Low-energy excitations of vapor-deposited amorphous ice and its annealing and methanol-doping effects studied by inelastic neutron scattering

A novel cryostat was developed for in situ neutron scattering studies on vapor-deposited amorphous samples. By the use of this cryostat, vapor-deposited amorphous ice was prepared at ca. 8 K. The neutron scattering spectra of the as-deposited sample and those annealed at ca. 120 K, 160 K, and 250 K were measured at 50 K in energy range below 100 meV. The four samples studied corresponded to vapor-deposited amorphous solid water (ASW), hyperquenched glassy water (HGW) (approximately), ice Ic, and ice Ih, respectively (according to historical nomenclature). The librational frequency of the water molecule is smaller in the order of ASW<HGW<Ih(≈Ic). This indicates that the strength of the intermolecular hydrogen bonds is Ih(≈Ic)>HGW>ASW. The absolute value of the vibrational density of states G(E) was obtained from the analysis combining the heat capacity and neutron scattering data. G(E) of ASW and HGW integrated below 6 meV were larger than that of ice Ih(≈Ic) by 0.060 and 0.039 degrees of freedom per water molecule, respectively. Similar experiments were performed on the vapor-deposited amorphous ice doped with 5% and 10% of methanol (CD3OH). The magnitude of G(E) below 6 meV increased with increasing fraction of methanol. All of the results in this study indicate that the low energy excitation [G(E) below 6 meV] of amorphous ice is enhanced by the defects and distortion of the hydrogen bonds.