Electron absorption spectroscopy , doped

Electronic Absorption Spectroscopy. Doping with iodine and SO3 had a significant effect on the absorption spectrum of /3-carotene. Triplet-triplet absorption spectra have been obtained for six carotenoids, e.g. canthaxanthin carotene-4,4 -dione (181)] in benzene, " and bimolecular rate constants for energy transfer from singlet oxygen to carotenoids evaluated. U.v. spectra of retinal, retinyl acetate, and axerophtene (182) in solid films have been determined. Several papers discuss the light absorption spectra of retinal derivatives as rhodopsin models. ... 

Pavlov, R.S., Marza, V.B., and Carda, J.B. (2002) Electronic absorption spectroscopy and colour of chromium-doped solids. 

A nonelectronic method of measuring impurity concentrations is that of absorption spectroscopy. From Eq. (36a) it is seen that ani = avnini0, where a i is the absorption constant due to electronic transitions from level i to the conduction band. The total impurity concentration Nt can be related to ni0 by a knowledge of EF. The photon-capture cross section doping experiments or by independently measuring Nt in some sample. This process has been carried out for Cr impurity (Martin, 1979) as well as (EL2) (Martin, 1981) in GaAs. The same considerations hold for photoconductivity measurements, except that t also needs to be known, as seen from Eq. (35). 

The oxide layer of a metal such as copper may be seen as a semiconductor with a band gap, which may be measured by absorption spectroscopy or photocurrent spectroscopy and photopotential measurements. Valuable additional data are obtained by Schottky Mott plots, i.e. the C 2 E evaluation of the potential dependence of the differential capacity C. For thin anodic oxide layers usually electronic equilibrium is assumed with the same position of the Fermi level within the metal and the oxide layer. The energetic position of the Fermi level relative to the valence band (VB) or conduction band (CB) depends on the p- or n-type doping. Anodic CU2O is a p-type semiconductor with cathodic photocurrents, whereas most passive layers have n-character. 

Previously, many aspects of the electronic structure of some of the diphenylpolyenes have been studied by optical absorption spectroscopy [15]. Doping-induced optical absorption studies of p-doping of polyenes of various lengths and with various end-groups [9], and even -carotene [16], have been reported. In addition, the diphenylpolyenes with x S 6 have been studied, in the gas phase, by UPS [17], and, for x <4, the data was analysed using the CNDO/S2 model [18]. Finally, diphenylpolyenes with an odd number of carbons in the polyene part of the molecule, have been studied by Tolbert et al. [19]. 

ESR spectroscopy directly proves the magnetic nature of charged conjugated molecules. Thus, combining in situ Vis-NIR and ESR spectroelectrochemistry gives a deep insight into the nature of the doped states. Indeed, this rather sophisticated technique was crucial for the assignment of the electronic absorption bands mentioned above [i25, 126]. 

Polarons, bipolarons, and solitons are different from each other as described above. It is expected that we can detect these excitations separately by electronic absorption, ESR, and vibrational spectroscopies. In particular, geometric changes induced by doping can be studied by vibrational (Raman and infrared) spectroscopy. 

It is widely accepted [7, 10] that the major species generated by chemical doping in nondegenerate polymers is bipolarons, except for polyaniline. However, the results obtained from Raman spectroscopy indicate the existence of polarons at heavily doped polymers. These results are inconsistent with the established view. The experimental basis for claiming the existence of bipolarons has been electronic absorption and ESR spectra [7, 9, 10]. We will comment on these experimental results. 

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