Electronic spectroscopy definition

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. 

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... 

Until recently, experimental studies of AI were limited to the identification of the process and, in some cases, to the determinations of cross sections or rate constants. It was not possible to draw definite conclusions from this experimental information regarding the involved mechanisms. In recent studies of the AI systems R -H, with R = Ar(3P20), Kr(3P20), Xe(3P2 o), it was verified by electron spectroscopy that the mechanism of Fig. 34b is dominant for these systems.99-101 In all three cases the observed electron spectra extended to the rather high energies of e —1.45 eV (Ar),= 1.0 eV (Kr), and 1.2 eV (Xe) and showed structure resulting from population of different vibrational rotational states as expected for the mechanism of Fig. 34b. As an example, the AI electron spectrum for Ar(3F2.0)-H is shown in Fig. 35. 

Core electron spectroscopy for chemical analysis (ESCA) is perhaps the most definitive technique applied to the differentiation between nonclassical carbocations from equilibrating classical species. The time scale of the measured ionization process is of the order of 10 16 s so that definite species are characterized, regardless of (much slower) intra- and intermolecular exchange reactions—for example, hydride shifts, Wagner-Meerwein rearrangements, proton exchange, and so on. 

Methods such as nuclear magnetic resonance (NMR), electron spectroscopy for chemical analysis (ESCA), electron spin resonance (ESR), infrared (IR), and laser raman spectroscopy could be used in conjunction with rate studies to define mechanisms. Another alternative would be to use fast kinetic techniques such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4), where chemical kinetics are measured and mechanisms can be definitively established. 

Single-crystal X-ray crystallography remains the only definitive technique for the structural characterization of heteronuclear gold cluster compounds, although other techniques, in particular Mossbauer, NMR, IR, and fast atom bombardment mass spectroscopies (FABMS), have yielded valuable information, especially concerning the nature of these species in solution. Electron spectroscopy, which has proved to be of great value in the identification of homonuclear gold cluster compounds (210) has received little attention by workers in this area,... 

A variety of model catalysts have been employed we start with the simplest. Single-crystal surfaces of noble metals (platinum, rhodium, palladium, etc.) or oxides are structurally the best defined and the most homogeneous substrates, and the structural definition is beneficial both to experimentalists and theorists. Low-energy electron diffraction (LEED) facilitated the discovery of the relaxation and reconstruction of clean surfaces and the formation of ordered overlayers of adsorbed molecules (3,28-32). The combined application of LEED, Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), field emission microscopy (FEM), X-ray and UV-photoelectron spectroscopy (XPS, UPS), IR reflection... 

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbornyl cation has also been extensively smdied at low temperatures there is much evidence that under these conditions the ion is definitely nonclassical. Olah and co-workers have prepared the 2-norbomyl cation in stable solutions at temperamres below 150°C in SbFs—SO2 and FSO3H SbF5 S02, where the stmcmre is static and hydride shifts are absent Studies by proton and NMR, as well as by laser Raman spectra and X-ray electron spectroscopy, led to the conclusion that under these conditions the ion is nonclassical. A similar result has been reported for the 2-norbomyl cation in the sohd state where at 77 and even 5 K, NMR spectra gave no evidence of the freezing out of a single classical ion. ... 

Since in electron spectroscopy the time scale of the measured ionization processes is on the order of 10 16 sec, definite ionic species are characterized, regardless on their possible intra- and intermolecular interactions (e. g., Wagner-Meerwein rearrangements, hydride shifts, proton exchange, etc.). Thus, electron spectroscopy gives an undisputible, direct answer to the long debated question of the non-classical nature of the norbornyl cation independent of any possible equilibration process. 

It also includes Auger electron spectroscopy microprobe identification of the surface crud in the pit, or definition of the surface composition gradient previously omitted from the speculation on atom transport processes. 

The definition of the powder surfaces tends to be in terms of the process for making it (23). This limits interpretation of their properties and also limits generalization of properties to surfaces which have been generated by atmospheric corrosion. Conversely, the LEED, Auger electron spectroscopy, photo emission properties, etc, are measured on fresh surfaces in UHV, because interpretation is difficult for more complex configurations and because experience has shown that these properties are profoundly modified and confused by exposure to real atmospheres. 

Various techniques have been utilized to determine the existence of a metal-metal bond in the solid state. Single crystal x-ray analysis and neutron diffraction (17) are the most accurate methods available and are often facilitated by the strong scattering of the metal atoms. Polarized electronic spectroscopy (430), electron spin resonance, magnetic susceptibility, and dc conductivity have been used to indicate some solid state interaction. These techniques are, however, not definitive (144). Recent work indicates that resonance enhancement in Raman spectroscopy may provide a useful tool (101, 382) in elucidating metal-metal interactions. 

In spatially resolved microanalysis, measurements are obtained for many small neighbouring locations, and spatial and gradient relationships can be determined. The essence of microanalysis is the achievement of good spatial resolution. For the surface sensitive electron spectroscopies this includes both lateral resolution on the sample surface and resolution in depth. Because of the small amount of material available, some of the accuracy and precision is given up for obtaining better spatial definition. 

Another important result from the photophysical studies on conjugated polymer/fullerene composites is the complete absence of any photoinduced interaction between fullerenes and either polyacetylene (PA) [54] or the polydiacetylenes (PDAs) [62], respectively. The results of absorption and emission spectroscopy, subpicosecond and millisecond photoinduced absorption spetroscopy, and picosecond transient and near steady state photoconductivity show that in contrast to the high quantum efficiency photoinduced electron transfer definitively established in the PPV and the P3AT composites with Cgo, photoinduced electron transfer from PA and PDAs onto Ceo is inhibited [62]. 

For definitions of acronyms, see Table XII. [From Thompson, M., Baker, M. D., Christie, A., and Tyson, J. F. (1985). Auger Electron Spectroscopy, Wiley-Interscience, New York.]... 

Some problems, such as the determination of the support sites adopted by organometallics, may be tackled definitively only by recourse to single crystal surface science techniques. Many electron spectroscopies are inappropriate to the study of metal species on insulating or semiconductor supports. Recent results using specular reflectance FTIR and SEXAFS techniques, however, are encouraging and may be applied to oxide surfaces. These experiments also give data which can be directly compared with results obtained on high-surface-area supports and therefore provide a fruitful interrelationship. 

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