1,720,965 research outputs found
Anharmonicity and NLO responses: an exact diagonalization study
No full textAbstract: We present the exact numerical diagonalization of the Mulliken donor-acceptor (DA) dimer with Holstein coupling. The resulting eigenstates are introduced in sum-over-states expressions of static optical susceptibilities. The careful partitioning of the sum, and the comparison with spectral properties give important clues on the role of electron-phonon (e-ph) coupling. Anharmonicity does not appreciably affect vibrational spectra. nor linear electronic spectra, and is irrelevant for the static linear polarizability. By contrast, huge anharmonic contributions to hyperpolarizabilities are found: the harmonic approximation is unreliable for the calculation of non-linear responses, even for systems where it hardly affects linear optical spectra. (C) 2001 Elsevier Science B.V. All rights reserved
Understanding non-linearity: a simple model for push-pull chromophores
No full textThe spectral properties of the push-pull chromophores were analyzed and discussed in detail. The analysis accounted for the large anharmonicity induced by electron-phonon coupling. Two electronic states were considered where both were separated by an energy difference and were linearly coupled to one or more harmonic vibrational modes
Vibronic contributions to resonant NLO responses: two-photon absorption in push-pull chromophores
No full textwo-photon absorption (TPA) spectra of push-pull chromophores are described in terms of a two-state model accounting for electron-vibration coupling. Vibrations have two main effects in TPA spectra. The most obvious one is the appearance of a vibrational structure in the spectrum; in this respect we find large Herzberg-Teller (HT) contributions. The second effect was not recognized so far: vibrational states contribute a new channel to TPA process, that shows up with a blue-shift and a distortion of the spectrum
Vibrational and environmental effects on NLO responses of molecular systems: what we can learn from a two-state model
No full textUnderstanding the properties of molecular materials for NLO is an obvious prerequisite for their use in advanced applications. But a proper modeling of these systems is challenging: their large optical nonlinearity implies large and non-linear responses to several interactions besides applied electric fields, spoiling standard (linear) perturbative approaches to electron-vibration (e-ph) coupling and/or to environmental effects. The Holstein donor-acceptor dimer is a simple two state model, that, relevant to push-pull chromophores, contains he main ingredients to understand non-linearity in molecular materials. Exact solutions of the coupled e-ph problem are easily obtained for this toy-model and are used to test several common approximation schemes for the calculation of NLO responses. The non-linearity of the interaction between electronic and slow degrees of freedom shows up with the anharmonicity of the exact potential energy surfaces relevant to the ground and excited state, as well as with a large dependence of electronic properties on slow coordinates. Anharmonic corrections, negligible in vertical (coherent) linear and non-linear processes, are prominent in incoherent non-linear processes and are responsible for the large amplification of static NLO responses as due to the coupling of electronic an bosonic degrees of freedom. The dependence of the electronic dipole moments and polarizabilities on slow coordinates shows up with non-Condon effects in coherent processes, that are responsible for the large infrared and Raman intensity of the coupled vibrational modes, as well as for the opening of new vibrational channel contributions to NLO processes, with no counterpart in linear spectroscopy
Infrared intensity and local vibrations of charged solitons
No full textAbstract: We present a simple microscopic model for infrared active vibrations (IRAV's) of charged solitons in trans-polyacetylene (PA), by combining the Su-Schrieffer-Heeger model for pi electrons and a force field based on pristine PA. The model rationalizes the one-to-one correspondence between Raman modes of pristine PA samples and IRAV's of doped or photoexcited samples and provides a microscopic basis for the phenomenological amplitude mode formalism. The softening of the IRAV modes relative to Raman frequencies and their huge intensities are clearly due to their coupling to pi electrons, and specifically to the large pi-electron fluctuations induced by small nuclear displacements along a single local coordinate. The model accounts for effective mass and infrared spectra of photoexcited samples, and qualitatively, for dopant-induced spectra. Our microscopic picture also clarifies two long-standing puzzles. A soliton defect extending over similar to 14 sites generates many local modes, but only special ones are coupled strongly to pi electrons. At the same time, local vibrations are sensitive to chain lengths much longer than the defect due to the long electronic coherence length
Cooperative and non-linear phenomena at the neutral-ionic phase transition
No full textAbstract: The complex interplay among on-site energy, Hubbard U, and coupling to Holstein and Peierls phonons at the neutral-ionic phase transition is discussed using diagrammatic valence bond calculations. The charge transfer and dimerization amplitudes and the infrared intensity of molecular vibrations are studied in the transition region. (C) 2002 Elsevier Science B.V. All rights reserved
Polyacetylene oligomers,: pi-electron fluctuations, vibrational intensities, and soliton confinement
No full textStatic nonlinear optical susceptibilities: testing approximation schemes against exact results
No full textThe reliability of the approximations commonly adopted in the calculation of static optical (hyper) polarizabilities is tested against exact results obtained for an interesting toy-model. The model accounts for the principal features of typical nonlinear organic materials with mobile electrons strongly coupled to molecular vibrations. The approximations introduced in sum over states and finite field schemes are analyzed in detail. Both the Born–Oppenheimer and the clamped nucleus approximations turn out to be safe for molecules, whereas for donor–acceptor charge transfer complexes deviations from adiabaticity are expected. In the regime of low vibrational frequency, static susceptibilities are strongly dominated by the successive derivatives of the potential energy and large vibrational contributions to hyperpolarizabilities are found. In this regime anharmonic corrections to hyperpolarizabilities are very large, and the harmonic approximation, exact for the linear polarizability, turns out totally inadequate for nonlinear responses. With increasing phonon frequency the role of vibrations smoothly decreases, until, in the antiadiabatic (infinite vibrational frequency) regime, vibrations do not contribute anymore to static susceptibilities, and the purely electronic responses are regained
- …
