Electronic absorption spectra transition, vibrational structure

Absorption spectra can provide information relating to the energy of an excited singlet state. This corresponds to the lowest 0-0 vibrational transition in the electronic absorption spectrum. When the vibrational fine structure is evident, the energy of the excited singlet state is readily determined, but when the 0-0 band cannot be located, the value can be taken from the region of overlap of the absorption and fluorescence spectra. 

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. 

At low enough temperatures vibrational fine structure of aromatic chromophores may be well resolved, especially if they are embedded in a suitable matrix such as argon or N2, which is deposited on a transparent surface at 15 K. This matrix isolation spectroscopy77166 may reveal differences in spectra of conformers or, as in Fig. 23-16, of tautomers. In the latter example the IR spectra of the well-known amino-oxo and amino-hydroxy tautomers of cytosine can both be seen in the matrix isolation IR spectrum. Figure 23-16 is an IR spectrum, but at low temperatures electronic absorption spectra may display sharp vibrational structure. For example, aromatic hydrocarbons dissolved in n-heptane or n-octane and frozen often have absorption spectra, and therefore fluorescence excitation spectra, which often consist of very narrow lines. A laser can be tuned to excite only one line in the absorption spectrum. For example, in the spectrum of the carcinogen ll-methylbenz(a)anthrene in frozen octane three major transitions arise because there are three different environments for the molecule. Excitation of these lines separately yields distinctly different emission spectra.77 Likewise, in complex mixtures of different hydrocarbons emission can be excited from each one at will and can be used for estimation of amounts. Other related methods of energy-... 

The solvent perturbation technique has been used to study the absorption bands attributable to singlet-triplet transitions in benzo-[6]thiophene, using ethyl iodide as the perturbing solvent.173 Attempts to obtain information on the -electron conjugation in the 5-membered ring of benzo[6]thiophene by examination of the vibrational structure of the electronic spectrum were unsuccessful.171 UV evidence suggests that the valence shell of the sulfur atom can expand to a 10-electron structure in benzo[6]thiophene.156... 

The reaction between Mo(H20)63+, prepared and purified following the procedure of Bowen and Taube (18), and nitrate was followed spec-trophotometrically under strict anaerobic conditions with nitrate in excess. The absorption spectrum of nitrite in 1.0M HPTS (p-toluene sulfonic acid) exhibits a multicomponent band (vibrational fine structure) between 350 and 400 nm which is attributable to the xBi <— 1A1 electronic transition. Purified Mo(H20)63+ in 1.0M HPTS has a low absorption at 293 nm, indicating the purity of the preparation (18). When Mo(H20)63+ is mixed with nitrate (constant concentration in large excess) and the reaction is allowed to go to completion, the nitrite fine structure appears between 350 and 400, concomitant with an increase in absorbance at 293 nm. The molybdenum species resulting from the oxidation of... 

The PPV spectra of Fig. 16 show all the signatures of exciton absorption and emission, such as in typical molecular crystals. The existence of well-defined structure in the absorption spectrum is not so easily accounted for in a band-to-band absorption model. In semiconductor theory, the main source of structure is in the joint density of states, and none is predicted in one-dimensional band structure calculations (see below). However, CPs have high-energy phonons (molecular vibrations) which are known (see, e.g., RRS spectra) to be coupled to the electron states. The influence of these vibrations has not been included in previous theories of band-to-band transition spectra in the case of such wide bands [176,183]. For excitons, the vibronic structure is washed out in the case of very intense transitions, corresponding to very wide exciton bands, the strong-coupling case [168,170]. Does a similar effect occur for one-electron bands Further theoretical work would be useful. 

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