Collective excitations in liquid : Neutron scattering and molecular-dynamics simulations

E. Guarini; U. Bafile; F. Barocchi; F. Demmel; F. Formisano; M. Sampoli; G. Venturi

doi:10.1209/epl/i2005-10322-9

All issues

Volume 72 / No 6 (December 2005)

Europhys. Lett., 72 6 (2005) 969-975

Abstract

Issue		Europhys. Lett. Volume 72, Number 6, December 2005


Page(s)		969 - 975
Section		Condensed matter: structural, mechanical and thermal properties
DOI		https://doi.org/10.1209/epl/i2005-10322-9
Published online		04 November 2005

Europhys. Lett., 72 (6), pp. 969-975 (2005)

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: 13 October 2005

Abstract

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.

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

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