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 va por-deposited amorphous samples. By the use of this cryostat, vapor-deposit ed 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 studi ed corresponded to vapor-deposited amorphous solid water (ASW), hyperquench ed glassy water (HGW) (approximately), ice I-c, and ice I-h, respectively ( according to historical nomenclature). The librational frequency of the wat er molecule is smaller in the order of ASW<HGW<I-h(approximate toI(c)). Thi s indicates that the strength of the intermolecular hydrogen bonds is I-h(a pproximate toI(c))>HGW>ASW. The absolute value of the vibrational density o f states G(E) was obtained from the analysis combining the heat capacity an d neutron scattering data. G(E) of ASW and HGW integrated below 6 meV were larger than that of ice I-h(approximate toI(c)) by 0.060 and 0.039 degrees of freedom per water molecule, respectively. Similar experiments were perfo rmed 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 fract ion of methanol. All of the results in this study indicate that the low ene rgy excitation [G(E) below 6 meV] of amorphous ice is enhanced by the defec ts and distortion of the hydrogen bonds. (C) 2001 American Institute of Phy sics.