Marilena Ricci

Diffusive and oscillatory dynamics of liquid iodobenzene measured by femtosecond optical Kerr effect

The dynamics of liquid iodobenzene are studied by means of time resolved heterodyne detected optical Kerr effect in a wide temperature range (263–371 K). In the picosecond regime the relaxation is characterized by a biexponential decay, attributed to the rotational diffusion of an anisotropic rotator. The temperature dependence of the two relaxation times agrees only in part with the prediction of the hydrodynamic theory. The subpicosecond dynamics is essentially oscillatory in nature; the Raman spectra obtained by Fourier transform of the time domain data show the contribution of two intramolecular low-frequency vibrations, and that of the intermolecular dynamics. The intermolecular spectra at different temperatures are interpreted on the basis of the Brownian oscillator model, and consist of the superposition of overdamped and underdamped modes. The intermolecular spectrum of the liquid shows a close resemblance with the low-frequency Raman spectrum of crystalline iodobenzene, and suggests an interpreta...

Optical Kerr-effect investigation of the reorientational dynamics of CS2 in CCl4 solutions

The reorientational relaxation of CS2 in solution in CCl4 and its dependence on both concentration and temperature is investigated with the use of the optical Kerr effect (OKE). The comparison of the single-particle reorientational relaxation time extracted from OKE data and obtained by Raman spectroscopy supports the hypothesis that the vibrational and reorientational relaxation processes for CS2 are not correlated. It is shown that the reorientational dynamics of CS2 is influenced by the solvent configuration around the solute molecule (cage effect), and that it is also influenced by the pair orientational correlation between solute molecules, even in dilute solutions.

Translation-rotation coupling in transient grating experiments : Theoretical and experimental evidences

The results of a Transient Grating experiment in a supercooled molecular liquid of anisotropic molecules and its theoretical interpretation are presented. These results show the existence of two distinct dynamical contributions in the response function of this experiment, density and orientation dynamics. These dynamics can be experimentally disentangled by varying the polarisation of the probe and diffracted beams and they have been identified and measured in a Heterodyne Detected experiment performed on m-toluidine. The results of the theory show a good qualitative agreement with the measurements at all temperatures.

Time resolved optical Kerr effect analysis of urea–water system

The nuclear dynamics of urea aqueous solution was analyzed by time resolved optical Kerr effect (OKE). The data analysis was achieved in time and in frequency domains. Three relaxation times characterize the time decay of the OKE signal at high mole fractions of urea, while only two relaxation times characterize this decay for the low mole fractions. The observed slowest relaxation time increases with increasing the mole fraction of urea. The comparison between this relaxation time and the ones determined by Raman and nuclear magnetic resonance spectroscopies suggests that the slow relaxation time is related to the reorientation of an axis lying in the plane of the urea molecule. At high mole fractions, the power spectra derived from the Fourier transform of the OKE signal are characterized by one broad peak at around 70 cm−1 and by a shoulder at around 160 cm−1 in the high frequency part of the former peak. This shoulder is related to the hydrogen bond interactions which involve urea molecules. Molecular...

The fast dynamics of benzene in the liquid phase. Part I. Optical Kerr effect experimental investigation

Femtosecond heterodyne detected optical Kerr effect experiments are performed on liquid benzene in a wide temperature range. Besides the diffusive orientational relaxation, at short times an oscillatory behaviour is observed, superimposed to a faster quasi-exponential decay. The spectral densities obtained by Fourier transform are characterised by a broad band in the region 0–150 cm−1, and their profiles show a marked temperature dependence. Similar behaviour is observed in a 1:6 molar solution of benzene in carbon tetrachloride. The experimental observations can be interpreted by assuming that the basic microscopic system, able to account for the main dynamical properties of the liquid at short times, consists of a benzene molecule librating and oscillating in a “ solvent” cage; consequently, the width of the observed intermolecular vibrational band has an essentially inhomogeneous origin. With the assumption of a bi-modal structure of the frequency distribution, a Kubo treatment is able to reproduce the main features of the experimental spectra at different temperatures. In particular, it is confirmed that the peculiar low frequency shape of the spectrum, corresponding to the fast quasi-exponential decay observed in the time-domain experiments, can be attributed to the motional narrowing effect. The value of the Kubo correlation time τc thus estimated ranges between 0.70 and 0.25 ps on going from temperatures close to the benzene melting point to near the boiling temperature.

Alternative SERRS probes for the immunochemical localization of ovalbumin in paintings: an advanced mapping detection approach.

In the field of analytical chemistry, many scientific efforts have been devoted to develop experimental procedures for the characterization of organic substances present in heterogeneous artwork samples, due to their challenging identification. In particular, performances of immunochemical techniques have been recently investigated, optimizing ad hoc systems for the identification of proteins. Among all the different immunochemical approaches, the use of metal nanoparticles - for surface enhanced Raman scattering (SERS) detection - remains one of the most powerful methods that has still not been explored enough for the analysis of artistic artefacts. For this reason, the present research work was aimed at proposing a new optimized and highly efficient indirect immunoassay for the detection of ovalbumin. In particular, the study proposed a new SERRS probe composed of gold nanoparticles (AuNPs) functionalised with Nile Blue A and produced with an excellent green and cheap alternative approach to the traditional chemical nanoparticles synthesis: the laser ablation synthesis in solution (LASiS). This procedure allows us to obtain stable nanoparticles which can be easily functionalized without any ligand exchange reaction or extensive purification procedures. Moreover, the present research work also focused on the development of a comprehensive analytical approach, based on the combination of potentialities of immunochemical methods and Raman analysis, for the simultaneous identification of the target protein and the different organic and inorganic substances present in the paint matrix. An advanced mapping detection system was proposed to achieve the exact spatial location of all the components through the creation of false colour chemical maps.

Time-resolved optical Kerr effect experiments on supercooled benzene and test of mode-coupling theory

We have performed time-resolved optical Kerr effect experiments with heterodyne detection on benzene. We succeeded in supercooling benzene by 21 K below the melting point T m = 279 K, and we investigated the full range of existence from the supercooled phase up to the boiling point T b = 353 K. Our time-resolved data show clearly the complex relaxation pattern of benzene that is characterized by different time scales. These dynamic features, common to many other molecular liquids, to date have not been addressed and there is not a unique theoretical model able to explain them, even in a simple molecular liquid such as benzene. We compare our data with the predictions of a schematic mode-coupling model, fitting the experimental relaxations with a numerical solution of the time-dependent correlation functions. Although the temperature range investigated is clearly outside the asymptotic scaling regime, we found the mode-coupling model able to describe properly the measured dynamics in large time and temperature ranges.

Orientational Dynamics in the Isotropic Phase of a Nematic Mixture: Subpicosecond Time Resolved Optical Kerr Effect Experiments on ZLI-1167 Liquid Crystal

Abstract We have investigated the orientational dynamics of the ternary mixture of propyl, pentyl and heptyl cyano-cyclohexyls (ZLI-1167) in the isotropic phase over a wide temperature range (372-453 K) by means of transient optical Kerr effect spectroscopy. The orientational dynamics of ZLI-1167 isotropic phase shows a complex relaxation pattern where several dynamical processes take place. We substantially observed three time scales of relaxation: fast (from hundreds of fsec to few psec), intermediate (from tens to hundreds of psec) and slow (from hundreds of psec to few nsec). The slow orientational relaxation shows a temperature dependence characterized by strong pre-transitional effects. The critical slowing down of these relaxation times can be correctly interpreted by Landau-de Gennes theory. The fast and intermediate decay times show a different temperature dependence without any observable pre-transitional effects. We suggest an interpretation of the experimental results in term of averaged singl...

Ablation of transition metal oxides by different laser pulse duration and thin films deposition

Thin films of transition metal oxides are of interest in many applications such as anticorrosion coatings and optical and electrochromic devices. In this work, the effect of different wavelength and pulse duration on ablation of oxides target has been investigated. The plume has been characterized by mass spectrometry and optical spectroscopy. Ablation thresholds have been measured by detecting ion emission from the target and from the onset of the plume luminescence. Morphology and optical properties of thin films deposited in different conditions are presented.

Temperature dependence of the reorientational dynamics and low-frequency response of aqueous urea solutions investigated by femtosecond optical Kerr-effect spectroscopy and molecular-dynamics simulation

The time resolved optical Kerr effect (OKE) has been applied to aqueous urea (χurea = 0.18) at different temperatures ranging from 283 to 333 K in order to investigate the behaviour of the different reorientational relaxation times and the low frequency OKE response of the system. The relaxation times follow the Debye–Stokes–Einstein (DSE) theory. These results reveal that at a microscopic level the reorientation of a molecule of urea is anisotropic. This anisotropy is expressed as an orientational correlation between urea molecules, as revealed by both the asymptotic decay of the Nurea–Nurea radial distribution function and by the angular distribution probability at different distances between urea molecules. It is speculated that this anisotropy plays an important role in the denaturation of proteins in aqueous urea solutions. As the molecular dynamics simulations suggest, the low-frequency OKE response is analysed in terms of the translational dynamics of the molecules in the solution. The slight shift of the position of the peak associated with the hydrogen bond interactions between water molecules suggests that urea acts as a slight structure breaker.