Inelastic neutron scattering studies of collective modes in molten CsCl

Abstract

The authors have measured the inelastic neutron scattering spectra of three samples of molten CsCl with different isotopic chlorine compositions (including the naturally occurring one) using an incident wavelength of 4.1 AA, and also of molten CsCl with natural Cl using 1.08 AA. The experiments were performed on the time-of-flight spectrometers IN6 and IN4 at the Institut Laue-Langevin. By a first-order subtraction they have substantially separated the contributions from mass (acoustic mode) and charge (optic mode) current correlations. A quasi-dispersion curve has been derived from the positions of the peaks in these functions. The acoustic and optic branches are well separated in energy over the measured region (1 AA-1<Q<4 AA-1) with relatively flat dispersion. The modes are strongly overdamped and although the energy of the low-Q optic mode corresponds to the crystal LO there is no evidence of a side peak in Sqq(Q, omega ). Thus the observed optic mode is not propagating, even at the lowest values of Q. Indeed the IN4 measurements show that Sqq(Q, omega ) is very broad (full width 18 meV) at low Q while Smm(Q, omega ) is much narrower. Small maxima in the widths of ST(Q, omega ) at Q approximately=1.7 AA-1 indicate the possible existence of a soft acoustic mode. Classical de Gennes narrowing may be observed at Q approximately=2.0 AA-1, corresponding to a change in diffusion mechanism. The statistical accuracy of the spectra was too low and the differences between scattering lengths too small to enable a second-order separation to be made using results from all three samples.