Rotational spectrum of the hydrogen isotopes at 0.2 GPa after 15 h, exhibiting evidence for proton exchange. The assignments of the rotational modes are indicated in the energy level diagram in the inset. The HD rotons are much broader than the D2 and H2 modes, quite likely due to lifetime broadening.
(a) Low-frequency Raman spectra of D2-H2O mixtures taken from the water-rich region, showing the pressure-induced spectral changes. (b) High-frequency Raman spectra of the D2-H2O mixtures in the water-rich region, showing the spectral signatures for H2O, D2, HD, H2, and HOD. (c) Low-frequency Raman spectra of the mixtures taken from the deuterium-rich region at high pressures, showing the spectral evidences for the hydrogen isotopes but the rotational details. (d) High-frequency Raman spectra of the mixtures in the deuterium-rich region at high pressures, showing the pressure-induced vibron splittings of the hydrogen isotopes.
(a) The pressure-induced shifts of the rotational modes of the hydrogen isotopes of the D2-H2O mixtures (solid symbols), showing an overall agreement with those of pure hydrogen isotopes (solid lines) Refs. 15–17. (b) The pressure-induced shifts of the vibrational modes of the hydrogen isotopes of the mixtures, showing contrasting behaviors with those of pure hydrogen isotopes8 and spectral evidences for the proton-ordering transition between around 28 GPa and 50 GPa (see the text).
The bond length of hydrogen isotopes calculated as a function of pressure using the Morse potential and the vibrational frequencies measured at the D2-rich, water-rich, and the boundary regions. It shows a little difference between the boundary and the deuterium-rich regions, although the bond lengths of the hydrogen isotopes in the water-rich region are consistently shorter (less than ∼0.5%) than those in the deuterium-rich region. The solid line represent the bond length of solid H2 and D2 mixtures,20 different from the present fluid D2-water mixtures.
(a) Low-frequency Raman spectra of D2 in the D2-rich region at 0.2 GPa, showing the evolution of hydrogen isotopes with the proton exchange reaction in the D2-H2O mixtures. (b) High-frequency Raman spectra in the D2-rich region at 0.2 GPa, showing the spectral changes associated with the proton exchanges and the rotational-vibrational modes (three from the S1 branch and one from the O1 branch) of D2 – but not of H2 or HD. (c) and (d) Time-dependent Raman spectral changes measured at the boundary at 2 GPa, showing the rotational modes of D2 and H2 and the translational modes (labeled νT) of the water13 in (c) and the evidences for which the water has solidified into a phase similar to the C1 clathrate in (d). (e) and (f) Time-dependent Raman spectral change measured at the ice-rich region, showing the intensity enhancements of the H2 S0(1) and H2 S0(0) rotons in (e) and of the HD and H2 vibrons in (f).
The normalized Raman intensity changes of hydrogen isotopes plotted as a function of time, showing the kinetics associated with the proton exchange reactions between (a) water and fluid D2 at 0.2 GPa, (b) ice and fluid D2 at 2 and 4 GPa, and (c) ice and solid D2 at 8 GPa. These time-dependent intensity changes are fitted to different kinetic equations (solid curves, see the text) to yield the rate constants marked on the plots.
The D2 vibrons of the D2-H2O mixtures (black lines) plotted with those of pure D2 (blue lines) at several pressures, to illustrate the attractive nature of water-D2 interaction and the repulsive interaction of ice-D2 interactions which increases with pressure.
The integrated intensity ratio of two vibrons, νS H at higher frequency and νS L at lower frequency, of the hydrogen isotopes split above 28 GPa, showing evidence for the proton-ordering transition of the mixtures between 28 and 50 GPa. This transition is likely associated with the proton-ordering transition of ice VII occurring over a large pressure range between 40 and 80 GPa.
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