Vibronic contributions to resonant NLO responses: two-photon absorption in push-pull chromophores

Two-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 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-channel contributions to other NLO responses are shortly discussed

Effects of Lattice and Molecular Phonons on Photoinduced Neutral-to-Ionic Transition Dynamics in Tetrathiafulvalene-pp-Chloranil

For electronic states and photoinduced charge dynamics near the neutral-ionic transition in the mixed-stack charge-transfer complex tetrathiafulvalene-$p$-chloranil (TTF-CA), we review the effects of Peierls coupling to lattice phonons modulating transfer integrals and Holstein couplings to molecular vibrations modulating site energies. The former stabilizes the ionic phase and reduces discontinuities in the phase transition, while the latter stabilizes the neutral phase and enhances the discontinuities. To reproduce the experimentally observed ionicity, optical conductivity and photoinduced charge dynamics, both couplings are quantitatively important. In particular, strong Holstein couplings to form the highly-stabilized neutral phase are necessary for the ionic phase to be a Mott insulator with large ionicity. A comparison with the observed photoinduced charge dynamics indicates the presence of strings of lattice dimerization in the neutral phase above the transition temperature.Comment: 9 pages, 7 figures, accepted for publication in J. Phys. Soc. Jp

Supramolecular interactions in clusters of polar and polarizable molecules

We present a model for molecular materials made up of polar and polarizable molecular units. A simple two state model is adopted for each molecular site and only classical intermolecular interactions are accounted for, neglecting any intermolecular overlap. The complex and interesting physics driven by interactions among polar and polarizable molecules becomes fairly transparent in the adopted model. Collective effects are recognized in the large variation of the molecular polarity with supramolecular interactions, and cooperative behavior shows up with the appearance, in attractive lattices, of discontinuous charge crossovers. The mean-field approximation proves fairly accurate in the description of the gs properties of MM, including static linear and non-linear optical susceptibilities, apart from the region in the close proximity of the discontinuous charge crossover. Sizeable deviations from the excitonic description are recognized both in the excitation spectrum and in linear and non-linear optical responses. New and interesting phenomena are recognized near the discontinuous charge crossover for non-centrosymmetric clusters, where the primary photoexcitation event corresponds to a multielectron transfer.Comment: 14 pages, including 11 figure

Giant infrared intensity of the Peierls mode at the neutral-ionic phase transition

We present exact diagonalization results on a modified Peierls-Hubbard model for the neutral-ionic phase transition. The ground state potential energy surface and the infrared intensity of the Peierls mode point to a strong, non-linear electron-phonon coupling, with effects that are dominated by the proximity to the electronic instability rather than by electronic correlations. The huge infrared intensity of the Peierls mode at the ferroelectric transition is related to the temperature dependence of the dielectric constant of mixed-stack organic crystals.Comment: 4 pages, 4 figure

Asteroids' physical models from combined dense and sparse photometry and scaling of the YORP effect by the observed obliquity distribution

The larger number of models of asteroid shapes and their rotational states derived by the lightcurve inversion give us better insight into both the nature of individual objects and the whole asteroid population. With a larger statistical sample we can study the physical properties of asteroid populations, such as main-belt asteroids or individual asteroid families, in more detail. Shape models can also be used in combination with other types of observational data (IR, adaptive optics images, stellar occultations), e.g., to determine sizes and thermal properties. We use all available photometric data of asteroids to derive their physical models by the lightcurve inversion method and compare the observed pole latitude distributions of all asteroids with known convex shape models with the simulated pole latitude distributions. We used classical dense photometric lightcurves from several sources and sparse-in-time photometry from the U.S. Naval Observatory in Flagstaff, Catalina Sky Survey, and La Palma surveys (IAU codes 689, 703, 950) in the lightcurve inversion method to determine asteroid convex models and their rotational states. We also extended a simple dynamical model for the spin evolution of asteroids used in our previous paper. We present 119 new asteroid models derived from combined dense and sparse-in-time photometry. We discuss the reliability of asteroid shape models derived only from Catalina Sky Survey data (IAU code 703) and present 20 such models. By using different values for a scaling parameter cYORP (corresponds to the magnitude of the YORP momentum) in the dynamical model for the spin evolution and by comparing synthetics and observed pole-latitude distributions, we were able to constrain the typical values of the cYORP parameter as between 0.05 and 0.6.Comment: Accepted for publication in A&A, January 15, 201

Anharmonicity and NLO responses: an exact diagonalization study

Abstract: 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

The 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

Vibrational and environmental effects on NLO responses of molecular systems: what we can learn from a two-state model

Understanding 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

Vibronic contributions to resonant NLO responses: two-photon absorption in push-pull chromophores

wo-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