Reflection Anisotropy Spectroscopy RAS

Reflection anisotropy spectroscopy (RAS) probes the difference between the reflection coefficients measured at near-normal incidence for two mutually perpendicular polarizations. Let ip be an azimuthal angle between the plane of incidence and one of the principal axes of the sample surface. Then the RAS signal normalized to the mean reflection coefficient, r, can be written as 

For an isotropic sample, Ar = 0, and therefore RAS measures the optical anisotropy. Another conclusion from here is that for an isotropic substrate only the overlayer contributes to the RAS signal. Such conditions are realized, for example, for the (001) surface of a cubic crystal, where one would measure the reflection coefficients for light polarized along the [110] and [IlO] directions (see Problem 5.3). 

As with SDR, the basic components of the experimental setup for RAS measurements resemble those used in SE with the addition of a photoelastic modulator. The latter device advances and retards, sinusoidally, the phase of the reflected light. Then a Fourier analysis of the signal allows one to obtain the real and imaginary parts of the reflection anisotropy, Ar/r. Detection of changes in the RAS signal at a fixed photon energy takes less than 100 ms. 

If a substrate is transparent in the frequency range under consideration, the spectrum of an overlayer can also be measured in transmission (Fig. 5.6). In the optical region, this condition is fulfilled for dielectrics and therefore adsorption on dielectric surfaces can be studied in such a geometry. The sensitivity of the spectrum to adsorbate properties can be increased by passing light several times through similar surfaces. 

The measured dependencies of the surface absorption coefficient on both the vapor pressure and the surface temperature corresponded to the Lang- 

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Surface states were first detected by optical techniques in semiconductors and are now studied mainly by ARUPS, KRIPES, STM, SDR, and reflectance anisotropy spectroscopy (RAS). 

Surface anisotropy has given rise to the technique of reflectance anisotropy spectroscopy (RAS), in which linearly polarized light is modulated between two principal directions (of the surface tensor) and the difference... 

Whereas SE measures the ratio of reflection coefficients for different polarizations, various reflection difference techniques probe relative differences in reflectivity. Among these techniques one distinguishes surface differential reflectivity (SDR), surface photoabsorption (SPA) and reflection anisotropy spectroscopy (RAS). 

Reflection anisotropy spectroscopy (RAS) probes the electronic structure of surfaces and interfaces using visible and near-ultraviolet photons. From its origins in the 1980s as an in situ real-time monitor of semiconductor growth processes, RAS has evolved into a technique that has been applied to surfaces in UHV, surfaces under high pressure of ambient gas and solid/liquid interfaces in the field of electrochemistry, together with more spedaUst applications such as liquid crystal devices. (Note that the technique was also known as reflection difference spectroscopy (RDS) in the early years.) Most optical probes are not surface sensitive... 

KRIPES K-resolved inverse photoelectron RAS reflectance anisotropy spectroscopy... 

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