Vibrational dynamics of amorphous ice structures studied by high-resolution neutron spectroscopy

The dynamics of amorphous water ice structures of different densities have been studied by high-resolution neutron time-of-flight and backscattering spectroscopy. An accurate determination of the vibrational density of states G(ω) in the energy range of phonons ℏω≲40 meV of a many fold of structures comprising the low-density amorphous (LDA, ρ≈31 molecules/nm³), high-density amorphous (HDA, ρ≈39 molecules/nm³), very-high-density amorphous (vHDA, ρ≈41 molecules/nm³) and modifications of intermediate density in respect to HDA and vHDA has been achieved. Unlike the G(ω) of high-density crystalline phases IX, V, and XII, which have been measured as reference systems, the G(ω) of all high-density amorphous counterparts proves to be textureless except for a predominant peak at low energies. In vHDA this peak is centered at about 10 meV and redshifted upon density decrease to 7 meV in LDA. A concomitant upshift of the low-energy librational band edge from 34 to 45 meV is detected in deuterated samples. Mean-square displacement and Debye temperatures T_D for vHDA, HDA^′—a structure obtained as a transient product of the temperature induced vHDA to LDA transformation—and LDA are extracted from the highest resolution backscattering experiments. T_D values indicate the absence of a dominant excess of low-energy modes in G(ω), referred to as boson peak in the literature, being in agreement with the G(ω) properties directly monitored by the time-of-flight technique. Having applied deuterated sample material we are able to display phase coherence effects within the phonon system in the second and third pseudo-Brillouin zone (1 Å⁻¹≤Q≤5 Å⁻¹) of the amorphous samples. A phase coherent signal from acoustic phonons can be followed up to energies of at least 15 meV.