Dynamics in supercooled glycerol by high resolution stimulated Brillouin gain spectroscopy

Acoustic dissipation and density of states in liquid, supercooled, and glassy glycerol

Combined Brillouin spectra collected at visible, ultraviolet, and x-ray frequencies are used to reconstruct the imaginary part of the acoustic compliance J'' over a wide frequency range between 0.5 GHz and 5 THz. For liquid, supercooled, and glassy glycerol, J'' is found to be linearly dependent on the tagged-particle susceptibility measured by incoherent neutron scattering up to ≃1  THz, giving evidence of a clear relation between acoustic power dissipation and density of states. A simple but general formalism is presented to quantitatively explain this relation, thus clarifying the connection between the quasielastic component observed in neutron scattering experiments and the fast relaxation dynamics probed by Brillouin scattering.

Breakdown of the Debye approximation for the acoustic modes with nanometric wavelengths in glasses

On the macroscopic scale, the wavelengths of sound waves in glasses are large enough that the details of the disordered microscopic structure are usually irrelevant, and the medium can be considered as a continuum. On decreasing the wavelength this approximation must of course fail at one point. We show here that this takes place unexpectedly on the mesoscopic scale characteristic of the medium range order of glasses, where it still works well for the corresponding crystalline phases. Specifically, we find that the acoustic excitations with nanometric wavelengths show the clear signature of being strongly scattered, indicating the existence of a cross-over between well-defined acoustic modes for larger wavelengths and ill-defined ones for smaller wavelengths. This cross-over region is accompanied by a softening of the sound velocity that quantitatively accounts for the excess observed in the vibrational density of states of glasses over the Debye level at energies of a few milli-electronvolts. These findings thus highlight the acoustic contribution to the well-known universal low-temperature anomalies found in the specific heat of glasses.

Ultrafast Electronic and Vibrational Energy Relaxation of Fe(acetylacetonate)3 in Solution

Transient mid-infrared spectroscopy is used to probe the dynamics initiated by excitation of ligand-to-metal (400 nm) and metal-to-ligand (345 nm) charge transfer states of FeIII complexed with acetylacetonate (Fe(acac)3, where acac stands for deprotonated anion of acetylacetone) in solution. Transient spectra in the 1500-1600 cm-1 range show two broad absorptions red-shifted from the bleach of the nu(CO) (approximately 1575 cm-1) and nu(C=C) (approximately 1525 cm-1) ground state absorptions. Bleach recovery kinetics has a time constant of 12-19 ps in chloroform and tetrachloroethylene and it decreases by 30-40% in a 10% mixture of methanol in tetrachloroethylene. The transient absorptions experience band narrowing simultaneously with blue-shifting of the absorption maxima. Both phenomena have time constants of 3-9 ps with no evident dependence on the solvent. The experimental observations are ascribed to fast conversion of the initially excited charge transfer states to the ligand field manifold, and subsequent vibrational cooling on the lowest ligand field excited state prior to electronic conversion to the ground state. The analysis of time dependent bandwidths and positions of the transient absorptions provides some evidence of mode specific vibrational cooling.

Inelastic ultraviolet scattering from high frequency acoustic modes in glasses

The dynamic structure factor of vitreous silica and glycerol has been measured as a function of temperature and of the momentum transfer up to Q=0.105 nm(-1) using a novel experimental technique, the inelastic ultraviolet scattering. As in the case of Brillouin light scattering and ultrasonic measurements, the temperature dependence of the acoustic attenuation shows a plateau below the glass transition whose amplitude scales as Q2. Moreover, a slight temperature dependence of attenuation has been found in vitreous silica at about 130 K, which seems to be reminiscent of the peak measured at lower Qs. These two findings strongly support the idea that anharmonicity is responsible for sound attenuation at ultrasonic and hypersonic frequencies. Finally, we demonstrate that the attenuation mechanism should show a change of regime between 0.105 and 1 nm(-1).

Experimental evidence of the acousticlike character of the high frequency excitations in glasses

The dynamic structure factor S(Q,E) of glassy glycerol has been measured by inelastic x-ray scattering as a function of momentum transfer Q and at constant energy transfer E. This allows one to establish independently from specific models of S(Q,E) the following: (i) Propagating collective excitations exist in glasses at high Q. (ii) Their dispersion up to E higher than E(BP) (the boson peak energy) confirms that E(BP) is not the onset of modes localization. (iii) The observation of an almost Q independent plateau on the high Q side of the Brillouin peak supports numerical simulations on glasses, describing the vibrational eigenvectors in terms of acousticlike and "random" components.