Philosophical Magazine, 1 May–1 June 2004 Vol. 84, Nos. 13–16, 1507–1512 Study of the dynamic structure factor of hydrogen fluoride by inelastic X-ray scattering R. Angelini y}, P. Giuray, G. Monacoy, G. Ruoccoz and F. Settey y European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France z Dipartimento di Fisica and Istituto Nazionale per la Fisica della Materia, Universita` di Roma La Sapienza, I-00185 Roma, Italy Abstract The dynamic structure factor S(Q, !) of liquid hydrogen fluoride is investigated by inelastic X-ray scattering at T ¼ 239 K in a momentum transfer region between 2 and 31 nm 1 . The extracted apparent sound velocity c(Q) shows a transition due to the structural or a relaxation with a relaxation time  a (Q) on the subpicosecond (less than 10 12 s) time scale. Nevertheless, from a comparison with existing molecular dynamics simulations, it arises that a second faster relaxation is present. This allows us to discuss our experimental results in terms of a two-relaxation-process scenario. } 1. Introduction Once a fluid is perturbed by an acoustic propagation, it tends to restore an equilibrium configuration through a relaxation process. The relaxation can be classi- fied on the basis of the relationship between the frequency O of the sound wave and the inverse of the relaxation time  characteristic of the process. The so-called viscous regime is probed whenever O  1/. In this regime, the rearrangements can be regarded as instantaneous processes. If, on the contrary, O  1/ , the rearrangement process is very slow and the fluid response turns out to be ‘frozen’. Within this short time scale the perturbation induced by the sound wave propagates elastically. This is the so-called elastic regime. In the intermediate region the dynamic behaviour is usually called viscoelastic. The presence of an active relaxation process can be deduced from the dynamic structure factor by looking at the behaviour of the energy position  hO(Q) of the inelastic features. When a relaxation process is active, that is O(Q)  1/(Q) is fulfilled, the apparent sound velocity, defined as c(Q) ¼ O(Q)/Q, shows a transition from a low- frequency value c 0 to a higher value. A recent inelastic X-ray scattering (IXS) study on liquid water (Cunsolo et al. 1999, Monaco et al. 1999) has shown this transition, which has been associated with the structural relaxation of the hydrogen bond network characteristic of the liquid. For this system the values of the sound velocity for the relaxed and unrelaxed state are c 0 ¼ 1500 m s 1 and c 1a ¼ 3200 m s 1 respectively, Philosophical Magazine ISSN 1478–6435 print/ISSN 1478–6443 online # 2004 Taylor & Francis Ltd http://www.tandf.co.uk/journals DOI: 10.1080/14786430310001644279 } Author for correspondence. Email: angelini@esrf.fr.