RICERCA ITALIANA     

Research program

Vibrational dynamics and relaxation in densified glasses and confined disordered systems

University Co-ordinator

Università degli Studi de L'AQUILA - FISICA - ()

Research Unit Leader

Michele Nardone

Description

In our research group we have recently set up a complex and innovative instrumentation devoted to Brillouin scattering experiments with visible as well as ultraviolet excitation (HIRESUV spectrometer). The instrument uses monochromatic radiation at 266 nm from frequency doubling of a commercial 532 nm diode pumped laser as exciting source and a custom made double grating monochromator capableof an ultimate resolution of 0.57 GHz (0.33 at 532 nm) and with a contrast as high as 10 to the 10-th above 30 GHz. With such an instrument we have been able to collect for the first time Brillouin spectra obtained using UV excitation both in transparent and opaque samples thus significantly extending the capabilities of this kind of spectroscopic technique.
One of the advantages of this extension to the ultraviolet is that it allows us to reach, for transparent samples, values of the exchanged momentum which are twice as large as those typically reached using visible radiation. This in turn allows us to perform a more extended and accurate study of all wavevector dependent quantities entering the Brillouin spectra. Among these in particular there is the linewidth of the Brillouin peaks which, at least in the hydrodynamic regime, varies as the second power of the exchanged wavevector. We plan to exploit this characteristic, as far as this project is concerned, in undertaking a thorough comparative study of sound propagation and attenuation in densified glasses as a function of the exchanged wavevector.
In our research group we also have a micro-Raman spectrometer which will be used to characterize the samples in terms of their vibrational density of states ( essentially the so called “Boson peak” position) and in order to veryfy the sample homogeneity on the scale of the laser beam dimensions which will be used for Brillouin spectroscopy.
In particular the research group in Aquila will:
1-Perform a systematic study of the Brillouin linewidths as a function of temperature and of the exchanged momentum in the hard glasses SiO2, GeO2 and B2O3 wich have undergone different densification processes. The normal density vitreous phase of all three sample have already benn studied by our group with this technique using both visible and UV excitation. This makes us conscious that particulr care will be required for the UV excitation measurements on B2O3 wich are less transparent to UV radiation. Nevertheless the larger momentum exchanged by UV radiation should allow, besides the already quoted extension of the momentum transfer range, also to perform a more reliable linewidth measurement particularly at low temperatures where using visible excitation this is only slightly above instrumental resolution. We thus hope, thanks to a detailed wavector and temperature dependence study, to achieve a better understanding of the attenuation of sound in densified glasses.
2-We will also attemp to determine the non ergodicity factor in the same samples. This requires the determination of the ratio between the brilouin peak intensity and the intensity elastically scattered from the bulk. The latter measurement in visible and UV scattering is particularly subtle since strongly perturbed by the presence of elastic scattering from the surfaces and/or from macroscopic inhomogeneities of the glassy samples. For this purpose we plan, thanks to a CCD detector recently installed in HIRESUV, to exploit the possibility of performing rapid acquisition of moderate resolution spectra changing the illuminated sample region in order to maximaize the aforementioned ratio.
These activities will be pursued in collaboration wth the groups in Messina and Trento which will also sulpply the densified samples
3-During the second yar of the project, in collaboration with the group in Camenrino we plan to investigate using both Raman and Brillouin techniques, polimorphs of SiO2 and GeO2 obtainable in a diamond anvil cell. In this case we will have to exploit the limiting contrast properties of the instrument attempting to determine only the Brillouin peak position in order to derive elastic constants. Also in this case we will exploit the possibility of the larger Brillouin shift offered by UV excitation.