Reflection anisotropy spectroscopy

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 

RAS measures the difference in reflectance (t) of normal-incidence plane-polarized light between two orthogonal directions in the surface plane x, y) normalized to the mean reflectance (t) 

The interpretation of RAS spectra from single-crystal surfaces is not as straightforward as it is for, for example, XPS. This is because the response of the surface depends on the complex dielectric function (a quantity that is difficult to calculate from first principles even for well-characterized materials) for both the bulk of the sample and the surface region. Also, in common with other techniques that are sensitive to surface electronic structure, the existence of intrinsic surface states and surface-modified bulk states compHcates matters. However, absence of a firm theoretical framework for predicting RAS spectra has not necessarily impeded the application of RAS in various fields. An empirical approach, often supported by other techniques that provide information on the electronic transitions responsible for RAS spectral features, can allow surface changes to be studied even without a complete understanding of the RAS spectra. 

The anisotropy in reflectance of a surface is a result of anisotropy in the surface electronic structure (and hence a difference in the response of a surface to Hght polarized in two different directions). The anisotropy in the surface electronic structure may be the result of (i) a surface having an anisotropic geometry or (ii) intrinsically isotropic material being arranged into structures that are aligned in a preferential direction on an isotropic surface. To date, the appHcations of RAS have concentrated on the first category. The RAS response from clean 

Although RAS is sensitive to the structure of surfaces at atomic (nanometers) or morphological (micrometers) spatial scales, it should be remembered that RAS spectrometers use light beams whose spot size on the sample is typically approximately millimeters, and so a RAS signal will be observed only if the surface anisotropy extends over these macroscopic dimensions. A good example of this point can be seen with the Si(OOl) surface. 

Fundamentals. When the reflectivity at normal or near normal incidence is measured relative to various crystallographic directions, anisotropies may be observed. 

This method is sometimes called reflectance difference spectroscopy (RDS) and, because of considerable overlap, this method is sometimes also considered to be a variation of electroreflectance spectroscopy (see p. 50 for further details). 

Instrumentation. An RDS setup (as commercially supplied [111, 114]) is equivalent to a normal incidence ellipsometer. It directly delivers the real and the imaginary parts (or their ratio) of and rooi- These are the complex reflectances along the respective crystallographic directions. 

Various possible optical configurations have been compared elsewhere [115]. In order to record reflectance-difference spectra beyond a light source (e.g. a Xe 

RA spectra obtained with a Au(llO) surface exposed to an aqueous solution of 0.1 M Na2S04 [118] are displayed in Fig. 5.26. 

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Frederick B G, Power J R, Cole R J, Perry C C, Chen Q, Flaq S, Bertrams T, Richardson N V and Weightman P 1998 Adsorbate azimuthal orientation from reflectance anisotropy spectroscopy P/rys. Rev. Lett. 80 4490-3... 

Adsorption of pyridine on Au(llO) electrodes was also studied using reflection anisotropy spectroscopy [255]. Reflection anisotropy of pyridine/Au(110) system has been attributed to n-jT transitions, the band of which was shifted compared to their spectral position in the gas phase, due to the interaction of the lone electron pair orbitals at N atom with the gold surface. 

Spectroscopies such as UV-visible absorption and phosphorescence and fluorescence detection are routinely used to probe electronic transitions in bulk materials, but they are seldom used to look at the properties of surfaces [72]. As with other optical techniques, one of the main problems here is the lack of surface discrimination, a problem that has sometime been b q)assed by either using thin films of the materials of interest [73, 74], or by using a reflection detection scheme. Modulation of a parameter, such as electric or magnetic fields, stress, or temperature, which affects the optical properties of the sample and detection of the AC component of the signal induced by such periodic changes, can also be used to achieve good surface sensitivity [75]. This latter approach is the basis for techniques such as surface reflectance spectroscopy, reflectance difference spectroscopy/reflectance anisotropy spectroscopy, surface photoadsorption... 

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... 

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

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 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... 

Frederick, B., Cole, R., Power, J., Perry, C., Chen, Q., Richardson, N., Weightman, R, Verdozzi, C., Jennison, D., Schultz, R, and Sears, M. (1998). Molecular orientation with visible light Reflectance-anisotropy spectroscopy of 3-thiophene carboxylate on Cu(llO) surfaces. Phys. Rev. B, 58 10883 -10889. 

Weightman, P. (2001). The potential of reflection anisotropy spectroscopy as a probe of molecular assembly on metal surfaces, phys. stat. sol. (a), 188 1443 -1453. 

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