu85Pd15(110) surface; Clues from the study of bulk CuxPd1-x alloys

The (110) surface of Cu85Pd15 adopts an ordered structure which can be controlled by Ar bombardment and thermal treatment. A summary is given of the large body of theoretical and experimental work on the physical and electronic structure of bulk CuxPd1−x alloys. The electronic structure of the ordered Cu85Pd15(110) surface is discussed in terms of the conclusions drawn from the study of bulk CuPd alloys.

An ultrasonic real-time scanner with pulsed doppler and T-M facilities for foetal breathing and other obstetrical studies

Abstract An ultrasonic system for foetal breathing studies is described. The system is composed of a Real-Time scanner with T - M mode and pulsed Doppler facilities. Possible approaches to Real-Time Scanning for studying foetal breathing are reviewed. Reasons for employing a rotating transducer scanner of small dimensions are presented. The roles of the three ultrasonic facilities, i.e. Real-Time, T - M and Doppler techniques are discussed. Finally the requirements of an ultrasonic system for studying foetal breathing are identified. Future developments and applications of the system are indicated.

A comparison of surface sensitive reflection spectroscopies

The surface sensitivity (in the sub-nanometre regime) of reflection spectroscopies is discussed. Simulations are used to illustrate the strengths and limitations of 45 degree reflectometry (45DR). Particular emphasis is placed upon the comparison with spectroscopic ellipsometry.

Azimuth dependent reflection anisotropy of oriented thin films

A simplified mathematical description of reflection anisotropy spectroscopy (RAS) measurements is presented. The RAS signals of a nanometer film with arbitrarily aligned dielectric axes are investigated. While RAS spectra are found to be relatively insensitive to tilting of the dielectric axes out of the surface plane, the variation of RAS signals with sample azimuthal orientation angle, θs, reveals a distinct sin θs effect, superimposed on the previously observed sin 2θs and sin 4θs terms, which provides a measure of the tilt angle.