arXiv subjects

Luca Del Freo

Publications and source records attributed to Luca Del Freo.

4 recordsLinked to original sources

arXiv2002

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.

cond-mat.str-el↗

arXiv2001

Cooperative and non-linear phenomena at the neutral-ionic phase transition

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.

cond-mat↗

arXiv2001

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.

physics.chem-ph↗

arXiv2001

Static NLO susceptibilities: testing approximation schemes against exact results

The 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.

physics.chem-ph↗