Normal-incidence reflection measurements

Measured normal incidence reflectances of a-SiC for incident electric field perpendicular to the hexagonal axis are shown in Fig. 9.6 these are unpublished measurements made in the authors laboratory, but they are similar to those pubhshed by Spitzer et al. (1959). Also included in this figure are both sets of optical constants—n, k and e, e"—calculated from the best fit of a one-oscillator model to the experimental data. Note that the model curve is almost a perfect representation of the data over the entire range shown for this solid, the technique of fitting data with a one-oscillator model is both a simple and accurate method for extracting optical constants. 

Fig. 9.2. Normal incidence reflectivity for various XUV/SXR mirrors. All of the mirrors, except the SiC mirror, are multilayer-coated. The solid lines show the theoretical values and the plots are measured data...

Usually, the absorption coefficient of the conducting crystals is so high that producing a crystal sufficiently thin and suitable for absorption measurements presents a great difficulty. If this is so, the bulk optical constants of a solid may be computed from the normal-incidence reflectivity of that material over an extended range of frequencies, followed by a Kramers-Kronig analysis of the measurements [12,14]. In this method the real, n, and imaginary, k, parts of the complex index of refraction... 

Figure 8.6. Measured normal incidence reflectivity of two-layer Jaumann RAM using PPy treated cotton cloth composite layers. Reprinted with permission from ref 84.

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

Hou, H. Q., W. G. Breiland, B. E. Hammons, and H. C. Chui. 1996. In situ growth rate measurements by normal-incidence reflectance during MOCVD growth. Electrochemical Society Proceedings 96(2) 27-35. 

Specular reflection IR spectroscopy has been used by Cole and coworkers to study the orientation and structure in PET films [36,37]. It has allowed characterizing directly very highly absorbing bands in thick samples, in particular the carbonyl band that can show saturation in transmission spectra for thickness as low as 2 pm. The orientation of different conformers could be determined independently. Specular reflection is normally limited to uniaxial samples because the near-normal incident light does not allow measuring Ay. However, it was shown that the orientation parameter along the ND can be indirectly determined for PET by using the ratio of specifically selected bands [38]. This approach was applied to the study of biaxially oriented PET bottles [39]. 

Reflectivity spectra can be registered in two different modes (i) direct reflectivity or (ii) dijfuse reflectivity. Direct reflectivity measurements are made with well-polished samples at normal incidence. Diffuse reflectivity is generally used for unpolished or powdered samples. Figure 1.7 shows the experimental arrangements for measuring both types of spectra. 

Measurement of the transmittance and reflectance of a slab for light at near-normal incidence. The samples must be sufficiently transparent for measurable transmission in thin slabs, but not as transparent as required in method 1. 

This technique involves a reflectivity measurement, therefore, the Kramers-Kroenig transform is required to convert the resulting reflectance spectrum to a pseud-absorbance spectrum. This is a simple procedure for normal incidence measurements. Optically thick samples are desirable. Additional interfaces, such as air pockets, in the sample can induce reflection absorption bands - resulting in a spectrum of convolved reflection and reflection absorption features. In some cases, optically smooth protein and starch films can be prepared (without cryogenic slicing),... 

Fig. 7.29. Top Reflectivity at normal incidence of two PLD grown Bragg mirrors with 5.5 and 10.5 YSZ-MgO layer pairs obtained from the ellipsometry model analysis. By doubling the layer number, the reflectivity was increased from 90 to 99%. The UV-vis ellipsometry data were fitted best with layer thicknesses of 38-46 nm YSZ/48-54nm MgO for the 5.5 layer pair Bragg, and 46.4 0.7 nm YSZ and 51.9 0.5 nm MgO for the 10.5 pair Bragg. Measured and calculated by R. Schmidt-Grand. Bottom SNMS isotope intensity depth profile of this 5.5 x YSZ/MgO Bragg structure...

The reflectivity is measured from a flat surface for normal incidence of the incoming light. Electromagnetic theory shows that the reflectivity R = IKa/ hn is given in terms of the real and imaginary parts n and k, respectively, of the complex index of refraction (cf. Bohren Huffman 1983),... 

Light (or near-ir and uv radiation) that is incident on opaque minerals is partly absorbed and partly reflected by them. There are two kinds of reflection processes that occurring when light is reflected from a flat polished surface of the mineral (specular reflectance) and that occurring when the light is reflected from the mineral after it has been finely powdered (diffuse reflectance). The latter arises from radiation that has penetrated the crystals (as in an electronic absorption spectrum) and reappeared at the surface after multiple scatterings in this case there will also be a specular component to the reflectance from light that is reflected from the surfaces of the particles. The specular reflectance of a flat polished surface of an opaque mineral measured at normal incidence can be related to the n and k terms of the complex refractive index (N) in which ... 

Light (uv-visible-near-ir radiation) reflected back from a flat, highly polished surface of a solid measured in intensity as a function of varying wavelength. Measurements can be made using plane-polarized light, at normal incidence, and on oriented faces of single crystals... 

Fig. 6.23 Normal incidence polarized reflection spectra of the two forms molecule 6-XVII (R = Et, R = OH). For each crystal modiflcation there are two spectra measured with the light polarized along each of the two directions (the so-called principal directions), as indicated in the upper right hand corner, which also shows the projection of the molecule(s) onto the crystal face studied, (a) Triclinic polymorph, (100) face (b) monoclinic polymorph, (100) face. (From Tristani-Kendra and Eckhardt 1984, with permission.)...

Besides ellipsometry, reflectometry has proven its value. By this technique adsorbed masses can conveniently be obtained and. if the measurements are carried out with polarized light, also the orientation of the adsorbed molecules. Experiments are usually done at near-normal Incidence, when // Another variant, pertaining to adsorption from solution and sketched in fig. 2.15, can be made fast enough for the kinetics of adsorption to be followed. In the mode shown, fluid is admitted to the surface from bottom to top ("impinging jet") equations are available for the rate of supply in the stagnation point (the "core" of the fluid flow, which hits the surface perpendicularly). The quotient of the reflected Intensities = S is obtained by electronic division, it is... 

---