We investigated the vibrational dynamics and the structural relaxation of
water nanoconfined in porous silica samples with pore size of 4 nm at different
levels of hydration and temperature. We used as spectroscopic technique the
time-resolved optical Kerr effect, which enables to investigate the ultrafast
water dynamics in a wide time (0.1-10 picosecond) or frequency (10-0.1 THz)
window. At low levels of hydration, corresponding to two complete superficial
water layers, no freezing occurs and the water remains mobile at all the
investigated temperatures, while at the fully hydration we witness to a partial
ice formation at about 248 K that coexists with the part of surface water
remaining in the supercooled state. At low hydration, both structural and
vibrational dynamics show significant modifications compared to the bulk liquid
water due to the strong interaction of the water molecules with silica
surfaces. Inner water, instead, reveals relaxation dynamics very similar to the
bulk one.Comment: 10 pages 9 figure
