Brillouin scattering of neutrons from liquids

Abstract

Inelastic coherent neutron scattering is called Brillouin scattering, if the momentum transferred to the scattering system is entirely taken up by collective particle motions. With this scattering, single excited states of such motions can be observed in polycrystals and in liquids in a similar way as in single crystals, provided the measurements are not too much contaminated by multiple scattering processes combining Bragg and inelastic scattering. Liquid and solid lead are investigated in this way. The comparison of the measured dispersion laws shows that at 340°C, for frequencies between 6-4 x 1011 and 2.3 x 1012 sec-1 and wavelengths between 7 and 21 A, sound waves propagate in the liquid by the same mechanism as in the solid. The contaminating double scattering is predominantly determined by maxima in the distribution of frequencies of collective motions. The observed positions of such maxima indicate that transverse modes exist also in the liquid.

Microscopic dynamics in liquid metals: The experimental point of view

“…A longitudinal mode, compatible with the higher frequency excitation previously reported by Dorner (Dorner et al, 1967) was reported, whose sound velocity is consistent with hydrodynamic value. The accessible kinematic range is too limited to asses any evidence of positive dispersion effect.…”

“…Such processes, i.e. the presence of inelastic modes characterized by wavevectors which differ by multiples of the reciprocal lattice spacing, have been early reported by means of INS in liquid lead (Cocking and Egelstaff, 1965a;Dorner et al, 1965;Randolph and Singwi, 1966) and more recently in liquid lithium by means of IXS (Scopigno et al, 2001b). As correctly pointed out by Faber, the presence of these excitations does not imply the existence of genuine high Q modes, it rather indicates that umklapp processes may occur in liquid as much as in solid (Faber, 1972).…”

“…In this way, one is able to have a direct sight over the so called umklapp modes, i.e. excitations characterized by wavevectors which differ by multiple of the reciprocal lattice spacing, which have been early reported by means of INS in liquid lead (Cocking and Egelstaff, 1965a;Dorner et al, 1965;Randolph and Singwi, 1966).…”

“…Details of the experiments performed up to 1975 (Cocking and Egelstaff, 1968;Dorner et al, 1965Dorner et al, , 1967Randolph and Singwi, 1966) have been exhaustively reviewed by Copley and Rowe (Copley and Lovesey, 1975). One of the most interesting results has been the evidence of both a longitudinal and a transverse branch in the dynamic structure factor, though this result was presented with some caution as the transverse mode could be an artifact arising from multiple scattering effects.…”

“…Despite the success of hard sphere model, there is still an obscure point concerning the real origin of such extended heat mode: while in the low Q limit it tends to the en-tropy fluctuation mode, indeed, at finite Q's it certainly involves mass diffusion processes. Looking at things from the constant energy point of view, umklapp modes resembling crystalline phonons in Brillouin zones higher than the first seems to be still poorly understood (Cocking and Egelstaff, 1965a;Dorner et al, 1965;Randolph and Singwi, 1966;Scopigno et al, 2001b).…”