Grazing angle specular reflectance

Though a powerfiil technique, Neutron Reflectivity has a number of drawbacks. Two are experimental the necessity to go to a neutron source and, because of the extreme grazing angles, a requirement that the sample be optically flat over at least a 5-cm diameter. Two drawbacks are concerned with data interpretation the reflec-tivity-versus-angle data does not directly give a a depth profile this must be obtained by calculation for an assumed model where layer thickness and interface width are parameters (cf., XRF and VASE determination of film thicknesses. Chapters 6 and 7). The second problem is that roughness at an interface produces the same effect on specular reflection as true interdiffiision. 

Reflection-Absorption IR spectroscopy (RAIRS) where the linearly polarized IR beam is specularly reflected from the front face of a highly reflective sample, such as a metal single crystal surface (Figure 3.1(a)). This is also sometimes referred to as IRAS (IR reflection absorption). The IR beam comes in at grazing angle (i.e. almost parallel to the surface), and although absorption bands in RAIRS have intensities that are some two orders of magnitude weaker than in transmission studies on... 

Figure 8. (a) FTIR spectrum of 1 in CC14, (b) C-H stretching region of the grazing angle external specular reflection IR spectrum of a monolayer of 1 on an aluminized silicon wafer, and (c) as in (b) for a monolayer of OTS on an aluminized silicon wafer. Spectra are recorded at 76° incidence, 1000 scans, 2 cm resolution, and the baselines have been adjusted to zero absorbance. 

The technique that is commonly used nowadays for the investigation of the structure of adsorption layers is the specular neutron reflection. Neutron reflection studies are based on the measurement of the intensity of a reflected beam, generated from a collimated beam of neutrons with the wavelength X falling on the air-solution interface at a grazing angle 0. The quantity that is measured is referred to as the reflectance, R, which is given by the ratio of the reflected intensity to the intensity of the incident beam [17]. From reflectance measurements one can estimate the adsorption and obtain information on the orientation of molecules in the adsorption layer. 

The thin films or coatings can be studied nondestructively, with no sample preparation other than deposition on a polished metal surface if necessary. Specular reflectance has been used to study lubricant films on computer disks, oxide layers on metal surfaces, paint curing as a function of time, and molecules adsorbed on surfaces. For example, the IR absorption spectrum of proteins adsorbed onto a polished gold surface can be studied. This spectrum from an adsorbed layer can form the basis of sensors for compounds that will bind to the proteins and change the spectrum. Use of a polarizer in conjunction with grazing angle analysis can provide information about the orientation of molecules adsorbed onto surfaces. 

FTIR spectroscopy can be used in diffuse reflectance, specular reflectance, and external reflectance in grazing angle reflectance mode [23]. It enables the analysis of functional group and specific bonds of surface species. Particles can be measured separately (diffuse reflectance mode) after being scraped from composite electrodes [24]. 

It is recommended to prepare electrodes comprising only A1 foil and the active mass— particles embedded by light pressnre into soft A1 foil electrodes. Such electrodes can be polarized, cycled, and then measnred by FTIR spectroscopy (reflectance modes such as grazing angle reflectance or specular reflectance modes in which there are commercial accessories available), Raman, and XPS. 

Figure 6. Specular reflection FT-IR spectra of ultra-thin polymer films obtained at grazing angle using parallel polarized infrared radiation (26).

Specular reflectance (Fresnel) Diffuse reflectance (DRIFTS) Reflection-absorption (RA) Grazing angle... 

Infrared Blacks Aluminum can be anodized, but the result in the IR is not as black as the visible appearance suggests, and the reflectance at grazing angles is very high. One method of obtaining black anodized aluminum, for example, yields a specular reflectance of nearly 80% at 2 pm, while other treatments for aluminum cut this to 10%. Black chrome is described by Driver et al. (1977). 

Figure 1 Vector diagram of specular reflection in reciprocal space with vectors outside the sample, a, and a/are glancing angles of incidence onto and scattering from the sample. / Is the incident, while kf is the outgoing wave vector / ,- /r, sina, and p/= /f/Sina/are their projections onto the normal to the surface. Oz=g/+P/is the momentum transfer petpendicular to the surface. The vector Qd represents surface diffraction in grazing-incidence geometry. The footprint of the sample surface Sample is marked with S, ( ,j, used in the discussion of eqn [1 ] in Box 1.

In Total Reflection X-Ray Fluorescence Analysis (TXRF), the sutface of a solid specimen is exposed to an X-ray beam in grazing geometry. The angle of incidence is kept below the critical angle for total reflection, which is determined by the electron density in the specimen surface layer, and is on the order of mrad. With total reflection, only a few nm of the surface layer are penetrated by the X rays, and the surface is excited to emit characteristic X-ray fluorescence radiation. The energy spectrum recorded by the detector contains quantitative information about the elemental composition and, especially, the trace impurity content of the surface, e.g., semiconductor wafers. TXRF requires a specular surface of the specimen with regard to the primary X-ray light. 

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