Dynamical response of liquid water in confined geometry by laser and neutron spectroscopies

Abstract

Detailed spectroscopic studies performed by quasi-elastic and inelastic light scattering (Rayleigh wing and Raman) and a preliminary analysis by quasi-elastic and inelastic neutron scattering (incoherent quasi-elastic neutron scattering (IQENS) and incoherent inelastic neutron scattering (IINS)) in water, in the bulk and in confined geometry at different hydration levels and at room temperature, are presented. The possibility to compare the experimental results obtained by both the complementary spectroscopic techniques and analysed in the framework of existing models for confined liquids, has allowed us to identify the clear influence of the confinement on the mobility of this fundamental liquid, which shows a dramatic frozen-in effect with respect to the bulk state. In addition, the presence of an α-relaxation process, related to a distribution of relaxational times which recalls the mode coupling theory (MCT), characteristic of dense supercooled liquids, has been confirmed. Furthermore inelastic features in both spectroscopies show a loss of low frequency collective vibrational bands connected with the presence of tetrahedral environments for physisorbed water. Finally Raman results show a systematic lowering of the OH stretching bands for water involved in tetrahedral C_2v symmetry.