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

Abstract

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.

Inelastic neutron scattering and low-energy excitation of amorphous SF6 hydrate prepared by vapor-deposition technique

“…2 are quite similar to those of pure and methanol-doped amorphous ices [11]. Four peaks lying in the energy range between 7 and 40 meV are known to be the translational (TA, LA, LO, and TO) modes in crystalline ices [18,19].…”

“…The purpose of this work was to evaluate the phonon excitation of amorphous SF 6 hydrate and to investigate the effect of structural relaxation and crystallization. It is also important to compare the present results with those of pure amorphous ices and also with those of methanol-doped amorphous ices described above [11]. Figure 1 shows the block diagram of the sample supply system and the cryostat used in this work; a more detailed description of the cryostat is given in ref.…”

“…(A review of recent structural and spectroscopic works is given in ref. 11.) Recently, we measured inelastic neutron scattering of vapor-deposited amorphous ice and found interesting annealing effects and a methanol-doping effect on the intensity of low-energy excitation [11].…”

“…In the present study, we prepared amorphous SF 6 hydrate by vapor deposition and measured the inelastic neutron scattering of the deposited samples by using a novel cryostat designed for vapordeposited samples [11]. The purpose of this work was to evaluate the phonon excitation of amorphous SF 6 hydrate and to investigate the effect of structural relaxation and crystallization.…”

“…Figure 1 shows the block diagram of the sample supply system and the cryostat used in this work; a more detailed description of the cryostat is given in ref. 11. The sample supply system is mostly made of Pyrex glass and partly of stainless steel.…”