Reflectance specular

Specular-reflectance accessories are another class of devices that are built for a specific type of spectrophotometer. In such an accessory, a transfer optical system is used to impinge the radiation on the sample and refocus the reflected radiation on the slit. In current practice the radiation is also focused on the sample, as opposed to the previous practice of spreading it out over a large area. 

Micro- as well as macro- reflectance accessories are available. Macroaccessories can be used for micro- (small-spot) work by placing a mask over the sample to reduce the irradiated area. The finish on the 

By moving the sample across the opening in the mask or over the illuminated area in a micro-accessory, it is possible to obtain a reflection profile of the sample. If much of this work is to be done, a platform with micrometer screws for positioning the sample can be substituted for the sample holder. This technique is especially useful in the semiconductor industry, where obtaining an epitaxial layer-thickness profile may be required. 

These accessories use mirrors as transfer optics they may also use condensing lenses for illuminated-area reduction. Except for those using condensing lenses, such accessories can be used over the entire range of the spectrophotometer. If the aluminum coating on the mirrors is of sufficient quality, they may also be used on ultraviolet-and visible-range spectrophotometers. 

Normally accessories of this type are used in pairs one unit in the sample beam and the other in the reference beam. When they are used in pairs, energy losses in the two beams are equalized. This also allows the operator to compare a sample to a reference without making two runs. 

FIGURE 4.37 An example of reflectance-absorbance, where a light beam passes through a sample, reflects from a snbstrate, passes through the sample a second time, and then is focused onto the infrared detector. 

FIGURE 4.38 Left A picture of a specular reflectance accessory. Right Its optical diagram. 

FIGURE 4.39 The reflection-absorbance spectrum of the paint on the outside of a full soda can. 

In external reflectance the incident radiation is focused on to the sample, and two forms of reflectance can occur, namely specular and diffuse. External reflectance measures the radiation reflected from a surface. The material must therefore be reflective, or be attached to a reflective backing. A particularly useful application of this technique is the study of surfaces. 

For most materials the reflected energy is only 5-10%, but in regions of strong absorptions the reflected intensity is greater. The data obtained appear different from normal tra(nsmission spectra, as derivative-iike bands result from the superposition of the normal extinction coefficient spectrum with the refractive index dispersion (based upon the Fresnel relationships from physics). However, the reflectance spectrum can be corrected by using the Kramers-Kronig (K-K) transformation. The corrected spectrum appears similar to the familiar transmission spectrum. 

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. The use of a polarizer in conjunction with grazing angle analysis can provide information about the orientation of molecules adsorbed onto surfaces. 

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The use of Lamb waves offers the possibility of rapid long-range in-service inspection. Receiver and transmitter probes are positioned single sided - access is only required from one side of the specimen - in a pitch-catch-arrangement, the receiver being outside tbe field of the specular reflection. 

Adsorption nil specular reflection accommodation coefficient zero... 

Figure A3.9.4. The ratio of specular reflectivity to incident beam intensity ratio for D2 molecules scattering from a Cii(lOO) surface at 30 K [21],...

Figure Bl.6.10 Energy-loss spectrum of 3.5 eV electrons specularly reflected from benzene absorbed on the rheniiun(l 11) surface [H]. Excitation of C-H vibrational modes appears at 100, 140 and 372 meV. Only modes with a changing electric dipole perpendicular to the surface are allowed for excitation in specular reflection. The great intensity of the out-of-plane C-H bending mode at 100 meV confimis that the plane of the molecule is parallel to the metal surface. Transitions at 43, 68 and 176 meV are associated with Rli-C and C-C vibrations.

One of the spots in such a diffraction pattern represents the specularly reflected beam, usually labelled (00). Each other spot corresponds to another reciprocal-lattice vector = ha + kb and is thus labelled (hk), witli integer h and k. 

Adzic R R, Yeager E and Cahan B D 1977 Specular reflectance studies of bromine adsorption on gold J. Electroanal. Chem. 85 267-76... 

If, instead of assuming diffuse reflection at the wall, it is postulated that a fraction f of the incident molecules is scattered diffusely and the rest suffer specular reflection, the right hand side of equation (2.8) must be multiplied by a factor (2 - f)/f. ... 

When there is no specular reflectance, the third term in the denominator drops out, in agreement with Eqs. (5-134) and (5-135). When the reflectance is exclusively specular, the denominator becomes 1/Ai i -I- p g/Aifl — P59), easily derivable from first principles. 

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. 

Infrared spectroscopy, including Fourier-transform infrared (FTIR) spectroscopy, is one of the oldest techniques used for surface analysis. ATR has been used for many years to probe the surface composition of polymers that have been surface-modified by an etching process or by deposition of a film. RAIR has been widely used to characterize thin films on the surfaces of specular reflecting substrates. FTIR has numerous characteristics that make it an appropriate technique for... 

A powerful characteristic of RAIR spectroscopy is that the technique can be used to determine the orientation of surface species. The reason for this is as follows. When parallel polarized infrared radiation is specularly reflected off of a substrate at a large angle of incidence, the incident and reflected waves combine to form a standing wave that has its electric field vector (E) perpendicular to the substrate surface. Since the intensity of an infrared absorption band is proportional to / ( M), where M is the transition moment , it can be seen that the intensity of a band is maximum when E and M are parallel (i.e., both perpendicular to the surface). / is a minimum when M is parallel to the surface (as stated above, E is always perpendicular to the surface in RAIR spectroscopy). 

In the process of MBE, the surface structure can be investigated by reflected high energy electron diffraction (RHEED). During MBE growth, one often observes an oscillation in the intensity of the specular reflected beam as a function of time. This is interpreted to be due to the layer-by-layer growth of a two-dimensional island. 

Electropolishing which exploits a generally similar type of solution, but introduces anodic currents as an additional means of dissolution thereby providing better control of rapid processing. Electrosmoothing and electrobrightening are terms used to describe inferior finishes which may have lustre but have lower specular reflectivity. 

Chemical polishing, yielding a surface of high specular reflectivity, exploits fully optimised bright dip solutions often achieved by the further addition of phosphoric acid at the expense of the residual water. Because phosphoric acid is relatively viscous at lower temperatures (e.g. less than 40°C) it can act as diffusion layer promoter (C), but its presence increases the chemical costs considerably. 

Reflectivity The total and specular reflectivities of an anodised aluminium surface are controlled by both the condition of the metal surface, polished... 

The general brightness of a surface is chiefly dependent upon the total reflectivity T, while specular reflectivity S controls the character of the reflected image. In assessing the subjective brightness of a surface the eye tends to be influenced more by the S/Tratio or image clarity than by the total reflectivity. 

Bright Plating electroplating under conditions whereby the electrodeposit has a high degree of specular reflectivity. 

Brightener an addition agent used specifically to produce an electrodeposit of high specular reflectivity. 

Electropolishing surface finishing of a metal by making it the anode in an appropriate solution, whereby a bright and level surface showing specular reflectivity is obtained. 

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