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Abstract
Picosecond acoustic interferometry is used to monitor in time the motion of the phase transition boundary between two water ice phases, VII and VI, coexisting at a pressure of 2.15 GPa when compressed in a diamond anvil cell at room temperature. By analyzing the time-domain Brillouin scattering signals accumulated for a single incidence direction of probe laser pulses, it is possible to access ratios of sound velocity values and of the refractive indices of the involved phases, and to distinguish between the structural phase transition and a recrystallization process. Two-dimensional spatial imaging of the phase transition dynamics indicates that it is initiated by the pump and probe laser pulses, preferentially at the diamond/ice interface. This method should find applications in three-dimensional monitoring with nanometer spatial resolution of the temporal dynamics of low-contrast material inhomogeneities caused by phase transitions or chemical reactions in optically transparent media.
Highlights
26 May 2017The transformation front velocity decreases by more than half, from 67 nm h−1 (19 pm s−1) at around 0.31 ns (vertical axis ∼13 h) to 29 nm h−1 (8 pm s−1) at around 0.21 ns (vertical axis ∼29 h)
The experiments were conducted with water ice formed upon compression of liquid water in a diamond anvil cell (DAC), schematically presented in figure 1(a)
The first arrival of the acoustic pulse on the ice/diamond interface is accompanied by a change in the amplitude and the phase of the Brillouin signal, which is clearly visible for the lateral positions between 4 μm and 14 μm in the zoomed signals presented in figure 1(c)
Summary
The transformation front velocity decreases by more than half, from 67 nm h−1 (19 pm s−1) at around 0.31 ns (vertical axis ∼13 h) to 29 nm h−1 (8 pm s−1) at around 0.21 ns (vertical axis ∼29 h) Both experiments reveal the deceleration of the front as it goes away from the anvil, indicating that the ice transformation process is due to the direct light interaction with the diamond and not with the water ice. For exactly the same pump and probe powers the transformation of ice starts in the second experimental configuration more than 10 h earlier, manifesting strong lateral inhomogeneity of the diamond/ice interface, this time not for sound reflection but for a possible heat and/or charge carriers transfer across it. This identification, as well as revealing the presence of relatively small quantities of ice VII near the surface of the upper anvil, as it is qualitatively indicated in figure 1(a), was accomplished by the PAI
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