Optical range

On the other hand, fluorine s high electronegativity and its ability to form mostly ionic chemical bonds, provide materials with several useful properties. First, compared to oxides, fluoride compounds have a wide forbidden zone and as a result, have low electroconductivity. In addition, fluorides are characterized by a high transparency in a wide optical range that allows for their application in the manufacturing of electro-optical devices that operate in the UV region [42,43]. 

Starfire Optical Range. Rayleigh LGSs with AO at the SOR... 

FIGURE 26. EA spectra of the radical cation of naphthalene in different media. Note the increase in resolution and optical range in argon matrices... 

Figure 1.1 The electiomagnetic spectrum, showing the different microscopic excitation sources and the spectroscopies related to the different spectral regions. XRF, X-Ray Fluorescence AEFS, Absorption Edge Fine Structure EXAFS, Extended X-ray Absorption Fine Structure NMR, Nuclear Magnetic Resonance EPR, Electron Paramagnetic Resonance. The shaded region indicates the optical range.

The short (UV) wavelength limit of the optical range is imposed by instrumental considerations (spectrophotometers do not usually work at wavelengths shorter than about 200 nm) and by the validity of the macroscopic Maxwell equations. These equations assume a continuous medium in other words, that there is a large number of ions within a volume of. The long (IR) wavelength limit of the optical range is basically imposed by experimental considerations (spectrophotometers work up to about 3000 nm). 

The term Tight is understood here in a broader sense than visible radiation, as it also includes the UV anti IR radiation belonging to the optical range. 

We have seen in the previous section that Raman spectra are complementary to infrared spectra. Both spectroscopies provide quite useful information on the phonon structure of solids. However, infrared spectra correspond to a range from about 100 cm to about 5000 cm that is, far away from the optical range. Thus, infrared absorption spectra are generally measured by so-called Fourier Transform InfraRed (FTIR) spectrometers. These spectrometers work in a quite different way to the absorption spectrophotometers discussed in Section 1.3. 

Figure 4.1 The spectral dependencies of (a) ei and 2 and (b) k and R. These curves have been represented for typical values within the optical range fia)o = 4eV, fiF = eV, and...

In general, spectral regions III and IV are far away from the optical range for good insulators, while these regions can be optically observed for some semiconductors. This is the reason why many semiconductors, such as Ge and Si, have a metallic aspect, while most of the good insulators, such as KCl and NaCl, are highly transparent in the visible. 

The previous assumption, >o = 0, can be also justified from the quantum point of view, as the most relevant transitions in metals take place within a band, usually the conduction band. The energy levels within a band of a sohd are separated by energies of about 10 eV, and so in the optical range (photon energies of some eV) the validity of assuming coj = 0 for transitions between these levels is evident. Consequently, the classical assumption >o = > = 0 is also justified from the quantum viewpoint. 

The absorption spectrum involving the valence band to first empty (conduction) band transitions is usually called the fundamental absorption spectrum. For many crystals, this spectrum lies within the optical range. 

Zinc (Zn) is a divalent metal with an atomic density of 6.6 x 10 cm . Determine the wavelength region within the optical range for which you expect this metal to be a good mirror. 

Now we assume that the wavelength of the electromagnetic wave is much larger than the atomic dimensions. This is, of course, true for the optical range, as the shortest wavelength is around 200 nm while the atomic dimensions are of the order of 0.1 nm. In this case, the electric field does not vary within the atomic volume and so E = E (0, t) = Eo sin ojt. Therefore, we can write... 

Divalent rare earth ions also have an outer electronic configuration of 4f"( including one more electron than for the equivalent trivalent rare earth). However, unlike that of (RE) + ions, the 4f " 5d excited configuration of divalent rare earth ions is not far from the 4f" fundamental configuration. As a result, 4f" 4f " 5d transitions can possibly occur in the optical range for divalent rare earth ions. They lead to intense (parity-allowed transitions) and broad absorption and emission bands. 

Planck s equation applies strictly to the emission into space at absolute zero, but for wavelengths in the visible and ultraviolet region from incandescent sources, this is substantially the same as emission into space at room temperature. For low temperatures and frequencies in the optical range ehvlkT > 1 the following simplification can be made ... 

In Chapter 4 we will see that two types of induced dipole functions are of a special importance the overlap-induced dipole, Eq. 4.2, an exponential with a range Rq O.lcr, and the multipole-induced dipole, Eq. 4.3, which falls off as R N (N = 4, 5,...). For these, the optical range becomes... 

These are the Fourier components of the polarization vector which are connected with the oscillations of the ions that are present in eqns. (44)-(46). In addition to this polarization which results from the motion of the nuclei, purely electronic polarization (i.e. the polarization of electrons at equilibrium positions of the nuclei) is also of importance. In the frequency region below the optical range, the purely electronic polarization can be expressed through the optical dielectric permeability (i.e. the dielectric permeability corresponding to the frequencies which are less than those in the optical absorption region, but exceed those of the nuclei vibrations). Optical frequencies considerably exceed those of the nuclear vibrations therefore, in the optical frequency region the nuclei do not, in practice, contribute to polarization. The connection of the Fourier component of purely electronic polarization with that of the induction of the electric field has the usual form... 

Figs. 6a und b. a) Typical observed interstellar extinction curve from the IR through the optical range to the UV. b) Schematic representation of the optical part of the extinction curve, defining the quantities A v, Ab and R... 

As an example of a modern commercial interferometer, the optical diagram of a Bruker IFS 66, is shown in Fig, 3.4-1. It allows working in the optical range from 40000 to 20 cm (250 nm to 500 im), to exchange different internal and external radiation sources and detectors, and to connect various accessories, such as a Raman module or an infrared or Raman microscope. 

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