Europhys. Lett.
Volume 72, Number 6, December 2005
Page(s) 969 - 975
Section Condensed matter: structural, mechanical and thermal properties
Published online 04 November 2005
Europhys. Lett., 72 (6), pp. 969-975 (2005)
DOI: 10.1209/epl/i2005-10322-9

Collective excitations in liquid $\chem{CD_4}$: Neutron scattering and molecular-dynamics simulations

E. Guarini1, U. Bafile2, F. Barocchi3, 1, F. Demmel4, F. Formisano5, M. Sampoli6, 1 and G. Venturi3, 1

1  CNR-INFM and CRS Soft, c/o Dipartimento di Fisica, Università di Firenze I-50019 Sesto Fiorentino, Italy
2  CNR-ISC, Sezione di Firenze - I-50019 Sesto Fiorentino, Italy
3  Dipartimento di Fisica, Università di Firenze - I-50019 Sesto Fiorentino, Italy
4  ISIS Facility, Rutherford Appleton Laboratory - Chilton, Oxfordshire, OX11 0QX, UK
5  CNR-INFM and CRS Soft, c/o Institut Laue Langevin - BP 156, Grenoble, France
6  Dipartimento di Energetica, Università di Firenze - I-50139 Firenze, Italy

received 18 July 2005; accepted in final form 13 October 2005
published online 4 November 2005

We have investigated the dynamic structure factor $S(Q,\omega)$ of liquid $\chem{CD_4}$ at $T = 97.7\un{K}$ in the wave vector range $2\le Q/\mathrm{nm}^{-1}\le15$ by means of neutron scattering and molecular-dynamics simulation, in order to study the centre-of-mass collective dynamics. The agreement between the experimental spectra and those simulated using a recent ab initio based intermolecular potential is good, particularly at low Q. Underdamped collective excitations, detected in the whole experimental Q-range, characterize the dynamics of liquid $\chem{CD_4}$ as markedly different from that of other molecular liquids. Also, the energy and damping of collective excitations in methane are shown to differ considerably, even at the lowest measured Q-values, from those of linearized hydrodynamic modes. An empirical relation, able to reconcile the different wave vector ranges of mode propagation observed in disparate liquids, is investigated.

61.12.Ex - Neutron scattering (including small-angle scattering).
61.25.Em - Molecular liquids.
61.20.Ja - Computer simulation of liquid structure.

© EDP Sciences 2005