Polyenes excited, dynamics

In ab initio methods (which, by definiton, should not contain empirical parameters), the dynamic correlation energy must be recovered by a true extension of the (single configuration or small Cl) model. This can be done by using a very large basis of configurations, but there are more economical methods based on many-body perturbation theory which allow one to circumvent the expensive (and often impracticable) large variational Cl calculation. Due to their importance in calculations of polyene radical ion excited states, these will be briefly described in Section 4. 

A two-photon allowed absorption of poly(dihexylsilane) has been observed at 0.9 eV above the one-photon excitation63 -65, in contrast to the two-photon absorption of linear polyenes, which is about 0.5 eV below the intense one-photon absorption. Absorption spectra and the dynamics of other high-lying excited states of the polysilane have been investigated by means of femtosecond time-resolved excited-state absorption spectroscopy66. 

Allan M, Neuhaus L and Haselbach E (1984) (All-E)-1,3,5,7-octatetraene Electron-energy loss and electron-transmission spectra. Helv Chim Acta 67 1776-1782 Andersson PO andGiUbro T (1995) Photophysics and dynamics of the lowest excited singlet state in long substituted polyenes with implications for the very long chain limit. J Chem Phys 103 2509-2519... 

Petek H, Bell AJ, Choi YS, Yoshihara K, Tounge BA and Christensen RL (1995) One- and two-photon fluorescence excitation spectra of the 2 Ag states of linear tetraenes in free jet expansions. J Chem Phys 102 4726 739 Ramasesha S and Zoos ZG (1984) Correlated states in linear polyenes, radicals, and ions Exact PPP transition moments and spin densities. J Chem Phys 80 3278-3287 Raubach RA and Guzzo AV (1973) Singlet-triplet absorption spectrum of uH-trans-retinal. J Phys Chem 75 983-984 Ricci M, Bradforth SE, Jimenez R and Fleming G (1996) Internal conversion and energy transfer dynamics of spheroidene in solution and in the LH-1 and LH-2 light-harvesting complexes. Chem Phys Lett 259 381-390... 

Figure 22.29 schematically shows the model originally proposed [182] for the ultrafast energy relaxation processes in PDPA films. It contains two relaxation channels [183,184] ionic, by IS and covalent, by 2Ag, which is populated following an ultrafast phonon-assisted internal conversion from the photogenerated excitons. PAg at short time is thus due to transitions from 2Ag (dark) excitons. As in long-chain polyenes [185] and f-(CH)jj [186] these excitons are subject to ultrafast recombination dynamics and this explains the ultrafast decay dynamics seen in Figure 22.28a. In degenerate ground state polymers 2Ag is unstable with respect to the formation of soliton excitations and therefore undergoes fission into two neutral SS pairs, 2Ag => 2(S° -I- S°) [18,184], followed by further separation...

The step-potential model is the conceptually simplest approach to treating electron-phonon coupling in tt-electron systems and is the logical extension of the free electron model. Scaled to the experimental bond lengths of butadiene and benzene, the static properties of nonlinear excitations in /ran5-polyacetyIene are well described. Based on standard adiabatic approximation, the extension of the step-potential model to the study of the dynamical behavior of nonlinear excitations in trans-polyacetylene is straightforward. The time unit is scaled to the frequency of the in-phase stretching mode of the polyene lattice. However, this does not affect the course of the dynamics shown. 

The analysis of the intensity pattern in the Raman spectra of polyenes makes it possible to select one particular degree of freedom in the vibrational space (the R coordinate) that plays the leading role in determining the Raman response. ECC theory requires that this particular degree of freedom be explicitly introduced in the treatment of the dynamical problem. R is thus chosen as one of the vibrational coordinates that form the basis for the description of the vibrational modes of the molecule. The key role of the R coordinate is due to the fact that the coupling between 7r-electron excitation and nuclear configuration acts mainly along the R direction. 

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