Electronic spectroscopies principle

Section BT1.2 provides a brief summary of experimental methods and instmmentation, including definitions of some of the standard measured spectroscopic quantities. Section BT1.3 reviews some of the theory of spectroscopic transitions, especially the relationships between transition moments calculated from wavefiinctions and integrated absorption intensities or radiative rate constants. Because units can be so confusing, numerical factors with their units are included in some of the equations to make them easier to use. Vibrational effects, die Franck-Condon principle and selection mles are also discussed briefly. In the final section, BT1.4. a few applications are mentioned to particular aspects of electronic spectroscopy. 

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. 

Instrumental Methods for Bulk Samples. With bulk fiber samples, or samples of materials containing significant amounts of asbestos fibers, a number of other instmmental analytical methods can be used for the identification of asbestos fibers. In principle, any instmmental method that enables the elemental characterization of minerals can be used to identify a particular type of asbestos fiber. Among such methods, x-ray fluorescence (xrf) and x-ray photo-electron spectroscopy (xps) offer convenient identification methods, usually from the ratio of the various metal cations to the siUcon content. The x-ray diffraction technique (xrd) also offers a powerfiil means of identifying the various types of asbestos fibers, as well as the nature of other minerals associated with the fibers (9). 

X-ray photoelectron spectroscopy (XPS), which is synonymous with ESCA (Electron Spectroscopy for Chemical Analysis), is one of the most powerful surface science techniques as it allows not only for qualitative and quantitative analysis of surfaces (more precisely of the top 3-5 monolayers at a surface) but also provides additional information on the chemical environment of species via the observed core level electron shifts. The basic principle is shown schematically in Fig. 5.34. 

Before reviewing recent advances in the field of luminescent dendrimers, it is worthwhile recaUing a few elemental principles of electronic spectroscopy. Interested readers are referred to several books and reviews for detailed discussions [2,11,12]. 

Nemkovich NA, Rubinov AN, Tomin VI (1991) Inhomogeneous broadening of electronic spectra of dye molecules in solutions. In Lakowicz JR (ed) Topics in fluorescence spectroscopy, principles, vol 2. Plenum, New York, pp 367 128... 

There is now available a substantial amount of information on the principles and techniques involved in preparing evaporated alloy films suitable for adsorption or catalytic work, although some preparative methods, e.g., vapor quenching, used in other research fields have not yet been adopted. Alloy films have been characterized with respect to bulk properties, e.g., uniformity of composition, phase separation, crystallite orientation, and surface areas have been measured. Direct quantitative measurements of surface composition have not been made on alloy films prepared for catalytic studies, but techniques, e.g., Auger electron spectroscopy, are available. 

In spectroscopy we may distinguish two types of process, adiabatic and vertical. Adiabatic excitation energies are by definition thermodynamic ones, and they are usually further defined to refer to at 0° K. In practice, at least for electronic spectroscopy, one is more likely to observe vertical processes, because of the Franck-Condon principle. The simplest principle for understandings solvation effects on vertical electronic transitions is the two-response-time model in which the solvent is assumed to have a fast response time associated with electronic polarization and a slow response time associated with translational, librational, and vibrational motions of the nuclei.92 One assumes that electronic excitation is slow compared with electronic response but fast compared with nuclear response. The latter assumption is quite reasonable, but the former is questionable since the time scale of electronic excitation is quite comparable to solvent electronic polarization (consider, e.g., the excitation of a 4.5 eV n — n carbonyl transition in a solvent whose frequency response is centered at 10 eV the corresponding time scales are 10 15 s and 2 x 10 15 s respectively). A theory that takes account of the similarity of these time scales would be very difficult, involving explicit electron correlation between the solute and the macroscopic solvent. One can, however, treat the limit where the solvent electronic response is fast compared to solute electronic transitions this is called the direct reaction field (DRF). 49,93 The accurate answer must lie somewhere between the SCRF and DRF limits 94 nevertheless one can obtain very useful results with a two-time-scale version of the more manageable SCRF limit, as illustrated by a very successful recent treatment... 

As representative techniques of the second group, we discuss two methods x-ray photoelectron spectroscopy (XPS), sometimes referred to as electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES). The main principle of the first method (XPS) is the excitation of electrons in an atom or molecule by x-rays. The resulting electrons carry energy away according to the formula... 

The other method. Auger electron spectroscopy, is considered appropriate for studying the chemical makeup (composition) of surfaces, with a sensitivity down to 1% of a single atomic layer (monolayer). It is also easier to perform than many other methods of surface studies of the present group. It is based on the principle that if an... 

Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) are the two principle surface analysis techniques. They are used to identify the elemental composition, i.e., the amount and nature of species present at the surface to a depth of about 1 nm. 

Photoelectron Spectroscopy. As a subdivision of electron spectroscopy, photoelectron or photoemission spectroscopy (PES) includes those instruments that use a photon source to eject electrons from surface atoms. The techniques of x-ray photoelectron spectroscopy (XPS) and uv photoelectron spectroscopy (UPS) are the principles in this group. Auger electrons are emitted also because of x-ray bombardment, but this combination is used infrequent-... 

On the other hand, it is pleasing to see that the organic chemist s standard evidence for structural identification such as NMR and IR spectral data can be computed quite accurately UV/vis spectra can also be computed with time-dependent methods, which, however, cannot yet be used to optimise the excited states as the Franck-Condon principle is a common assumption in electron spectroscopy, this is no serious drawbackfor computing... 

The principles of electronic spectroscopy have been discussed by Herzberg (1950) for diatomic molecules, and in a classic review by Sponer and Teller (1941) for the more general case of polyatomic structures. Recent developments are described in articles appearing regularly in the Annual Reviews of Physical Chemistry. Triatomic molecules and radicals have been intensively studied, the latter by the powerful method of flash photolysis (Herzberg, 1959). As triatomic structures have been comprehensively reviewed recently (Ramsay, 1962) we include in this article only those triatomic systems that are of particular interest in organic chemistry. Otherwise attention will be directed to molecules of four or more atoms, including all known representatives of the important chromophores. 

The first investigation into the excited states of ZnPc based on first-principles methods is the TDDFT/SAOP study by Ricciardi et al [135], where the UV-vis and the vacuum ultraviolet region of the electronic spectrum of ZnPc are described in detail. Subsequently, Nguyen and Pachter, in the context of a TDDFT/B3LYP study of the electronic spectroscopy of the zinc tetrapyrrole series [140], ZnP, ZnPz, ZnTBP, and ZnPc, came to a somewhat different interpretation of the Uv-vis spectrum of this phthalocyanine. 

A very useful source of complementary information is XPS (X - ray Photo - electron Spectroscopy), which is a typical surface analysis technique. XPS is often used as a valuable tool in studies of the interaction of silanes with silica40,41,42,43 or neoceramic coatings.44,45 The basic principles of the XPS technique are described in appendix B. 

Fig. 3.24 Electron energy scheme explaining the principle behind metastable atom electron spectroscopy. An excited atom M collides gently with an adsorbed molecule A the metastable atom is de-excited by electron transfer from the adsorbate to the core hole...

The second type of predissociation observed for diatomic molecules is known as electronic predissociation the principles are illustrated in figure 6.28. A vibrational level v of a bound state E lies below the dissociation asymptote of that state, but above the dissociation asymptote of a second state E2. This second state, E2, is a repulsive state which crosses the bound state E as shown. The two states are mixed, and the level v can predissociate via the unbound state. It is not, in fact, necessary for the potential curves of the two states to actually cross. It is, however, necessary that they be mixed and there are a number of different interaction terms which can be responsible for the mixing. We do not go into the details here because electronic predissociation, though an important phenomenon in electronic spectroscopy, seldom plays a role in rotational spectroscopy. Since it involves excited electronic states it could certainly be involved in some double resonance cases. 

Background on ESCA. ESCA is one form of the general class of electron spectroscopy (12-14). The fundamental principles are reviewed here. 

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