Structure and Single Proton Dynamics of Bulk Supercooled Water A. Botti a, ⁎ , F. Bruni a , M.A. Ricci a , A. Pietropaolo b , R. Senesi b , C. Andreani b a Dipartimento di Fisica “E. Amaldi”, Università di Roma Tre, via della Vasca, Navale 84, 00146 Roma, Italy b Dipartimento di Fisica, Universita ` degli Studi di Roma “Tor Vergata” via della, Ricerca Scientifica 1, 00133, Roma, Italy Available online 30 August 2007 Abstract Neutron diffraction measurements with isotopic substitution (NDIS) and Deep In- elastic Neutron Scattering (DINS) experiments have been performed on bulk liquid water in the supercooled regime. Supercooling of ultra-pure water has been obtained thanks to a PTFE coating of the sample container. From the structural point of view the effect of supercooling at ambient pressure results in a slight change of the water coordination shells with respect to the structure of ambient water, although similarities with Low Density Water can be observed. As a matter of fact, the present data compare well with previous results obtained at about the same temperature, applying external pressure and can be interpreted within the second critical point scenario. DINS measurements have been carried out on this system with the aim of determining the anharmonic character of the momentum distribution of the protons, complementing the structural information on supercooled bulk water. © 2007 Elsevier B.V. All rights reserved. Keywords: Supercooled water; Neutron diffraction; Neutron scattering; Momentum distribution PACS: 61.12.-q; 25.40.Fq; 61.20.Qg; 64.60.My 1. Introduction The thermodynamic, structural and dynamic properties of bulk water in its supercooled state is an ongoing issue, that have been approached mostly theoretically and computationally [1], due to the experimental difficulties in keeping bulk water in a metastable state, below its melting point. From the experimental side, different approaches have been adopted in order to circumvent this practical obstacle. Among all the possible choices, pressurization[2,3] and confinement [4–6] have played an important role; in the experiments presented here instead we have obtained supercooling of the bulk liquid at ambient pressure, by using PTFE coated containers. The result of the theoretical and experimental effort so far is rationalized in two opposite theories, namely the second critical point scenario and the singularity free hypothesis [7]. Despite their differences, both theories share the idea that molecules can organize in at least two different ways, giving origin to two water polymorphs: a high-specific volume and low entropy structure, called low density water (LDW), and a low-specific volume and high entropy structure, called high density water (HDW). These are the liquid counterpart of the low density amorphous (LDA) and very high density amorphous (VHDA) solid water, i.e. the glassy phases found below the temperature of glass transition, T g . The existence of two water polymorphs in the supercooled liquid state, originally formulated after the Mishima et al. experiments [8], has been further supported by two recent experiments, namely a neutron diffraction study of supercooled water under pressure [2,3] and measurements of the density of a quasi-liquid layer at the interface between amorphous SiO 2 and ice I h [4]. At thermodynamic states accessible to the experiments, that are well above the hypotized second critical point, water is assumed to continuosly evolve between the two extreme structures of LDW and HDW. Structural studies can be coupled to dynamical studies performed on the same bulk supercooled water sample towards a complete picture of this challenging system. In particular the Deep Inelastic Neutron Scattering technique allows to investi- gate the single-particle proton dynamics [9]. This technique is of great relevance for an accurate description of the microscopic interactions between water molecules. Indeed, it reflects both the role of the hydrogen bond network in determining the mean kinetic energy of the proton, bE k N, and the influence of the molecular symmetry on its momentum distribution, n(p). Journal of Molecular Liquids 136 (2007) 236 – 240 www.elsevier.com/locate/molliq ⁎ Corresponding author. E-mail address: botti@fis.uniroma3.it (A. Botti). 0167-7322/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2007.08.017