Optical absorptions and charge

Optical Absorptions and Charge Transfer States (Englman) Optical Activity, Theoretical Aspects of. I. Small Mole- 8 13... 

Film preparation plays a crucial role in determining the photoelectrochemical properties of phthalocyanine electrodes. Since the coupling of individual chromophores strongly depends on their relative orientation, the position of the absorption maximum and its width shows a clear dependence on the structure of thin films. Also the charge transport within phthalocyanine films, a fundamental necessity for the films to work as electrodes, depends upon the overlap of the frontier orbital wave functions. Beyond the microscopic structure of films also the morphology of films plays an important role. In the case of crystalline films, the orientation of crystallites relative to the electrode surface will be relevant because of anisotropies in optical absorption and charge transport. The size of the observed photocurrent directly depends on the real electrode surface area accessible by the electrolyte and this leads to a strong dependence on the porosity of the films. 

For a simplified case, one can obtain the rate of CL emission, =ft GI /e, where /is a function containing correction parameters of the CL detection system and that takes into account the fact that not all photons generated in the material are emitted due to optical absorption and internal reflection losses q is the radiative recombination efficiency (or internal quantum efficiency) /(, is the electron-beam current and is the electronic charge. This equation indicates that the rate of CL emission is proportional to q, and from the definition of the latter we conclude that in the observed CL intensity one cannot distii pish between radiative and nonradiative processes in a quantitative manner. One should also note that q depends on various factors, such as temperature, the presence of defects, and the... 

Dimerization causes shifts of optical absorption and emission bands of the order of several hundred cm (cf. Table I). This suggests that the triplet EPR data must be interpreted in terms of exciton and charge transfer effects. In the case of ZnTCP the effect of dim.erization on zfs values can be accounted for on the basis of rapid triplet excitation transfer between essentially unperturbed porphyrin moieties. If the exciton model applies the principal components of the zfs tensor in the dimer (X, , Z ) can be related... 

The results of this study suggest that silver atom and cluster optical absorption and fluorescent probes offer a very sensitive method for assessing the strength of a metal support interaction and a way of monitoring subtle alterations in electronic charge density at the trapping site of a metal guest. 

Electro-absorption (EA) spectroscopy, where optical absorption is observed under the application of an electric field to the sample, is another method that can distinguish between localised and inter-band excitations. The electric field produces a Stark shift of allowed optical absorptions and renders forbidden transitions allowed by mixing the wavefunctions of the excited states. Excitons show a quadratic Stark (Kerr) effect with a spectral profile that is the first derivative of the absorption spectrum for localised (Frenkel) excitons and the second derivative for charge transfer excitons, i.e. 

Candeias LP, Grozema FC, Padmanaban G, Ramakrishnan S, Siebbeles LDA, Warman JM. (2003) Positive charge carriers on isolated chains of MEH-PPV with broken conjugation Optical absorption and mobility. JPhys Chem B 107 1554-1558. 

Similar studies based on optical absorption and ESR spectra indicate that the reduction of polyacetylene by exposure to sodium naphthalide in tetrahydrofinan results in negatively charged (CH) carbanions and N" " counterions. 

Dye-sensitized systems are not suited to interpretation by conventional device physics methods on account of two features firstly, the mesoscopic phase separation of electron and hole conductors, which makes the porous material unable to sustain large electric fields and secondly, the separation, through the use of sensitizers, of optical absorption from charge transport in either material. Efforts to understand the photovoltaic action of the DSSC are leading to a reassessment of basic principles and the possibilities of novel photovoltaic designs. 

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