Electronic absorption and resonance Raman spectra of the radical cation of bithiophene are reported. The bithiophene radical cation was produced by γ-radiolysis in a glassy matrix at 77 K, and the Raman spectrum excited in resonance with the two absorption bands at 425 and 590 nm. The electronic states relevant to the observed electronic transitions were identified and characterized by CASSCF calculations. The optical absorption and resonance Raman spectra were calculated by wave packet propagation methods using the ab initio calculated molecular parameters. The calculated spectra agree well with the experimental ones. The importance of carrying out full wave packet propagation calculations is underlined by the fact that in one case the simple Savin formula gave a completely wrong prediction of the resonance Raman spectrum.

The radical cation of 1,3,6,8-tetraazatricyclo [4.4.1.1(3,8)]dodecane (TTD) has been studied using magnetic resonance and optical spectroscopic methods and computational techniques. With the help of deuterated isotopomers, assignments of EPR and resonance Raman spectra could be unequivocally established. The results demonstrate that the radical cation has D(2d) symmetry, and instantaneous electron delocalization over the four equivalent nitrogen atoms occurs. This extensive delocalization in a completely saturated system is a unique feature of the TTD radical cation. The spectroscopy of TTD, in contrast to that of simpler diamines such as 1,4-diaza[2.2.2]bicyclooctane, simultaneously reveals the consequences of orbital interactions through space and through bonds. The relationship between nitrogen pyramidalization and hyperfine coupling constants in nitrogen-centered radical cations with a number of different bonding arrangements is reviewed.

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The radical cation of 2,5-dimethyl-2,4-hexadiene (tetramethylbutadiene, TMB) is studied experimentally by time-resolved resonance Raman (RR) spectroscopy and theoretically by ab initio and density functional theory calculations (UHF, UBLYP, UB3LYP, and CASSCF using the 6-31G(d) basis set). The radical cation is produced in solution at room temperature by laser flash photolysis, and its RR spectrum is excited in resonance with the optically allowed, strong, second electronic 22Au ← 12Bg transition around 365 nm. Calculated transition energies and oscillator strengths agree qualitatively well with reported absorption data. On the basis of the molecular orbital calculations carried out, the observed RR spectrum is assigned to a mixture of s-trans and gauche conformers, the first being strongly predominant. A lower limit of 42 kJ mol-1 is estimated for the torsional energy barrier between the two conformers. Equilibrium geometries and electronic transitions are discussed.

A method is introduced for prediction of the true spectral absorbance at a given wavelength from an absorbance measurement that is affected by various deficiencies of the spectral measurement channel such as finite bandwidth, cross-over at unwanted wavelength, and signal bypassing the sample. These are unwanted effects that violate the validity of the Beer–Lambert law by introducing a nonlinear dependency between the concentration and the measured absorbance. The method is based on a simple physical model of the spectral measurement channel. It may be used even for applications where only one or a few spectral channels are used, e.g., for instruments based on optical filters. The method is applied to a wastewater NO x sensor consisting of two filter-based spectral channels for absorption measurement in the UV region. Simulations show that dramatic improvement in the accuracy is obtained by introducing only one additional parameter for nonlinear correction as compared to the linear model. Calibration of the model is discussed.

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A new method for characterization of uniaxial planar waveguides from their measured effective mode indices is presented. The theory is outlined and expressions for efficient computer analysis are given. Uniaxial waveguide samples have been made in c-cut LiNbO(3) by proton exchange with and without post annealing in order to test the method on both steplike and graded-index profiles. The resulting characterization of the samples is discussed in relation to the inverse WKB method. Finally, the importance of incorporating the effects of material birefringence in the characterization of these kinds of waveguides is investigated.