Vibronic absorption spectrum

Fragnito H L, Bigot J-Y, Becker P C and Shank C V 1989 Evolution of the vibronic absorption spectrum in a molecule following impulsive excitation with a 6 fs optical pulse Chem. Phys. Lett. 160 101 ... 

Guo, R., Balasubramanian, K., Wang, X. and Andrews, L. (2002) Infrared vibronic absorption spectrum and spin - orbit calculations of the upper spin-orbit component of the AU3 ground state. Journal of Chemical Physics, 117, 1614-1620. 

Each of the homogeneous lines in a vibronic absorption spectrum actually consists of a family of transitions between various rotational states of the molecule. The rotational fine structure in the spectrum can be seen for small molecules in the gas phase, but for large molecules the rotational lines are too close together to be resolved. 

Equation (B 1.1.8) gives the intensity of one vibronic band in an absorption spectrum. It is also of interest to consider... 

A different example of non-adiabatic effects is found in the absorption spectrum of pyrazine [171,172]. In this spectrum, the, Si state is a weak structured band, whereas the S2 state is an intense broad, fairly featureless band. Importantly, the fluorescence lifetime is seen fo dramatically decrease in fhe energy region of the 82 band. There is thus an efficient nonradiative relaxation path from this state, which results in the broad spectrum. Again, this is due to vibronic coupling between the two states [109,173,174]. 

A further clear establishment of the absoiption due to singlet excitons and the phonons coupled to them is the electroabsorption experiment reported in Ref. [18]. The main results are shown in Figure 9-14 the top panel shows the absorption spectrum of m-LPPP at 20 K. It becomes clear that the peaks at 2.7, 2.9, and 3.1 eV, representing A0, A i, and A2 (see Fig. 9-10) are not the only vibronic replicas. There are additional peaks between these dominant ones if the experiment is conducted at low temperature. The bottom panel in Figure 9-14 shows a so-called electroabsorption spectrum which is obtained as the modulation (or change) of the absorption under the application of an electric field. Below 3.2 eV the electroab-... 

Figure 16-36 shows the absorption spectra of thin films of four differently substituted five-ring OPVs. in contrast to the solution spectra, which show structureless low-energy absorption bands, the absorption bands of the films are structured. In the solid slate, the molecules are spatially constrained, whereas in solution different conformers exist, resulting in a distribution of accessible levels. As a consequence, some details appear in the absorption spectrum of the films which can be attributed to vibronic coupling, while, in dilute solution, the spectrum is a broad featureless band. For oct-OPV5 and Ooci-OPV5 films, the absorption maxima are red-shifted over approximately 0.1 eV relative to the solution (see Fig. 16-12). The low-energy absorption band of a thin film of Ooct-OPV5-CN" displays an appreciably larger...

Nonradiative transfer of excitation energy requires some interaction between donor and acceptor molecules and occurs if the emission spectrum of the donor overlaps the absorption spectrum of the acceptor, so that several vibronic transitions in the donor must have practically the same energy as the corresponding transitions in the acceptor. Such transitions are coupled, i.e., they are in resonance, and that is why the term resonance energy transfer (RET) or electronic energy transfer (EET) are often used. 

The vibronic coupling model has been applied to a number of molecular systems, and used to evaluate the behavior of wavepackets over coupled surfaces [191]. Recent examples are the radical cation of allene [192,193], and benzene [194] (for further examples see references cited therein). It has also been used to explain the lack of structure in the S2 band of the pyrazine absorption spectrum [109,173,174,195], and recently to study the photoisomerization of retinal [196],... 

Muller and Stock [227] used the vibronic coupling model Hamiltonian, Section III.D, to compare surface hopping and Ehrenfest dynamics with exact calculations for a number of model cases. The results again show that the semiclassical methods are able to provide a qualitative, if not quantitative, description of the dynamics. A large-scale comparison of mixed method and quantum dynamics has been made in a study of the pyrazine absorption spectrum, including all 24 degrees of freedom [228]. Here a method related to Ehrenfest dynamics was used with reasonable success, showing that these methods are indeed able to reproduce the main features of the dynamics of non-adiabatic molecular systems. 

Figure 2.6 Absorption spectrum of a solution of anthracene in benzene, and the vibronic transitions responsible for the vibrational line structure...

For some aromatic hydrocarbons such as naphthalene, anthracene and pery-lene, the absorption and fluorescence spectra exhibit vibrational bands. The energy spacing between the vibrational levels and the Franck-Condon factors (see Chapter 2) that determine the relative intensities of the vibronic bands are similar in So and Si so that the emission spectrum often appears to be symmetrical to the absorption spectrum ( mirror image rule), as illustrated in Figure B3.1. 

Weak coupling leads to minor alterations of the absorption spectrum (hypo-chromism or hyperchromism, Davidov splitting of certain vibronic bands). 

A detailed study of the electronic structure and optical properties was published for the spiro derivative of f-Bu-PBD, Spiro-PBD (40) [108]. The vibronic structure of the lowest energy absorption band is well resolved, in solution as well as in the amorphous him. The 0-0 transition is at 351 nm (3.53 eV), the 0-1 and 0-2 vibronic bands that have a higher oscillator strength, are at 336 nm (3.69 eV) and 318 nm (3.90 eV), respectively. The fluorescence spectrum of this compound is symmetrical to the absorption spectrum with a Stokes shift of 43 nm. 

Fluorescence always occurs from the lowest singlet state even if the initial excitation is to higher energy state (Kasha s rule). Azulene and some of its derivatives are exceptions to this rule. Because of vibrational relaxation of initially excited vibronic state, the fluorescence spectrum may appear as a minor image of the absorption spectrum for large polyatomic molecules. The shape of the emission spectrum is independent of the exciting wavelength. 

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