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By means of one- and two-dimensional transient infrared spectroscopy and femtosecond stimulated Raman spectroscopy, we investigated the excited state dynamics of peridinin, a carbonyl carotenoid occurring in natural light harvesting complexes. The presence of singly and doubly excited states, as well as of an intramolecular charge transfer (ICT) state, makes the behavior of carbonyl carotenoids in the excited state very complex. In this work, we investigated by time resolved spectroscopy the relaxation of photo-excited peridinin in solvents of different polarities and as a function of the excitation wavelength. Our experimental results show that a characteristic pattern of one- and two-dimensional infrared bands in the C=C stretching region allows monitoring the relaxation pathway. In polar solvents, moderate distortions of the molecular geometry cause a variation of the single/double carbon bond character, so that the partially ionic ICT state is largely stabilized by the solvent reorganization. After vertical photoexcitation at 400 nm of the S2 state, the off-equilibrium population moves to the S1 state with ca. 175 fs time constant; from there, in less than 5 ps, the non-Franck Condon ICT state is reached, and finally, the ground state is recovered in 70 ps. That the relevant excited state dynamics takes place far from the Franck Condon region is demonstrated by its noticeable dependence on the excitation wavelength.

The time-resolved optical Kerr effect spectroscopy (OKE) is a powerful experimental tool enabling accurate investigations of the dynamic phenomena in molecular liquids. We introduced innovative experimental and fitting procedures, that enable a safe deconvolution of sample response function from the instrumental function. This is a critical issue in order to measure the dynamics of liquid water. We report OKE data on water measuring intermolecular vibrations and the structural relaxation processes in an extended temperature range, inclusive of the supercooled states. The unpreceded data quality makes possible a solid comparison with few theoretical models: the multi-mode Brownian oscillator model, the Kubo's discrete random jump model, and the schematic mode-coupling model. All these models produce reasonable good fits of the OKE data of stable liquid water, i.e., over the freezing point. The features of water dynamics in the OKE data becomes unambiguous only at lower temperatures, i.e., for water in the metastable supercooled phase. We found that the schematic mode-coupling model provides the more rigorous and complete model for water dynamics, even if its intrinsic hydrodynamic approach does not give a direct access to the molecular information.

We have studied the effect of transient vibrational inversion of population in trans-β-apo-8(')-carotenal on the time-resolved femtosecond stimulated Raman scattering (TR-FSRS) signal. The experimental data are interpreted by applying a quantum mechanical approach, using the formalism of projection operators for constructing the theoretical model of TR-FSRS. Within this theoretical frame we explain the presence of transient Raman losses on the Stokes side of the TR-FSRS spectrum as the effect of vibrational inversion of population. In view of the obtained experimental and theoretical results, we conclude that the excited S2 electronic level of trans-β-apo-8(')-carotenal relaxes towards the S0 ground state through a set of four vibrational sublevels of S1 state.

Hydrogen bond interactions strongly affect vibrational properties and frequencies, the most common consequence being a redshift of the stretching vibration involved; there are, however, few exceptions to this general trend. In previous works, we have proved the effectiveness of ab initio simulations combined with wavelet analysis to investigate these effects and put them into relation to structural environment. In this work, we investigate the hydrogen bond effects on the structural and vibrational properties of 1,3-propanediol in acetonitrile by a combined experimental and computational approach. We explain the appearance of two spectral components in the O-H stretching band on the basis of intra- and intermolecular hydrogen bond interactions. We also elucidate the blueshift of the C≡N stretching band as due to a hydrogen bond interaction between the glycol and acetonitrile that modify the electron density distribution inside the CN group. This effect is well reproduced by ab initio molecular dynamics simulations and density functional calculations reported in this work.

Vibrational dynamics of liquid formamide is studied in the spectral region of the amide I mode by means of linear and two-dimensional infrared spectroscopies. The two-dimensional spectrum has a complex structure to be connected to the partially excitonic nature of the vibrational states. The measurements performed on a 1:10 (12)C:(13)C formamide isotopic mixture allow separating the broadening contribution due to the inhomogeneous frequency distribution of the local oscillators from that of excitonic origin. A model based on the Kubo picture of the line broadening is used, together with the dynamic information obtained from a molecular dynamics simulation, to fit the spectra of the (12)C formamide impurity in the isotopic mixture. The relevant dynamical information, such as the amplitude of the frequency fluctuations, lifetime of the second vibrational excited state, and anharmonicity, is thus recovered. By appropriately combining the outcomes of experiments and molecular dynamics simulation, we demonstrate that motional narrowing determines the line shape of the amide I resonance to a large extent. The same analysis provides an estimate of the transition dipole moment of formamide, which results in good agreement with an ab initio calculation. The calculated frequency fluctuation correlation time is found to be comparable to the hydrogen-bond lifetime, which defines the basic structural relaxation rate of the networked liquid.

Dynamics of single molecules in liquids, inspected in the picosecond time scale by means of spectroscopic measurements or molecular-dynamics (MD) simulations, reveals a complex behavior which can be addressed as due to local confinement (cage). This work is devoted to the analysis of cage structures in liquid benzene, obtained from MD simulations. According to a paradigm proposed for previous analysis of atomic and molecular liquids [see, for example, A. Polimeno, G. J. Moro, and J. H. Freed, J. Chem. Phys. 102, 8094 (1995)], the istantaneous cage structure is specified by the frame of axes which identifies the molecular configuration at the closest minimum on the potential-energy landscape. In addition, the modeling of the interaction potential between probe molecule and molecular environment, based on symmetry considerations, and its parametrization from the MD trajectories, allows the estimation of the structural parameters which quantify the strength of molecular confinement. Roto-translational dynamics of probe and related cage with respect to a laboratory frame, dynamics of the probe within the cage (vibrations, librations, re-orientational motions), and the restructuring processes of the cage itself are analyzed in terms of selected time self-correlation functions. A time-scale separation between the processes is established. Moreover, by exploiting the evidence of fast vibrational motions of the probe with respect to the cage center, an orientational effective potential is derived to describe the caging in the time scale longer than approximately 0.2 ps.