Strong electronic transitions

Reflectance. Both internal and external reflectance spectroscopy are relatively simple experiments to perform. Commercially available attachments for standard UV-visible spectrometers can be used. For films with strong electronic transitions reasonable spectra can be obtained. The theory for external and internal reflectance is the same as that for the IR and can be found elsewhere (2, 37). The techniques have not been very popular in their applications to surface analysis. The major reason appears to be... 

TABLE 6.4 The fitted parameters of the smoothed SO2 Abs. XS/E from a single fit involving two electronic transitions. The lower electronic transition is fitted with a Gaussian (3 parameters, first column) and the second electronic transition is fitted with Formula (29) (4 parameters, second column). The uncertainties are 2 S.D. (Standard Deviation). Note that the uncertainty of the s parameter of the upper transition is only 1%. Note that the lower (weak) electronic transition is much narrower than the upper strong electronic transition because the widths are proportional to the slopes of the PES of these two states at the G.S. (Ground State) equilibrium geometry see Formula 6.18 and 6.19 of Ref. [15]... 

It is likely that the enormous sensitivity of laser spectroscopy (109) will in the near future make possible the detection of reactive intermediates as well as the reactants and products of atomic reactions. It has been estimated that 10 molecules/cm can be detected by laser absorption spectroscopy under realistic laboratory conditions for a moderately strong electronic transition in the visible region of the spectrum (IIO). If this sensitivity can be attained, then even the direct detection of recoil species appears to be possible. 

There are two physical reasons for the enhancement in RRS, the Franck—Condon enhancement and the vibronic enhancement Both mechanisms are complicated, a detailed description is given in [30], An appHcation of RRS is the investigation of biological molecules like metalloporphyrins and carotenoids. These molecules have very strong electronic transitions in the VIS. The vibrations of the chromophoric part become resonance enhanced but the vibrations of the surrounding protein matrix do not This allows observation of the chromophoric site without spectral interference from the surrounding protein. RRS is suitable... 

Example 16-8. The [CoC ] " ion exhibits an intense blue color, with a strong electronic transition at 15,000 cm and a weaker one at 23,000 cm . Two other strong bands occur in the near-IR and the IR at 5800 and 3200 cm , respectively. Calculate A and P from these data. 

Strong (electronic) transitions have Ag values of the order 10 -10 s , i.e. lifetimes are typically 1-10 ns. Note that the apparent lifetimes can be shorter than this quantum calculation, e.g. the lifetime can be shortened by collisions or stimulated emission. 

The interaction of two macroscopic bodies can sometimes be obtained by summing the dispersion energy between all pairs of molecules or unit cells in the two bodies. There is no electrostatic or induction contribution when the material is uncharged and isotropic. If the separation of the units is large compared to the reduced wavelength associated with the strong electronic transitions, the dispersion interaction is retarded and therefore weakened it varies as R rather than [34]. If the dispersion energy between the units is... 

The process of doping is accompanied by the appearance of gap levels in the electronic spectrum at low energies (near IR) at various Amax with a simultaneous blue shift and bleaching of the HOMO-LUMO (tt-tt ) strong electronic transition [61,62]. 

A strong point of EELS is that it detects losses in a very broad energy range, which comprises the entire infrared regime and extends even to electronic transitions at several electron volts. EELS spectrometers have to satisfy a number of stringent requirements. First, the primary electrons should be monochromatic. Second,... 

The structural unit associated with an electronic transition m UV VIS spectroscopy IS called a chromophore Chemists often refer to model compounds to help interpret UV VIS spectra An appropriate model is a simple compound of known structure that mcor porates the chromophore suspected of being present m the sample Because remote sub stituents do not affect Xmax of the chromophore a strong similarity between the spectrum of the model compound and that of the unknown can serve to identify the kind of rr electron system present m the sample There is a substantial body of data concerning the UV VIS spectra of a great many chromophores as well as empirical correlations of sub stituent effects on k Such data are helpful when using UV VIS spectroscopy as a tool for structure determination... 

Electronic transitions in molecules in supersonic jets may be investigated by intersecting the jet with a tunable dye laser in the region of molecular flow and observing the total fluorescence intensity. As the laser is tuned across the absorption band system a fluorescence excitation spectrum results which strongly resembles the absorption spectrum. The spectrum... 

Electronic Properties. What distinguishes polysdanes from virtually ad. other polymers is their backbone CJ-conjugation. This leads to strong electronic absorption in the near-uv from a O —O transition. For most homo- and copolymers the absorption maximum (/-j ) hes between 300 and 400... 

Shorter-wavelength radiation promotes transitions between electronic orbitals in atoms and molecules. Valence electrons are excited in the near-uv or visible. At higher energies, in the vacuum uv (vuv), inner-shell transitions begin to occur. Both regions are important to laboratory spectroscopy, but strong absorption by make the vuv unsuitable for atmospheric monitoring. Electronic transitions in molecules are accompanied by stmcture... 

RAIRS spectra contain absorption band structures related to electronic transitions and vibrations of the bulk, the surface, or adsorbed molecules. In reflectance spectroscopy the ahsorhance is usually determined hy calculating -log(Rs/Ro), where Rs represents the reflectance from the adsorhate-covered substrate and Rq is the reflectance from the bare substrate. For thin films with strong dipole oscillators, the Berre-man effect, which can lead to an additional feature in the reflectance spectrum, must also be considered (Sect. 4.9 Ellipsometry). The frequencies, intensities, full widths at half maximum, and band line-shapes in the absorption spectrum yield information about adsorption states, chemical environment, ordering effects, and vibrational coupling. 

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