A MOLECULAR-DYNAMICS INTERPRETATION OF THE INELASTIC NEUTRON-SPECTRA IN THE PLASTIC PHASE OF HEXACHLOROETHANE

A molecular dynamics calculation of the coherent and incoherent neutro n scattering spectra of the plastic phase of hexachloroethane has been carried out; the experimental neutron profiles S(Q, omega) can be des cribed as the sum of a broad 'quasielastic' contribution which fits a Lorentzian curve and an 'elastic' contribution which fits a Gaussian c urve displaying the instrumental energy resolution. The calculated and experimental neutron Bragg intensities are in good agreement, and the calculated coherent plus incoherent neutron profiles show remarkable agreement with the experimental 'quasielastic' Lorentzian fits in all cases; the experimental 'elastic' Gaussian term could not, however, be reproduced in the calculations. The previous interpretations of this 'elastic' contribution are discussed in the text. The integrated diffu se intensity has been calculated also for different directions in reci procal space, showing good agreement with experimental data, and the i ntermolecular correlations have been studied by calculating the single -molecule scattering contribution. The behaviour of the coherent inter mediate scattering function F(Q, t) shows that the collective excitati ons in the plastic phase of C2Cl6 have a short lifetime of less than 2 ps, and the theoretical existence of phonon-like acoustic modes has a lso been investigated by analysing the molecular centre-of-mass motion ; the results show that these collective excitations are heavily dampe d, in agreement with the lack of evidence of these modes in the experi mental neutron spectra.