Vibronic fine state

Comparison of the UV spectrum of polystyrene in the 2600 A region with that of toluene shows a close relationship in terms of both extinction coefficients and vibronic fine structure. The effect of para substituents is most conveniently characterized by the shift in the band corresponding to the a0-o transition. The comparison of substituent effects on the electronic excited states of thepara substituted polystyrenes parallels those for the corresponding para substituted toluenes. Such a correlation would only be expected if the tr - n transitions were effectively localized within a given pendant group of the polymer system. This conclusion is reinforced by the observation that polystyrene and toluene show similar shake up structure in their ESC A spectra with respect to both band profiles and intensities (when due... 

The present studies were undertaken as a prelude to an analyses of the electronic Si 7 S0 band spectrum of CB. A simulation of the vibronic fine structure in the jet-cooled ultraviolet system requires a knowledge of the puckering-wagging hypersurfaces of both the ground and excited electronic states as well as the molecular structures. This paper represents the first of a two part spectroscopic study of cyclobutanone and an extension to a recent calculation on the S0 and Si states [7]. 

Probable causes for the multiple emission are (a) vibronic fine structure of the squaraine emission, (b) emission from a relaxed excited state, and (c) emission from an exciplex (with solvent) or the excited state of the squaraine-solvent complex. To differentiate these possibilities, the effects of solvent, temperature, and structural changes on the multiple emission have been studied. Sq4 was chosen as a model for these investigations because of its high solubility in... 

Unfortunately, the experimental information for anthracene is not as rich as for naphthalene. Transitions to the lowest 7r-excited state occur at 1.1 eV and are dipole forbidden. In addition, PES [161,162] predicts transitions at 1.76,2.73, and 2.81 eV. The EA spectrum [166] of anthracene recorded in argon matrices agrees with the absorption maxima in organic glasses [140], and the vibronic fine structure is in accord with the Raman spectrum of polycrystalline anthracene [167] at room temperature. The CASSCF/CASPT2 calculations presented by... 

As depicted in Figure 5.5 PFs in film display an unstructured, long absorption maximum centered at 3.3 eV. The photoluminescence emission spectrum of PFs shows a vibronic fine structure with an energetic spacing of 180 meV (stretching vibration of the C = C-C = C structure of the polymer backbone) with the transition at 2.9 eV yielding a deep blue emission. In dilute solution the spectra are very similar to that of the solid state and only a small bathochromic shift of 20 meV is typically observed for both absorption and emission. 

Vibronic-coupling theory has been a well established area of research since many years. The basic elements of the theory are the concept of dia-batic electronic states, the normal-mode description of vibrational motion, and the application of symmetry selection rules to derive appropriate model Hamiltonians. The applications of vibronic-coupling theory cover the full range of molecular spectroscopy, including, in particular, optical absorption and emission and photoelectron spectroscopy. Typical spectroscopic phenomena associated with vibronic interactions are the appearance of nominally forbidden electronic bands, the excitation of nontotally symmetric modes, or unusual and complex vibronic fine structures of electronic spectra. A fairly comprehensive and up-to-date exposition of vibronic-coupling theory is provided by the monograph of Bersuker and Polinger. ... 

Aromatic hydrocarbons such as pyrene have also been employed as a luminescence probe of polarity and microviscosity in a variety of organized assemblies (109). Pyrene is a good excimer-forming probe due to the long lifetime of fluorescence and formation of excited-state dimers (excimers) at low concentration. Figure 9 shows an example pyrene luminescence spectrum. The ratio of excimer to monomer fluorescence intensity is often utilized as a measure of pyrene mobility and proximity. The vibronic fine structure of the pyrene monomer is sensitive... 

Excited states and, to a lesser extent, the ground state may be sensitive to changes in environmental parameters such as temperature, pressure, viscosity and polarity. This manifests itself in a number of quantifiable parameters, such as (i) spectral shifts (ii) change in the emission quanmm yield (iii) change in the nature of the emissive state (iv) change in non-radiative and radiative decay rates (v) variations in the intensity and resolution of vibronic fine structure and (vi) the emergence of excimer or exciplex emission. These response mechanisms can be used to design sensor platforms to monitor both microscopic and macroscopic environmental parameters. 

The use of poor solvents or solvent mixtures can further allow aggregation and crystallization directly from solution, which is the subject of the following section. This aggregation is accompanied by a bathochromic shift of the absorption maxima and the appearance of a vibronic fine structure (see also Sect. 3). In the extreme case of solid-state spectra of P3HT films, these effects are even more pronounced, with the spectra also becoming more structured for high molecular weights (see Sect. 4.1.1). 

If the experunental technique has sufficient resolution, and if the molecule is fairly light, the vibronic bands discussed above will be found to have a fine structure due to transitions among rotational levels in the two states. Even when the individual rotational lines caimot be resolved, the overall shape of the vibronic band will be related to the rotational structure and its analysis may help in identifying the vibronic symmetry. The analysis of the band appearance depends on calculation of the rotational energy levels and on the selection rules and relative intensity of different rotational transitions. These both come from the fonn of the rotational wavefunctions and are treated by angnlar momentum theory. It is not possible to do more than mention a simple example here. 

For the variational calculations of the vibronic spectrum and the spin-orbit fine structure in the X H state of HCCS the basis sets involving the bending functions up to 0i = 02 = 11 with all possible and I2 values are used. This leads to the vibronic secular equations with dimensions 600 for each of the vibronic species considered. The bases of such dimensions ensure full... 

For electronic or vibronic transitions there is a set of accompanying rotational transitions between the stacks of rotational levels associated with the upper and lower electronic or vibronic states, in a rather similar way to infrared vibrational transitions (Section 6.1.4.1). The main differences are caused by there being a wider range of electronic or vibronic transitions they are not confined to 2" — 2" types and the upper and lower states may not be singlet states nor need their multiplicities to be the same. These possibilities result in a variety of types of rotational fine structure, but we shall confine ourselves to 2" — 2" and — types of transitions only. 

Radiative transitions may be considered as vertical transitions and may therefore be explained in terms of the Franck-Condon principle. The intensity of any vibrational fine structure associated with such transitions will, therefore, be related to the overlap between the square of the wavefunctions of the vibronic levels of the excited state and ground state. This overlap is maximised for the most probable electronic transition (the most intense band in the fluorescence spectrum). Figure... 

The intensity distributions of well resolved vibronic spectra recorded in absorption and emission at low temperature are used to determine the geometric distortions of the electronically excited states of coordination compounds. In particular for complexes of lower symmetry, band analysis is necessary leading to results with which bond distance changes can be calculated. For spectra exhibiting no vibrational fine structure, a new technique is proposed which uses time resolved methods, considering deviations from the Poisson distribution of photons by recording time intervals between two successively emitted photons. 

The nature of the Jahn-Teller interaction is such as to yield several vibronic states, within just a few wave numbers of the ground state. These states have very different expectation values for the electronic and nuclear co-ordinates, and we shall now examine how the low-lying vibronic structure may be elucidated by the followings modem physical techniques high-field, multifrequency electron paramagnetic resonance (HFMF EPR) magnetic circular dichroism (MCD), and extended X-ray absorption fine stmcture spectroscopy (EXAFS). The recent advent of HFMF EPR... 

The transition from the ground to the excited state, where the excitation goes from v = 0 (in the ground state) to v = 2 (in the excited state), is the most probable for vertical transitions because it falls on the highest point in the vibrational probability curve in the excited state. Yet many additional transitions occur, so that the fine structure of the vibronic broad band is a result of the probabilities for the different transitions between the vibronic levels. 

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