Specific color absorbance

A second kind of electronic defect involves the electron. Let us suppose that the second plane of the cubic lattice has a vacancy instead of a substitutional impurity of differing valency. This makes it possible for the lattice to capture and localize an extraneous electron at the vacancy site. This is shown in the following diagram. The captured electron then endows the solid structure with special optical properties since it ean absorb photon energy. The strueture thus becomes optically active. That is, it absorbs light within a well-defined band and is called a "color-center" since it imparts a specific color to the crystal. 

Most materials exhibit specific colors because they absorb certain wavelengths (colors) from white, ordinary light. A red object exposed to white light, for example, appears red because atoms on its surface absorb all the other colors in the beam and reflect only red. If transparent materials contain coloring materials, such as dyes or pigments, they absorb the characteristic color of the coloring material. 

More common methods for elemental analysis - to determine the elemental contents of a sample - include spectroscopy and spectrometry. Spectroscopy measures changes in atoms that cause a specific light photon to be either absorbed (absorption spectroscopy) or emitted (emission spectroscopy). This light has a precise wavelength or energy, characteristic of a specific element in the periodic table. The simplest (and oldest) form of elemental analysis was not spectroscopic, in fact, but colorimetric. This method was based on the reaction of a strongly colored chemical in a solution. The appearance of a specific color in the solution revealed the identity of the element of interest. If the color intensity is proportional to the amount of that element present, the method can also be used to estimate the amount of the element present. 

As we will see in more detail in the next section, the colors of these flames result from atoms in these solutions releasing energy by emitting visible light of specific wavelengths (that is, specific colors). The heat from the flame causes the atoms to absorb energy we say that the atoms become excited. 

Substances that are intrinsically fluorescent can often be exeited with long-wavelength UV light. They absorb the radiation and then emit, usually in the visible region of the spectrum, so that they appear as bright luminous zones, whieh can frequently be differentiated by color. They, thus, set themselves apart from the multitude of substances that only exhibit absorption. This detection possibility is characterized by high specificity (Sec. 2.3). 

Paints are complex formulations of polymeric binders with additives including anti-corrosion pigments, colors, plasticizers, ultraviolet absorbers, flame-retardant chemicals, etc. Almost all binders are organic materials such as resins based on epoxy, polyurethanes, alkyds, esters, chlorinated rubber and acrylics. The common inorganic binder is the silicate used in inorganic zinc silicate primer for steel. Specific formulations are available for application to aluminum and for galvanized steel substrates. 

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