Spectroscopy, optical

Optical spectroscopy, like X-ray spectroscopy, is going through a period of rapid refinement of techniques. Among recent innovations, the use of synchrotron radiation has opened up the study of systems with large band gaps such as alkali halides, since most previous studies were subject to a cutoff at the largest available band gap (for windows) which was provided by LiF, at about 12 eV. Another important advance is the introduction of modulated techniques. 

This is in turn related to the complex index of refraction n by 

Thanks to the existence of Kramers-Kronig relations between Si((o) and 82(0)), a complete knowledge of either function serves to determine the other. In practice this means that a measurement of the reflectivity of a clean surface is sufficient to determine 82(0)). For a review of the basic theory see Phillips.  

The function 82(0)) is again given by the integral formula (38) where j is here an unoccupied state above the Fermi level and i is an occupied state below it. In practice, many-body effects again intrude and exciton structure indeed plays a prominent part in 82(a)) for many systems, especially those with large band gaps. All of this will, for present purposes, be swept under the proverbial rug. 

Even with so much simplification by omission, we are left with a fairly complicated formula for 82(0))- In this case, unless the electronic density has very prominent peaks corresponding to essentially flat bands, the simple interpretation in terms of a joint density of states may not be reliable. This is because the existence of the k-selection rule which is built into the full formula (38). Only initial and final states of the same k vector can contribute. In addition, since (38) involves a dipole matrix element, transitions between states of, say, predominantly s and p character will be enhanced. However, k selection is the principal effect. So-called indirect (non-k-conserving) transitions can 

The major limitation of the purely optical approach is that it is limited to states of low /. It is most effective for fine structure intervals, which may be determined to high accuracy by the relative wavelength measurements of the transitions to the two fine structure levels. 

Transition metals (with electrons in d-orbitals) and cofactors with unsaturated bonds (with electrons in p orbitals) give rise to optical absorption spectra. In haem proteins the most intense optical absorbances observed are from transitions within the porphyrin ring (tt — ir ) with further significant contributions from ligand to metal (tt — d) charge transfer bands (especially in the near infrared). The (d— d) transitions are much weaker. The conversion of Fem (or Fe11) 

Compared to the intensity of the absorbance of metal-porphyrin complexes, that of free radicals on amino acids is very low. Therefore optical spectra 

However, it is possible to detect a tyrosine radical optically in ribonucleotide reductase, as there is only a relatively weak competing absorption from the binuclear non-haem iron centre [164]. A distinct sharp peak is seen that is not present in proteins that have been treated with the radical scavenger hydroxyurea [165,166] nor is it present in proteins such as haemerythrin or methane monooxygenase, which have similar active-site structures, but lack 

There are now good theoretical descriptions of the electronic structures contributing to the optical absorption bands in spectra of porphyrin radicals and ferryl species [160,167] most charge-transfer bands in the latter are due to a transition from a porphyrin p orbital to an Fe-0 tt orbital [167], However, in the absence of a prior knowledge of the structure around the Felv site (and/or spectra of a variety of synthetic model compounds) it is not straightforward to assign an optical spectrum to a ferryl species. Thus the intermediate assumed to be the ferryl species in the binuclear haem c /Cub centre of cytochrome c oxidase [168] has a spectrum at 580 nm essentially identical [169] to that of low-spin ferric haem a3 compounds (e.g. cyanide). 

There are no reported optical spectra attributed to FeIV in non-haem iron proteins suggested to have ferryl intermediates. However, a low-intensity peak at about 600 nm is observed in a model non-haem ferryl compound generated by the addition of H202to a (p-oxo) diferric complex [130], 

The structural disorder formalism has been mostly utilized to discuss electronic transport in organic solids [29,38] (cf. Sec. 4.6), and only a few works show its applicability to interpret optical spectra [62,67], and, recently, quantum efficiency of organic LEDs [68]. The absorption spectrum of an organic material with impurities disorder, local electric fields, or strong exciton-phonon coupling exhibits an exponential tail, commonly referred to as the Urbach tail [69,70]. Such a spectrum can often be decomposed into broad bands featuring 

Gaussian profiles (Fig. 8). Gaussian profiles are ascribed to the Gaussian distribution of energies (e) of the emitting states  

The distribution parameter a reflects the root-mean square standard deviation of the non-resonance interaction energy D [cf. Eq. (13)] corresponding to the polarization energy of a charge carrier in a medium (cf. Sec. 2.3.1). The essential contribution to D is the difference of the van der Waals energies between an unexcited and excited molecule embedded in a medium of polarizability a. 

For a dipole-allowed singlet transition (i) characterized by an oscillator strength fu it can be expressed as 

Variation of the intermolecular distances by (Ar/r) causes an average relative fluctuation of the Z)-term by 

The original studies of optical spectra of micas arose from an interest in the variable colors of muscovite and phlogopite (e.g., Hall 1941), which formed the basis for commercial classification of their electrical properties. However, subsequent research has used optical spectroscopy to delve into the assignment of Fe and IVCT bands, as well as the issue of the site occupancy of Fe  

Some drawbacks remain with the commercial packages, however. Packages for anisotropic measurements are not currently available, and most of the polarization accessories available from the major companies are film polarizers that are not useful. Furthermore, only a few of the commercial systems (such as the one made by Jobin Yvon, Inc.) work in the NIR. Most basic systems are designed for silicon detectors that go from about 200-1150 nm (G.R. Rossman, pers. comm. 2000). For this reason, astute users are well served to build their own equipment. 

In a reflectance or ellipsometry experiment, measurements are always referred to the physical plane of incidence, as defined in Fig. 27.24. If the polarization is parallel to this plane of incidence, the parameters related to it are denoted by the subscript p. For polarization perpendicnlar to the plane, the subscript s is used. When a linearly polarized beam is reflected, one often finds that the parallel and perpendicular components nndergo changes in amplitude and phase. Thus, two beams that are in 

FIGURE 27.23 Electric (E) and magnetic (H) vectors in a linearly polarized light wave. The plane of polarization contains the electric field vectors in space. At a fixed focation, the tip of the electric vector traces a straight line as a function of time. (From Muller, 1973, with permission from Wiley-VCH.) 

Reflectance measurements involve measurements of the intensity of light reflected from a flat specular surface of an electrode in a spectroelectrochemical cell. The incident light is polarized either parallel (p) or perpendicular s) to the plane of incidence, as shown in Fig. 27.24. A detector monitors the intensity of the reflected beam. The light is monochromatic, but the spectrometers usually can be tuned over large wavelength ranges. There are excellent reviews of reflectance by McIntyre (1973) and Plieth et al. (1992). 

The reflectance, R, is defined as the ratio of the reflected light intensity to the intensity of the incident beam. Usually, one determines the change in reflectance, A/ , induced by some parameter, such as the electrode potential. Experimentally, one measures only the intensity of the reflected beam, 4. So if the incident intensity remains constant, the reflected beam gives hJl/R = A4/4. Experimental results are presented as plots of A/J/R vs. the parameter of interest, such as the frequency of the incident light or electrode potential. Modulation schemes, wherein the beam is chopped or the potential is modulated, are used to enhance the signal-to-noise ratio. 

Following this procedure, the UV spectra of carbonylated Ni (Fig. 2-34), Pt, and Os dusters of various dzes have been interpreted with acceptable accuracy, considering the limitations of the theoretical approach. [261, 266, 295] The lowest dipole allowed transitions were calculated to be around 2-3 eV (about 

Figere 2-34. Experimental UV-visible sj rum of [Nlt(CO),2p . Reproduced with permission from [266]. Copyright 1990 American Chemical Society. 

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We now proceed to some examples of this Fourier transfonn view of optical spectroscopy. Consider, for example, the UV absorption spectnun of CO2, shown in figure Al.6.11. The spectnuu is seen to have a long progression of vibrational features, each with fairly unifonu shape and width. Wliat is the physical interpretation of tliis vibrational progression and what is the origin of the width of the features The goal is to come up with a dynamical model that leads to a wavepacket autocorrelation fiinction whose Fourier transfonn... 

Mukamel S 1995 Prf/rc/p/es of Non-linear Optical Spectroscopy (New York Oxford University Press)... 

This is the classic work on molecular rotational, vibrational and electronic spectroscopy. It provides a comprehensive coverage of all aspects of infrared and optical spectroscopy of molecules from the traditional viewpoint and, both for perspective and scope, is an invaluable supplement to this section. 

A comprehensive discussion of wavepackets, classical-quantum correspondence, optical spectroscopy, coherent control and reactive scattering from a unified, time dependent perspective. 

Most ion-molecule techniques study reactivity at pressures below 1000 Pa however, several techniques now exist for studying reactions above this pressure range. These include time-resolved, atmospheric-pressure, mass spectrometry optical spectroscopy in a pulsed discharge ion-mobility spectrometry [108] and the turbulent flow reactor [109]. 

A diagrannnatic approach that can unify the theory underlymg these many spectroscopies is presented. The most complete theoretical treatment is achieved by applying statistical quantum mechanics in the fonn of the time evolution of the light/matter density operator. (It is recoimnended that anyone interested in advanced study of this topic should familiarize themselves with density operator fonnalism [8, 9, 10, H and f2]. Most books on nonlinear optics [13,14, f5,16 and 17] and nonlinear optical spectroscopy [18,19] treat this in much detail.) Once the density operator is known at any time and position within a material, its matrix in the eigenstate basis set of the constituents (usually molecules) can be detennined. The ensemble averaged electrical polarization, P, is then obtained—tlie centrepiece of all spectroscopies based on the electric component of the EM field. 

Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

As discussed in more detail elsewhere in this encyclopaedia, many optical spectroscopic methods have been developed over the last century for the characterization of bulk materials. In general, optical spectroscopies make use of the interaction of electromagnetic radiation with matter to extract molecular parameters from the substances being studied. The methods employed usually rely on the examination of the radiation absorbed. 

The power of optical spectroscopies is that they are often much better developed than their electron-, ion- and atom-based counterparts, and therefore provide results that are easier to interpret. Furtlienuore, photon-based teclmiques are uniquely poised to help in the characterization of liquid-liquid, liquid-solid and even solid-solid interfaces generally inaccessible by other means. There has certainly been a renewed interest in the use of optical spectroscopies for the study of more realistic systems such as catalysts, adsorbates, emulsions, surfactants, self-assembled layers, etc. 

Perhaps the best known and most used optical spectroscopy which relies on the use of lasers is Raman spectroscopy. Because Raman spectroscopy is based on the inelastic scattering of photons, the signals are usually weak, and are often masked by fluorescence and/or Rayleigh scattering processes. The interest in usmg Raman for the vibrational characterization of surfaces arises from the fact that the teclmique can be used in situ under non-vacuum enviromnents, and also because it follows selection rules that complement those of IR spectroscopy. 

One interesting new field in the area of optical spectroscopy is near-field scaiming optical microscopy, a teclmique that allows for the imaging of surfaces down to sub-micron resolution and for the detection and characterization of single molecules [, M]- Wlien applied to the study of surfaces, this approach is capable of identifying individual adsorbates, as in the case of oxazine molecules dispersed on a polymer film, illustrated in figure Bl.22,11 [82], Absorption and emission spectra of individual molecules can be obtamed with this teclmique as well, and time-dependent measurements can be used to follow the dynamics of surface processes. 

Sharma A and Khatri R K 1995 Surface analysis optical spectroscopy Encyciopedia of Anaiyticai Science ed A Townshend (London Academic) 8 4958-65... 

Chiarotti G 1994 Electronic surface states investigated by optical spectroscopy Surf. Sc/. 299/300 541-50... 

McGlip J F 1990 Epioptics linear and non-linear optical spectroscopy of surfaces and interfaces J. Phys. Condens Matter 2 7985-8006... 

Moerner W E 1996 Fligh-resolution optical spectroscopy of single molecules in solids Acc. Chem. Res. 29 563-71... 

Moerner W E, Plakhotnik T, Irngartinger T, Wild U P, Pohl D W and Hecht B 1994 Near-field optical spectroscopy of individual molecules in solids Phys. Rev. Lett. 73 2764-7... 

Bruckner V, Feller K-H and Grummt U-W 1990 Applications of Time-Resolved Optical Spectroscopy (New York Elsevier)... 

K. E. Peiponen, E. M. Vertiainen, and T, Asakura, Dispersion, Complex Analysis and Optical Spectroscopy. (Classical theory), Springer-Verlag, Berlin, 1999,... 

S. Mukamel, Principles of nordinear optical spectroscopy, Oxford University F ress, Oxford,... 

The focus of this chapter is photon spectroscopy, using ultraviolet, visible, and infrared radiation. Because these techniques use a common set of optical devices for dispersing and focusing the radiation, they often are identified as optical spectroscopies. For convenience we will usually use the simpler term spectroscopy in place of photon spectroscopy or optical spectroscopy however, it should be understood that we are considering only a limited part of a much broader area of analytical methods. Before we examine specific spectroscopic methods, however, we first review the properties of electromagnetic radiation. 

The first detector for optical spectroscopy was the human eye, which, of course, is limited both by its accuracy and its limited sensitivity to electromagnetic radiation. Modern detectors use a sensitive transducer to convert a signal consisting of photons into an easily measured electrical signal. Ideally the detector s signal, S, should be a linear function of the electromagnetic radiation s power, P,... 

Optical Spectroscopy Sampling Techniques Manual. Harrick Scientific Corporation Ossining, N.Y., 1987. 

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