Specular reflection, complex

A majority of traditional NIR measurements are made on solid materials and these involve reflectance measurements, notably via diffuse reflectance. Likewise, in the mid-IR not all spectral measurements involve the transmission of radiation. Such measurements include internal reflectance (also known as attenuated total reflectance, ATR), external reflectance (front surface, mirror -style, or specular reflectance), bulk diffuse reflectance (less common in the mid-IR compared to NIR), and photoacoustic determinations. In these cases more complex expressions may be required to describe the measured spectrum in terms of IR absorption. In the case of photoacoustic detection, which has been applied to trace-level gas measurements, the spectrum produced is a direct measurement of IR absorption. While most IR spectra are either directly or indirectly... 

The Pt + Pt intervalence transitions of such chain complexes occur in the regions 25,000-18,200 cm 1, 23,600-14,300 cm 1 and 20,600-7,500 cm 1 for chloro-, bromo-, and iodo-bridged complexes, respectively, the trend Cl > Br > I being the reverse of that of the conductivity of the complexes. The transition wavenumbers may be determined either by Kramers-Kronig analysis of specular reflectance measurements or from plots of the excitation profiles of Raman bands enhanced at or near resonance with the Pt I-PtIV intervalence band. The maxima have been found to be related to the Pt —PtIV chain distance, the smaller the latter the less being the intervalence transition energy (3). 

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 ... 

For mesitylene and durene, the kinetics have been followed by specular reflectance spectroscopy [17]. The results indicated that mesitylene produces a fairly stable radical cation that dimerizes. That of durene, however, is less stable and loses a proton to form a benzyl radical, which subsequently leads to a diphenylmethane. The stability of the radical cation increases with increasing charge delocalization, blocking of reactive sites, and stabilization by specific functional groups (phenyl, alkoxy, and amino) [18]. The complex reaction mechanisms of radical cations and methods of their investigation have been reviewed in detail [19a]. Fast-scan cyclovoltammetry gave kinetic evidence for the reversible dimerization of the radical cations of thianthrene and the tetramethoxy derivative of it. Rate constants and enthalpy values are reported for this dimerization [19b]. 

In the presence of a thin film, as shown in Fig. 3, the situation becomes more complex. For the case of specular reflection, with the electric field polarised in the x direction (i.e. perpendicular to the plane of reflection), the continuity conditions at the (1, 2) interface demand that... 

Complex Geometries with Specular Reflection. When processing metal parts, for example, specular reflection can be significant, and the directional dependence of the surface properties may impact the temperature of the material. Using a Monte Carlo approach, the effect of specular reflection has been considered for simple geometries, and specular behavior generally impacts the thermal evolution when open geometries are considered (e.g., Ref. 176). 

With newer computational tools such as finite element and boundary element methods, conduction and convection in arbitrary three-dimensional geometries can be handled in a relatively straightforward manner. Some recent efforts have been made to link the FEM method (for ease in handling complex geometries) with alternative methods to evaluate view factors in order to predict thermal processing of complex shapes with specular surface radiative properties or specular reflection [177]. 

Diffuse reflection is a complex process that occurs when a beam of radiation strikes the surface of a finely divided powder. W ilh this type of sample, specular reflection occurs at each plane surface. However, because there arc many of these surfaces and they are landomlv oriented, radiation is reflected in all direc-... 

The polarized single crystal specular reflectance and solution absorption spectra have been reported for 20 (M = Rh, Ir R = H) (44, 474). A high reflectivity at 19100 cm (e — 14000 1/mole cm) and 17600 cm (s 15000 1/mole cm) were reported for the rhodium and iridium complexes respectively. These reflectances account for the metallic luster of these complexes and are assigned to a -> L r transition. The final state is characterized as a Frenkel exciton consistent with the poorly conducting character of these complexes. 

If the complex refractive index of this flat surface is given by k = n-ik, and the light propagates in air (n l), the normal specular reflectivity is given by... 

Diaz, G., R.E. Clavijo, M.M. Campos-VaUette, M. Saavedra, S. Diez, L. Lopez, and R. Munoz (1997). Specular reflectance infrared spectra of the macrocycles cyclam and cyclamdione and their Cu(II) complexes deposited onto a smooth copper surface. Vib. Spectrosc. 15, 201. 

Specular reflection is the term used to describe mirror-like reflection, from the surface of a sample (angle of reflection equals angle of incidence). Specular reflected radiation ostensibly carries no information about the IR absorption of a sample and is a source of interference in diffuse reflection experiments when the sample is not completely matte, i.e., has an element of shininess about it. However, if the reflected intensity from a sample is due principally to reflection from the front surface of the sample, then an absorption index spectrum of the sample can be generated from the reflected intensity over the whole spectrum using the Kramers-Kronig transformation. (This complex transformation is an... 

In particular, the zeroth, order corresponds to specular reflection found in ordinary mirrors. For m sufficiently large the above relation can be satisfied only by a complex angle 6m = iB m sgn(m) (nIT). This means that the grating produces evanescent waves. 

The optical absorption coefficient a, calculated from the complex refractive index or dielectric constant of the film on Pt, exhibits two peaks at 3 00 and 365 mm, as pointed out by McIntyre and Kolb on the basis of their specular reflectance spectroscopic measurements over the wavelength range... 

The reflectivity is defined as the ratio of the intensities of the reflected and incident beams and should be differentiated from the reflectance which is the ratio of the amplitudes of the incident and reflected waves. The reflectance in general is a complex number because there is usually a change in phase of a wave on reflection whereas reflectivity is a real number varying from zero to unity. The specular reflection can provide information on the composition distribution normal to the surface. The reflectivity is a function of both the angle of incidence of the beam to the surface and the refractive index changes of the substrate. The reflectivity is a function of the length scale of interactions of... 

---