Spectrum, optical

The reflection spectra of sintered and freshly polished samples between 1 and 6 eV show the following maxima (in nm)  

Measurements of optical absorption and reflection spectra permit the study of the real (fij and imaginary ( 2) parts of the dielectric function 

Here /k) and ((k) are the energies of the virtual and occupied one-electron states, respectively Pj/k) is the transition matrix element  

Most properties of solids represent the response of the solid to some external electromagnetic field or mechanical force. The response of the electronic states to electromagnetic fields is so directly related to electronic structure that it is appropriate to discuss it first. We are interested in absorption and reflection of light as well as in the ordinary dielectric and diamagnetic properties of solids. When static or slowly varying electric fields act on a polar semiconductor, the two types of atoms in the semiconductor will move with respect to each other, giving a lattice 

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GHz spectral line surveys of tliree regions of the W3 giant molecular cloud complex [21]. From such studies, which reveal dramatic differences in the THz spectmm of various objects, molecular astrophysicists hope to classify the evolutionary state of the cloud, just as optical spectra are used to classify stars. 

Chemical reactions can be studied at the single-molecule level by measuring the fluorescence lifetime of an excited state that can undergo reaction in competition with fluorescence. Reactions involving electron transfer (section C3.2) are among the most accessible via such teclmiques, and are particularly attractive candidates for study as a means of testing relationships between charge-transfer optical spectra and electron-transfer rates. If the physical parameters that detennine the reaction probability, such as overlap between the donor and acceptor orbitals. 

Basche T, Ambrose W P and Moerner W E 1992 Optical spectra and kinetics of single impurity molecules in a polymer spectral diffusion and persistent spectral hole burning J. Opt. See. Am. B 9 829-36... 

Schaaff T G efa/1997 Isolation of smaller nanocrystal Au molecules robust quantum effects in the optical spectra J. Phys. Chem. B 101 7885... 

G. L. Hug, Optical Spectra ofNonmetallic Inorganic Transient Species in Aqueous Solution, NSRDS-NBS 69, National Bureau of Standards, Washington,... 

Fig. 5. Evolution of optical spectra of polypyrrole during electrochemical doping.

Since the susceptibilities can be extracted from the optical spectra of these active modes, a quantitative description based on dissipative tunneling techniques can be developed. Such a program should include the analysis of the motion of the reaction complex PES, with the dissipation of active modes taken into account. The advantage of this procedure is that it would allow one to confine the number of PES degrees of freedom to the relevant modes, and incorporate the environment phenomenologically. 

These aspects of the optical spectra of solids are illustrated in the upper portion of Figure 1, which displays the reflectance curve (R) at room temperature for a typical semiconductor, GaAs. The fundamental absorption edge around 1.4 eV produces only a weak shoulder. Some structure is apparent in the two features around 3 eV and the large, broad peak near 5 eV. However, the dominant aspect of the line shape is the slowly varyii background. The derivative namre of Modulation Spectroscopy suppresses the uninteresting background effects in favor of sharp, deriva-... 

As shown in Fig. 7, a large increase in optical absorption occurs at higher photon energies above the HOMO-LUMO gap where electric dipole transitions become allowed. Transmission spectra taken in this range (see Fig. 7) confirm the similarity of the optical spectra for solid Ceo and Ceo in solution (decalin) [78], as well as a similarity to electron energy loss spectra shown as the inset to this figure. The optical properties of solid Ceo and C70 have been studied over a wide frequency range [78, 79, 80] and yield the complex refractive index n(cj) = n(cj) + and the optical dielectric function... 

Electronic properties of CNTs, in particular, electronic states, optical spectra, lattice instabilities, and magnetic properties, have been discussed theoretically based on a k p scheme. The motion of electrons in CNTs is described by Weyl s equation for a massless neutrino, which turns into the Dirac equation for a massive electron in the presence of lattice distortions. This leads to interesting properties of CNTs in the presence of a magnetic field including various kinds of Aharonov-Bohm effects and field-induced lattice distortions. 

Although long-time Debye relaxation proceeds exponentially, short-time deviations are detectable which represent inertial effects (free rotation between collisions) as well as interparticle interaction during collisions. In Debye s limit the spectra have already collapsed and their Lorentzian centre has a width proportional to the rotational diffusion coefficient. In fact this result is model-independent. Only shape analysis of the far wings can discriminate between different models of molecular reorientation and explain the high-frequency pecularities of IR and FIR spectra (like Poley absorption). In the conclusion of Chapter 2 we attract the readers attention to the solution of the inverse problem which is the extraction of the angular momentum correlation function from optical spectra of liquids. 

Of course, knowledge of the entire spectrum does provide more information. If the shape of the wings of G (co) is established correctly, then not only the value of tj but also angular momentum correlation function Kj(t) may be determined. Thus, in order to obtain full information from the optical spectra of liquids, it is necessary to use their periphery as well as the central Lorentzian part of the spectrum. In terms of correlation functions this means that the initial non-exponential relaxation, which characterizes the system s behaviour during free rotation, is of no less importance than its long-time exponential behaviour. Therefore, we pay special attention to how dynamic effects may be taken into account in the theory of orientational relaxation. 

Eq. (2.68) may also be used to solve the inverse problem. The recovery of g(a>) from experimentally obtained optical spectra may prompt the origin of the maximum. To find g(oj), it is necessary to determine from the correlation function K( not only Ge but also... 

Owing to this relation the true shape of Kj(t) may be recovered from Kt(t) found from the optical spectra or MD simulations. 

Observed and Calculated Optical Spectra tor Discandium and Dititanium (107)... 

Vanadium atom depositions were further studied in alkane matrices 109) in an effort to observe the influence of other low-temperature, matrix environments on the optical spectra and clustering properties of metal atoms. Thus, vanadium atoms were deposited with a series of normal, branched, and cyclic alkanes over a wide range of temperature. The atomic spectra were somewhat broadened compared to those in argon, but the matrix-induced, frequency shifts from gas-phase values were smaller. As shown in Fig. 3, these shifts decrease with in-... 

The optical spectra of the nickel-triad metals have been reinvestigated (111) in Ar, Kr, and Xe matrices, and, although the data for Ni and Pt atoms correlated well both with previous studies and with the... 

Fig. 7. The optical spectra obtained on depositing (A) silver vapor with methane at 10-12 K at Ag CH4 = 1 10, and (B) silver vapor with water vapor at 10-12 K at Ag HjO = 1 10, brief warming to 77 K, and recooling to 10-12 K for spectral recording (146).

Fig. 8. The optical spectra obtained on depositing silver vapor with n-Ct2H< vapor at 10-12 K, and (A) Ag n-CuHu 1 10<, (B) Ag n-CttH = 1 rlO , and (C)-(E) showing the progress of annealing (B) at 30, 40, and 80 K, respectively (146).

Optical Spectra of Ag Atoms and Ag, Dimers in Methane and High-Molecular-Weight Paraffin Wax (n-CaH , n- C3,H ) Matrices (146) at 10-12 K... 

Theoretical analyses (75-77) of the matrix-induced changes in the optical spectra of isolated, noble-metal atoms have also been made. The spectra were studied in Ar, Kr, and Xe, and showed a pronounced, reversible-energy shift of the peaks with temperature. The authors discussed the matrix influence in terms of level shift-differences, as well as spin-orbit coupling and crystal-field effects. They concluded that an increase in the matrix temperature enhances the electronic perturbation of the entrapped atom, in contrast to earlier prejudices that the temperature dilation of the surrounding cage moves the properties of the atomic guest towards those of the free atom. 

The copper system appears to behave similarly to the silver system, and it may be used here in order to illustrate the idea of "selective, naked-cluster cryophotochemistry 150,151). A typical series of optical-spectral traces that illustrate these effects for Cu atoms is given in Fig. 15, which shows the absorptions of isolated Cu atoms in the presence of small proportions of Cu2, and traces of Cus molecules. Under these concentration conditions, the outcome of 300-nm, narrow-band photoexcitation of atomic Cu is photoaggregation up to the Cus stage. The growth-decay behavior of the various cluster-absorptions allows unequivocal pinpointing of UV-visible, electronic transitions associated with Cuj and Cus 150). With the distribution of Cui,2,3 shown in Fig. 15, 370-nm, narrow-band excitation of Cu2 can be considered. Immediately apparent from these optical spectra is the growth (—10%) of the Cu atomic-resonance lines. Noticeable also is the concomitant... 

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