Characterizing electronic transitions

CD spectroscopy appeared highly suitable because compounds 10-12 possess two chromophores, naphthalene and r-triazine, having electrically allowed, well-characterized electronic transitions. In Figure 10 is reported the absorption and CD spectra (acetonitrile solution) of 2,4,6-tris[(7 )-l(l-naphthyl)ethylamino]-l,3,5-triazine 12 <2000EJ01767>. 

F. Wang, J. Luo, K. Yang, J. Chen, F. Huang, Y. Cao, Conjugated fluorene and silole copolymers synthesis, characterization, electronic transition, light emission, photovoltaic cell and field effect hole mobility. Macromolecules, 38, 2253-2260 (2005). 

All of these time correlation functions contain time dependences that arise from rotational motion of a dipole-related vector (i.e., the vibrationally averaged dipole P-avejv (t), the vibrational transition dipole itrans (t) or the electronic transition dipole ii f(Re,t)) and the latter two also contain oscillatory time dependences (i.e., exp(icofv,ivt) or exp(icOfvjvt + iAEi ft/h)) that arise from vibrational or electronic-vibrational energy level differences. In the treatments of the following sections, consideration is given to the rotational contributions under circumstances that characterize, for example, dilute gaseous samples where the collision frequency is low and liquid-phase samples where rotational motion is better described in terms of diffusional motion. 

The ability to measure the energy of electronic transitions and their line widths accurately, in a convenient manner, is one of the most important aspects of semiconductor characterization. The former can be used to evaluate alloy compositions... 

X-ray absorption spectroscopy combining x-ray absorption near edge fine structure (XANES) and extended x-ray absorption fine structure (EXAFS) was used to extensively characterize Pt on Cabosll catalysts. XANES Is the result of electron transitions to bound states of the absorbing atom and thereby maps the symmetry - selected empty manifold of electron states. It Is sensitive to the electronic configuration of the absorbing atom. When the photoelectron has sufficient kinetic energy to be ejected from the atom It can be backscattered by neighboring atoms. The quantum Interference of the Initial... 

XANES spectroscopy shows that a narrow and intense pre-edge peak at 4967 eV, due to the Is 3pd electronic transition involving Ti atoms in tetrahedral coordination, is present in well-manufactured TS-1 (Fig. 2c). Conversely this electronic transition of Ti(IV) species in Ti02 (anatase or rutile) is characterized by a very low intensity due to the small pd hybridization in octahedral symmetry. Indeed the transitions l2g are symmetrically forbidden in the case of octahedral coordination of Ti (IV), but the transition Ai T2 is allowed in the case of tetrahedral coordination of Ti(IV), as in the case of [Ti04] units [52,58-61,63,68]. 

The light reflected by a powdered solid will consist of a specular reflection component and of a diffuse reflection component. The specular component represents reflection of the incident light by the surfaces of the component particles, and it is characterized by a complete absence of light transmission through the interiors of the particles. By contrast, diffuse reflectance is associated with the radiation that penetrates into the particles to some extent and that then emerges from the bulk solid. This light will exhibit spectral characteristics that are modified from those of the incident beam by the electronic transitions that took place within the solid phase and at the boundaries of the component particles. 

Apart from the more conventional transport measurements of molecular junctions at constant bias voltage, alkane(di)thiols-based molecular junctions were also characterized by transition voltage spectroscopy [258, 259], AC voltage modulation [260], and inelastic electron tunneling spectroscopies [261],... 

UV-VIS spectra result from electronic transitions occurring in the analyte molecules and complex ions. Since vibrational levels are superimposed on the electronic levels, many electronic transitions are possible (refer to Figure 7.15 and accompanying discussion), resulting in continuous spectra rather than the line spectra that characterize spectra of atoms. 

Their spectral patterns are characterized by the presence of several bands red-shifted relative to the 0 electronic Sj Sq origin of the bare chromophore Cr at 37618 cm (peak B). The most intense one is separated from the peak B of the chromophore by the A values reported in the figure. Since a red-shifted Sj Sq origin denotes an increase of attractive interactions in the complex by the electronic transition, the Av values of Fig. 10b, c, and d, indicate that bond strengthening follows the order [Cr-Mrr] > [C/j-M/js] > [C/j-Mss]. 

The electron transitions depicted in Fig. 10 correspond to transitions of the system between states characterized by different adsorption curves. Such adsorption curves which represent the energy of the system E as function of the distance r between the particle C and the adsorbent surface for the case when particle C is a monovalent atom are schematically depicted in Fig. 11 (3, 4)- The curve I represents adsorption on an unexcited crystal, i.e., on a crystal that does not contain free electrons and holes. Curve I represents curve I shifted a distance u upwards parallel to itseff that is, it corresponds to adsorption on an excited crystal containing a free electron (in the conduction band) and a free hole (in the valence band). Curves p and n represent the adsorption curves for, respectively, strong donor, and strong acceptor chemisorption (curve n can lie either below or above curve p). The minima of curves I, n, p, I correspond to the states OL, CbL d- pL, CpL eL, CL cL -1- pL. 

The complexity of the low temperature MCD spectra of the oxidized and reduced trinuclear cluster shows the multiplicity of the predominantly S — Fe charge transfer transitions that contribute to the absorption envelope. While MCD spectroscopy provides a method of resolving the electronic transitions, assignment cannot be attempted without detailed knowledge of the electronic structure. However, the complexity of the low temperature MCD spectra is useful in that it furnishes a discriminating method for determining the type and redox state of protein bound iron-sulfur clusters. Each well characterized type of iron-sulfur cluster, i.e. [2Fe-2S], [3Fe-4S], and [4Fe-4S], has been shown to have a characteristic low temperature MCD spectrum in each paramagnetic redox state (1)... 

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