Alessandro Gessini

Two-colour pump–probe experiments with a twin-pulse-seed extreme ultraviolet free-electron laser

Exploring the dynamics of matter driven to extreme non-equilibrium states by an intense ultrashort X-ray pulse is becoming reality, thanks to the advent of free-electron laser technology that allows development of different schemes for probing the response at variable time delay with a second pulse. Here we report the generation of two-colour extreme ultraviolet pulses of controlled wavelengths, intensity and timing by seeding of high-gain harmonic generation free-electron laser with multiple independent laser pulses. The potential of this new scheme is demonstrated by the time evolution of a titanium-grating diffraction pattern, tuning the two coherent pulses to the titanium M-resonance and varying their intensities. This reveals that an intense pulse induces abrupt pattern changes on a time scale shorter than hydrodynamic expansion and ablation. This result exemplifies the essential capabilities of the jitter-free multiple-colour free-electron laser pulse sequences to study evolving states of matter with element sensitivity.

EIS: the scattering beamline at FERMI

The Elastic and Inelastic Scattering (EIS) beamline at the free-electron laser FERMI is presented. It consists of two separate end-stations: EIS-TIMEX, dedicated to ultrafast time-resolved studies of matter under extreme and metastable conditions, and EIS-TIMER, dedicated to time-resolved spectroscopy of mesoscopic dynamics in condensed matter. The scientific objectives are discussed and the instrument layout illustrated, together with the results from first exemplifying experiments.

The mixed longitudinal?transverse nature of collective modes in water

We report high-resolution, high-statistics inelastic x-ray scattering measurements of the dynamic structure factor of water as a function of momentum and energy transfer in various thermodynamic conditions, including high-pressure liquid near the melting point, supercooled liquid and polycrystalline ice. For momentum transfer values below 8 nm−1, two collective excitations associated with longitudinal and transverse acoustic modes were observed. Above 8 nm−1, another excitation was detected in the liquid. Comparison with polycrystalline data and molecular dynamics simulations suggests that this mode is related to longitudinal–transverse mixing of mode symmetry.

Investigation of Acetic Acid Hydration Shell Formation through Raman Spectra Line-Shape Analysis

Raman spectra of acetic acid aqueous solutions in the 500-4000 cm(-1) range have been measured as a function of water concentration to investigate the hydration shell formation mechanism around the acetic acid molecules. A fitting procedure based on the Kubo-Anderson model has been applied to the spectra. This has allowed us to determine the average lifetime of the hydrogen bonds involving a given functional group, as well as their geometrical distribution as a function of water concentration. The comparison of our results with literature data has demonstrated that the fitting model is adequate to describe organic water mixtures. Finally, the role of water in the formation of the hydrophobic shell around the methyl group in diluted acetic acid water solutions has been discussed, evidencing how the methyl group hydrophobicity strongly influences the acetic acid behavior in aqueous solutions.

Probing matter under extreme conditions at Fermi@Elettra: the TIMEX beamline

FERMI@Elettra is a new free-electron-laser (FEL) facility, presently under commissioning, able to generate subpicosecond photon pulses of high intensity in the far ultraviolet and soft X-ray range (λ=100-20 nm for the present FEL1 source, extended in future to 4 nm with the FEL2 source). Here we briefly describe the present status of the TIMEX end-station, devoted to perform experiments on condensed matter under extreme conditions. The layout of the end-station, presently in the final stages of construction, is reported showing the details of the optics and sample environment. The potential for transmission, reflection, scattering, as well as pump-and-probe experiments is discussed taking into account that FEL pulses can heat thin samples up to the warm dense matter (WDM) regime. The calculated deposited energy in selected elemental films, including saturation effects, shows that homogeneous heating up to very high temperatures (1-10 eV for the electrons) can be easily reached with a suitable tuning of the energy and focus of the soft x-ray pulses of FERMI@Elettra. The results of the first test of the TIMEX end-station are also reported.

Probing the molecular connectivity of water confined in polymer hydrogels

The molecular connectivity and the extent of hydrogen-bond patterns of water molecules confined in the polymer hydrogels, namely, cyclodextrin nanosponge hydrogels, are here investigated by using vibrational spectroscopy experiments. The proposed spectroscopic method exploits the combined analysis of the vibrational spectra of polymers hydrated with water and deuterated water, which allows us to separate and selectively investigate the temperature-evolution of the HOH bending mode of engaged water molecules and of the vibrational modes assigned to specific chemical groups of the polymer matrix involved in the physical interactions with water. As main results, we find a strong experimental evidence of a liquid-like behaviour of water molecules confined in the nano-cavities of hydrogel and we observe a characteristic destructuring effect on the hydrogen-bonds network of confined water induced by thermal motion. More interestingly, the extent of this temperature-disruptive effect is found to be selectively triggered by the cross-linking degree of the hydrogel matrix. These results give a more clear picture of the molecular mechanism of water confinement in the pores of nanosponge hydrogel and open the possibility to exploit the spectroscopic method here proposed as investigating tools for water-retaining soft materials.

Determination of dynamical parameters in liquids by homodyne transient grating spectroscopy at large angles

We report on the possibility of extracting fast dynamical relaxation times from homodyne transient grating measurements. We demonstrate the validity of our approach by experimental measurements on liquid acetonitrile and by comparison with literature. This approach would be of tremendous help in the case of free-electron-laser-based transient grating experiments due to the overcoming of technical difficulties, such as large-angle geometries.

Experimental setups for FEL-based four-wave mixing experiments at FERMI

The recent advent of free-electron laser (FEL) sources is driving the scientific community to extend table-top laser research to shorter wavelengths adding elemental selectivity and chemical state specificity. Both a compact setup (mini-TIMER) and a separate instrument (EIS-TIMER) dedicated to four-wave-mixing (FWM) experiments has been designed and constructed, to be operated as a branch of the Elastic and Inelastic Scattering beamline: EIS. The FWM experiments that are planned at EIS-TIMER are based on the transient grating approach, where two crossed FEL pulses create a controlled modulation of the sample excitations while a third time-delayed pulse is used to monitor the dynamics of the excited state. This manuscript describes such experimental facilities, showing the preliminary results of the commissioning of the EIS-TIMER beamline, and discusses original experimental strategies being developed to study the dynamics of matter at the fs-nm time-length scales. In the near future such experimental tools will allow more sophisticated FEL-based FWM applications, that also include the use of multiple and multi-color FEL pulses.

Oxidative damage in DNA bases revealed by UV resonant Raman spectroscopy

We report on the use of the UV Raman technique to monitor the oxidative damage of deoxynucleotide triphosphates (dATP, dGTP, dCTP and dTTP) and DNA (plasmid vector) solutions. Nucleotide and DNA aqueous solutions were exposed to hydrogen peroxide (H2O2) and iron containing carbon nanotubes (CNTs) to produce Fentons reaction and induce oxidative damage. UV Raman spectroscopy is shown to be maximally efficient to reveal changes in the nitrogenous bases during the oxidative mechanisms occurring on these molecules. The analysis of Raman spectra, supported by numerical computations, revealed that the Fentons reaction causes an oxidation of the nitrogenous bases in dATP, dGTP and dCTP solutions leading to the production of 2-hydroxyadenine, 8-hydroxyguanine and 5-hydroxycytosine. No thymine change was revealed in the dTTP solution under the same conditions. Compared to single nucleotide solutions, plasmid DNA oxidation has resulted in more radical damage that causes the breaking of the adenine and guanine aromatic rings. Our study demonstrates the advantage of using UV Raman spectroscopy for rapidly monitoring the oxidation changes in DNA aqueous solutions that can be assigned to specific nitrogenous bases.

Thermodynamic hydration shell behavior of glycine

Glycine aqueous solutions have been studied as a function of temperature and concentration by means of UV Brillouin and Raman spectroscopes. Brillouin spectra provided information on the average relaxation time τα related to the mechanisms of hydrogen bonds (HBs) formation and breaking. The concentration-temperature behavior of τ has been compared to the vibrational dephasing lifetime of atoms involved in HBs, as derived by a lineshape analysis of Raman spectra. We point out how it is possible to trace the thermodynamic behavior of a selected HB from Raman data. In particular, our results confirm the predominant role played in the hydration process by the water molecules surrounding the hydrophobic groups and, furthermore, evidence how at low temperature the HB strength between these molecules is greater than those found in bulk water and between glycine and water molecules.