Soft Report - Dipartimento di Fisica - Sapienza

Time Resolved X-rays Scattering from Disordered SystemsThe tehnique on ID09B beamline at the ESRF.This technique allows to probe the X-rays scatteringintensity changes following a perturbation induced inthe sample by a laser pulse. The time resolution isgiven by the pulsed structure of SynchrotronRadiation: the pulse duration is determined by theelectrons bunch length and at the present timecorresponds to 100 ps. So the technique looks likean optical pump and probe experiment the uniquething being the microscopic structural informationprovided by an X-rays pulse as probe signal. Theexperiment is performed locking the laser phase onthe electrons bunch frequency in the ring. Themaximum repetition rate of the laser is much slowerrespect to the X-rays emission then a rotativechopper allows to reject the pulses to match the1Khz laser emission (or slower if needed). The timedelay is finely tuned by means of a fast timing diodefrom the value imposed by the repetition rate up tothe 100ps X-pulse resolution. For a fixed time delaythousands of pulses are collected on a CCD giving ascattering image: this is usually subtracted from ano-laser one to get a difference pattern which couldbetter give the microscopic structural information. Asthe detector has not time resolution the sample mustbe replaced at each pulse: a standard experiments isthen usually performed making the liquid flowing ina jet in order to recover the scattering volume eachmillisecond. The optic pump setup now consists of acryogenically cooled 13 passes Dragon amplifierwhich make available 100 femtosecond pulses at800 nm wavelength with an energy up to 2.5 mJ perpulse; a TOPAS system is also available to tune thepump in a wide range of wavelengths.Probing photochemical reactions in solutions.One of the most important application of thetechnique has been the observation of the laserinduced reactions in photoactive molecules. Thedifferent steps of the excited molecule reactionproduce a structural information that can bedetected in the difference pattern: ref [1] gives anexhaustive example of a typical application inphotochemistry. In this kind of the experiments theanalysis has to account for the global changes of thesolution due to the solvent rearrangement aroundthe solute and the heat released: numericalsimulations and laser heating [2] experiments onthe solvent play a crucial role to extract the soluteinformation. Once this information is achieved onecan develop a sort of molecular movie providingunique information on the structures involved in thereactions pattern and their lifetime.Impulsive Infrared Heating in simple liquids.The exigency of extracting solute information inphotoreactions dynamics turned our attention on thedynamic following an impulsive heating in a simpleliquid. An infrared pulse can be used toinstantaneously release heat in the sample producingan initial temperature and pressure jump at constantvolume which first induce a pressure wave to relax tomechanical equilibrium and then equilibrate thetemperature on larger timescales. Thishydrodynamics can be observed from a microscopicpoint of view by this technique allowing toinvestigate how the hydrodynamic theory accountsfor the temporal evolution of the signal on smalllength-scales. Typical difference patterns at differenttime delays are reported for methanol in the figurebelow: the increment of the signal around 10nsmarks the expansion dynamics. Moreover severalliquids cross a metastable negative pressure stateduring the expansion dynamics following the heating(corresponding to the peak region in the figure): thisstate together with cavitations effects can bestudied in his structural and dynamical features.Impulsive infrared heating in glassformers. Onthe way of using this technique as a sort of thermalstimulated scattering with a microscopic probe andtaking advantage of the extremely large timescaleswindow available our attention is now moving to thestudy of complex liquids and glasses: in this class ofsystems the presence of the slow structuralrelaxations dynamics allows to observe the structuralchanges before the thermodynamic equilibrium isreached. This project is still work in progress:developments of the sample environment have beennecessary to extend the technique to not flowingsystems and to implement a thermal control to heatand cryogenically cool the sample. A rotative cellsystem has been tested and is now available toreplace the scattering volume at the repetition rateneeded. The application of this technique to glassesmight offer a new tool for the study of the slowdynamics in this class of complex systems from amicroscopic point of view. Further developments inpotentially interesting directions for the SOFTcommunity might include polymers studies andcolloidal systems also considering that time resolvedsmall angle experiments have already beensuccessfully performed [3].References[1]H.Ihee et al., Science 309, 1223 (2005).[2] M.Cammarata et al., JCP 124, 124504 (2006).[3] A.Plech et al. CPL 401, 565 (2005).Authors:M. Cammarata(a), M. Lorusso(a), Q. Kong(a), E.Pontecorvo(b), G. Ruocco(b), F.Sette(a), M.Wulff (a).(a) ESRF, Grenoble, France.(b) Dipartimento di Fisica, Universita' di Roma 'LaSapienza' and CRS SOFT-INFM-CNR, Roma, Italy.59SOFT Scientific Report 2004-06