Study of hydrogen-mode dynamics and phase transition mechanism by coherent phonon excitation

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In the first year, 1994, of this project, we concentrated our effort to construct newly the experimental system for coherent phonon excitation. The light source of the excitation pulse was a mode-locked Q-switched YLF laser. Two coherent light pulses, produced from one pulse by a half-mirror, with a time-width of 60 picosecond were coincided each other temporally and also spatially in a small area selected in the specimen The temporal interference pattern produces an electronic field strong enough to excite a phonon mode which begins to propagate in two opposite directions with a definite wave vector after the excite pulses passed through the excitation area. The excited phonons are probed in real time as a change of the intensity of the light diffracted from the phonons. In this stage of the study, a cw argon laser was employed as a light source of the probe light. In the second stage of the present project (1995-1996), we improved the system by introducing a pulse-probe method. The new version of the experimental system gave us an excellent signal to noise ratio of the light diffracted from the excited phonons. In the final period, 1996-1997, we applied our system to investigate the phase transition dynamics in several cases ; typical uniaxial ferroelectrics tri-glycine sulphate and selenate, a hydrogen-bonded ferroelectric rubidium hydrosulfate, a hydrogen-bonded ferroelastic potassium trihydrogen selenate, an orientational order-disorder ammonium chloride and water as a typical hydrogen system. Ineach case, we succeeded to excite a coherent phonon as an excellent probe to observe the dynamical aspect of the order parameter coupled with phonons. Exact values of the characteristic relaxation time were given for each of these cases in addition to a new phenomenon observed at just above the freezing point of the supercooled water.