Broadband optical ultrafast reflectivity of Si, Ge and GaAs.

Ultrafast optical reflectivity measurements of silicon, germanium, and gallium arsenide have been carried out using an advanced set-up providing intense subpicosecond pulses (35 fs FWHM, [Formula: see text] = 400 nm) as a pump and broadband 340-780 nm ultrafast pulses as a white supercontinuum probe. Measurements have been performed for selected pump fluence conditions below the damage thresholds, that were carefully characterized. The obtained fluence damage thresholds are 30, 20.8, 9.6 mJ/[Formula: see text] for Si, Ge and GaAs respectively. Ultrafast reflectivity patterns show clear differences in the Si, Ge, and GaAs trends both for the wavelength and time dependences. Important changes were observed near the wavelength regions corresponding to the [Formula: see text], [Formula: see text] singularities in the joint density of states, so related to the peculiar band structure of the three systems. For Ge, ultrafast reflectivity spectra were also collected at low temperature (down to 80 K) showing a shift of the characteristic doublet peak around 2.23 eV and a reduction of the recovery times.

The amorphous Zn biomineralization at Naracauli stream, Sardinia: electron microscopy and X-ray absorption spectroscopy.

An amorphous Zn biomineralization ("white mud"), occurring at Naracauli stream, Sardinia, in association with cyanobacteria Leptolyngbya frigida and diatoms, was investigated by electron microscopy and X-ray absorption spectroscopy. Preliminary diffraction analysis shows that the precipitate sampled on Naracauli stream bed is mainly amorphous, with some peaks ascribable to quartz and phyllosilicates, plus few minor unattributed peaks. Scanning electron microscopy analysis shows that the white mud, precipitated in association with a seasonal biofilm, is made of sheaths rich in Zn, Si, and O, plus filaments likely made of organic matter. Transmission electron microscopy analysis shows that the sheaths are made of smaller units having a size in the range between 100 and 200 nm. X-ray absorption near-edge structure and extended X-ray absorption fine structure data collected at the Zn K-edge indicate that the biomineral has a local structure similar to hemimorphite, a zinc sorosilicate. The differences of this biomineral with respect to the hydrozincite biomineralization documented about 3 km upstream in the same Naracauli stream may be related to either variations in the physicochemical parameters and/or different metabolic behavior of the involved biota.

Portable laser-heating stand for synchrotron applications.

A compact, double-sided laser-heating system for diamond-cell synchrotron applications is described. The optical table, containing laser, spectrometer, and all optics for visual observation and measuring temperatures and pressures has an area of less than 1/2 m(2) and weighs less than 20 kg. All components can be remotely controlled at micron levels with simple dc motors and pneumatic drives. The design allows quick alignment of the laser-heated hot spot with the x-ray beam and the spectrometer. The prealigned system can be set up at most synchrotron beamlines within about 1 h. We carried out measurements on a variety of materials above one megabar and up to over 4000 K at both the x-ray diffraction beamline ID 27 and the x-ray absorption beamline ID 24 at the European Synchrotron Facility. A new measurement of the melting temperature of iron by x-ray absorption spectroscopy is presented.

Probing atomic displacements with thermal differential EXAFS.

Differential extended X-ray absorption fine structure (DiffEXAFS) is a novel technique for the study of small atomic strains. Here the development of this technique to the measurement of thermally induced strain is presented. Thermal DiffEXAFS measurements have been performed on alpha-Fe and SrF(2), yielding alpha = (11.6 +/- 0.4) x 10(-6) K(-1) and (19 +/- 2) x 10(-6) K(-1), respectively. These are in good agreement with accepted values, proving the viability of the technique. Analysis has revealed sensitivity to mean atomic displacements of 0.3 fm.

Effect of pressure on magnetoelastic coupling in 3d metal alloys studied with x-ray absorption spectroscopy.

Using x-ray absorption spectroscopy, we have studied the effect of pressure on femtometer-scale bond strain due to anisotropic magnetostriction in a thin FeCo film. At 7 GPa local magnetostrictive strain is found to be larger than at ambient, in agreement with spin-polarized ab initio electronic structure calculations, but contrary to the expected effect of compression on bond stiffness. The availability of high pressure data on local magnetostrictive strain opens new capabilities for validating theoretical predictions and can lead to the development of materials with the desired properties.

Investigation of undercooled liquid metals using XAFS, temperature scans and diffraction.

Novel techniques and the experimental station for experiments on condensed matter under extreme conditions that have been developed at the BM29 beamline of the European Synchrotron Radiation Facility (ESRF) are described. The experimental setup includes facilities to collect high-quality extended X-ray absorption fine structure (EXAFS) spectra, to perform controlled temperature scans while monitoring the sample absorption for the direct detection of phase transitions, and to collect high-resolution energy-scanning X-ray diffraction (ESXD) data, with recent enhancements through the installation of a two-channel collimator detector system. Facilities for X-ray absorption temperature scans, introduced five years ago, are now exploited for a wide variety of purposes. A method for the measurement of the nucleation rate in undercooled liquids has been proposed recently. All these advances in the experimental setup and techniques, combined with a simple but rigorous X-ray absorption fine structure (XAFS) data analysis scheme for disordered matter, have contributed to make feasible challenging experiments on undercooled liquid matter that were not even conceivable only a few years ago. An example of the application of these methods to undercooled liquid indium (In) is presented.