Reflectivity, of metals

Hohifeld J, Conrad U, Muller Wellershoff S S and Matthias E 1998 Femtosecond time-resolved linear and second-order reflectivity of metals Nonlinear Optics in Metals ed K H Bennemann (Oxford Clarendon) pp 219-67... 

Perhaps the most obvious metallic property is reflectivity or luster. With few exceptions (gold, copper, bismuth, manganese) all metals have a silvery white color which results from reflecting all frequencies of light. We have said previously that the electron configuration of a substance determines the way in which it interacts with light. Apparently the characteristic reflectivity of metals indicates that all metals have a special type of electron configuration in common. 

Figure 6.20 shows an example in which QEXAFS has been used in combination with XRD to study the temperature programmed reduction of copper oxide in a Cu/ZnO/Al203 catalyst for the synthesis of methanol [43,44]. Reduction to copper metal takes place in a narrow temperature window of 430-440 K, and is clearly revealed by both the EXAFS pattern and the appearance of the (111) reflection of metallic copper in the XRD spectra. Note that the QEXAFS detects the metallic copper at a slightly lower temperature than the XRD does, indicating that the first copper metal particles that form are too small to be detected by XRD, which requires a certain extent of long range order [43,44],... 

Polycrystalline oxide materials, both undoped and doped, have been extensively examined for use as photoanodes. Ti02 electrodes have been prepared by thermal oxidation of a Ti plate in an electric furnace in air at 300-800°C (15-60 min) and in a flame at 1300°C (20 min) [27-30]. XRD analysis of thermally oxidized samples indicates the formation of metallic sub-oxide interstitial compounds, i.e. TiOo+x (x < 0.33) or Ti20i y (0 < y < 0.33) and Ti30 together with rutile Ti02 [27]. The characteristic reflection of metallic titanium decreases in intensity after prolonged oxidation (60 min) at 800° C indicating the presence of a fairly thick oxide layer (10-15 pm). Oxidation at 900°C leads to poor adhesion of the oxide film... 

Electrons in metals and semiconductors give rise to free-carrier absorption, the absorption coefficient being proportional to the square of the incident wavelength (hence high in the infrared region for most metals). The reflectivity of metals is related to the plasma frequency, cOp, by the relation... 

Sample (K) at 1253 K contained Mo metal in flowing H2. The Mo metal in the sample had predominant (110) and (211) reflections. Haddix et al.24 reported that the (100) reflection of metallic Mo was never directly observed in their study. Moreover, the high-temperature peak appeared to be due to the desorption of N2 adsorbed on Mo (110) as reported by Bafrali and Bell25 and Mahnig and Schmidt,26 who found desorption temperatures of about 1350 K25 and 1460 K,26 respectively. 

Figure 14.4 Metal binding on inorganic particles showing typical adsorption edges of selected anions and cations on Fe oxide particles reflective of metal-like and ligandlike complexes. (Modified from Santschi et al., 1997.)...

There is no clear definition of what magnitude of enhancement entitles a system to be classified as a SERS-active system. In this review we will arbitrarily set the demarcation line for SERS at a 100-fold enhancement level. Any enhancement higher than that will be considered as SERS, while lower enhancements will be ignored. The reason for this limit is that it is within simple surface coverage effects (roughness factor) and trivial enhancements resulting from reflectivity of metal surfaces and possible orientation effects. " ... 

The high reflectivity of metals is also due to the free electrons. When light photons strike the metal surface, those electrons near to the Fermi surface can absorb the photons, as plenty of empty energy states lie nearby. However, the electrons can just as easily fall back to the lower levels originally occupied, and the photons are re-emitted. A detailed explanation of reflectivity of a metal requires knowledge of the exact shape of the Fermi surface and the number of energy levels (density of states) at the Fermi surface. 

The high electrical conductivity and reflectivity of metals is attributed to ... 

These expressions can be used to derive the nearly total reflectance of metals below their plasma frequency. A similar characteristic frequency dependence of a(o)) and e((o) may be seen in semiconductors where oip depends the electron density in the fliled valence band. The conduction electrons can oscillate as a collective mode (plasma oscillation). A plasmon is a quantized plasma oscillation. The frequency and wavevector dependence of plasmons in one-dimensional metals have been predicted (458, 576) to be qualitatively different from those of three-dimensional metals. Recent direct measurements (552) of plasmons in the one-dimensional organic metal tetrathiofulvalinium-tetracyano-quinodimethanide (TTF)(TCNQ) are qualitatively consistent with some of the predictions assuming a tight-binding band (576). 

Figure 13-2. Changes during the time in reflections of metal ion>exchanged material ( Starting material a-ZP-bipy, 1.5H O batch conditions ...

On the other hand, it is easy to prepare metals in the form of evaporated thin films on a large variety of substrates. Their geometries are readily dictated by the evaporation masks employed. By making the evaporated films very thin, heat capacities can be made to be very low, and the speed of response can be high. Because of the high reflectivity of metals it is necessary to overcoat the films with an evaporated film of a material such as platinum or gold black which will absorb the incident radiation. Like thermistor bolometers, most metal film bolometers operate at room temperature. See Dewaard and Wormser [2.79] for additional details. 

Materials fall into three classes of microwave-material interaction reflectors (bulk metals), transmitters (quartz), and absorbers (water). The reflectivity of metals presents both drawbacks and benefits to processing them in a microwave. On the one hand, a bulk reaction is hard to achieve because microwave energy is generally absorbed only in a thin shell of a metal. This region is called the skin depth, which Equation 5.2 defines in terms of the physical properties of the material and electric fields... 

Potential-modulated UV-vis reflectance spectroscopy, often referred to as electroreflectance (ER), was originally developed in solid-state physics to characterize surfaces and was applied to studies of the electronic band structure of semiconductors. The ER technique has also been used to characterize metal electrode surfaces in the absence and presence of adsorbates. The reflectivity of metal electrodes is a function of the surface charge density of the electrodes. ER technique has also been used to investigate electrode reactions of organic species adsorbed on the electrode surfaces. Several review articles on ER are available [21-24]. 

In the infrared region, several materials (for example, CaF2, NaCl, and KBr crystals) are transparent up to 30 jim (Fig. 4.6). However, because of the high reflectivity of metallic coated mirrors and gratings in the infrared region, grating spectrometers with mirrors are preferred over prism spectrographs. 

Figure 1. X-ray difiraction patterns of copper chromite 1. parent sample 2. reduced in H2 at 320°C 3. sample 2 calcin at 320 C for 10 h. The reflections of metal Cu (o), silicon (+) are given in Figure 1. Positions of the reflections of a cubic type CUC12O4 spinel are marked on the top.

In addition to size, shape, and distribution and etchability of the phases, light reflectivity is a criterion for distinguishing and identifying the phases in a ceramic material. The reflectivity of ceramics is considerably lower than the reflectivity of metals. As an aid to microstructural examination, Figs. 53 and 54 plot the reflectivity... 

Fig. 148. Schematic frequency dependence of the reflectivity of metals experimentally (solid line) and according to three models (dotted line).

In this section, the reflectance of metallic PANI-CSA is compared with that of the semiconducting polyaniline (emeraldine base) and the conventional emeraldine salt, PANI protonated with H2SO4 (PANI-H2SO4), which is classified as a typical insulating-regime material in a doped polyaniline system [1161]. In particular, in order to better understand the role of disorder in the conducting emeraldine salt, the discussion focuses on a comparison of the data obtained from PANI-H2SO4 with those obtained from PANI-CSA [1161]. 

Electrons in conduction bands can also absorb thermal energy the ready transport of energy by these electrons accounts for the high thermal conductivity of metals. The absorption and reemission of photons of visible light by conduction electrons accounts for the high reflectivity of metals. Metals are ductile and malleable because under mechanical stress the positive ions of the crystal can move past each other with very little resistance and without breaking any metallic bonds. 

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