CENTRO DE QUIMICA INORGANICA "DR. PEDRO J. AYMONINO"
Unidad Ejecutora - UE
congresos y reuniones científicas
Proyectos : Sub -Picosecond Ultra Fast Transient Spectroscopy/On the Need of Having a Good TeraHertz Source in LatinAmerica.
NESTOR E. MASSA
Laboratorio Nacional de Luz Sincrotron
Workshop; 2nd Workshop LNLS-2; 2009
Laboratorio Nacional de Luz Sincrotron
The understanding of charge, spin, and lattice dynamics in general, and in particular, at and near a phase transition, and the correlations of the elementary excitations is a subject fundamental interest in the study of solids. Strong changes affecting any those variables, as a drastic conductivity alteration turning an ordered insulator into a metal oxide, may be revealed by ultra short photoexciting pump pulses and the use of a femtosecond probe. The same reasoning may be extended to the diffraction studies of charge transfer complexes, molecular excited states, organic and inorganic solutions, and the dynamics of structures in general. The overall technical requirements are by now fairly well understood for time-resolved X-ray spectroscopy (Extended X-ray Absorption Fine Structure-EXAFS or X-ray Absorption Near-Edge Structure-XANES) in the picosecond time domain and further work is being carried out for shorter times. The principle of the measurements is always based on the pump-probe scheme, where an ultrashort laser pulse induces a structural modification in a chemical, physical or biological system. The hard x-ray pulse from a synchrotron probes the evolution of the structure in real-time. Typically, the technique applies to processes that repeat and recover, such as electron and vibrational relaxation processes, and chemical reactions in which the recovery is by replenishment. Thus, the aim of this project is the used of a pico- and femto-second hard X-ray pulse from a synchrotron to probe the evolution of a structure in real-time by means of an adjustable time delay between the pump, an ultrashort laser pulse, and the probe pulse. By temporal monitoring, as in XFAS experiments, the light intensity (reflectance or transmission) and/or polarization changes one may get important information on dynamical structural changes of the electronic structure, magnetization, orbital ordering and phase coexistence. Reliable femtosecond detection will also help to push forward the new frontier of synchrotron radiation research since for solid-state phase transitions, for the kinetic pathways of chemical reactions, and for the efficiency and function of biological processes, the fundamental timescale of a molecular vibration is at about 100 fs. It is also important to realize that this also implies, because its brightness, that a synchrotron must be developed as an excellent terahertz source. This, beyond the use as conventional infrared source, will allow radiation to probe H+ migrations, metastable domains (magnetic or ferroelectric, electronic density maps), as well as electron-molecular couplings helping to materialize results of multidimensional vibrational and electronic spectroscopy. It is also important to mention that femtosecond-resolved near-field microscopy allows transport studies in nanostructured materials as well as the study of carrier dynamics in single nanostructures with very good temporal and spatial resolution. X-ray pulse structure at synchrotron sources is capable of resolving structural dynamics on the nanometer scale to high precision. Measurements on single nanostructures are a powerful means for the characterization of the inhomogeneity of nanostructure ensembles. As today, a good and interesting candidate for this kind of studies is the multiferroic rhombohedral R3c distorted perovskite BiFeO3. It is the paradigmatic example of a family of compounds currently understood as having two order parameters, spontaneous polarization (antiferroelectric, ferroelectric, ferrilectric), and spontaneous magnetization (antiferromagnetism, ferromagnetism, ferrimagnetism) triggering, by magnetoelectric coupling, one order by the other. The application of the techniques commented above would reveal insights on the room temperature spin ordering (TN~640 K) in the ferroelectric phase (TC~1090 K) as well as the temperature dependent creation of ferroelectric domains and, being a net antiferromagnetic, the spiral formation. The search of the potential influence of the spin ordering on the lattice stability, below TN, will promote key findings for novel materials sharing both properties, magnetic ordering and ferroelectricity, at 300 K.