| Co-Investigator(Kenkyū-buntansha) |
KANASAKI Jyun'ichi Nagoya University, Dept. of Physics, Research Associate, 理学部, 助手 (80204535)
TANIMURA Katsumi Nagoya Univ., Dept. of Physics, Associate Professor, 理学部, 助教授 (00135328)
NAKAI Yasuo Nagoya Univ., Dept. of Physics, Associate Professor, 理学部, 助教授 (40022719)
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| Research Abstract |
The purpose of this research project is to understand basic atomic processes induced by electronic excitation in several types of non-metallic solids. Two major directions of research are taken : one to reach complete understanding of the processes of the self-trapping of excitons and of defect formation arising from exciton relaxation and to apply the concept obtained in alkali halides to other materials including amorphous silica and to the processes in the bulk and on surfaces induced by dense electronic excitation. Several new experimental apparatus are developed : femtosecond cascade excitation spectroscopy, Raman spectroscopy for excited states and resonance ionization spectroscopy of neutral particles emitted by laser irradiation.
Major results of the studies are summarized as follows.
1) The dynamical de-excitation processes on the adiabatic potential energy surfaces of the lowest excited state of NaCl, KI and KBr were examined using femtosecond time-resolved spectroscopy. For Na
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Cl, femtosecond laser pulses were used to excite the lowest state of the self-trapped excited to next higher excited states. This cascade excitation technique turns out to be useful to examine the de-excitation processes at the lowest adiabatic potential energy surface of the self-trapped excitons in alkali halides. Specific coherent oscillation in a double-well potential energy surface is newly found. It is found also that the lowest adiabatic potential energy surface has a multi-well structure with small phonon frequencies.
2) The experimental results of resonant Raman spectra revealed that the self-trapped excitons is a composite comprising an F center and an H center. Using the results of current observation by Kan'no et al. that the excitons in alkali halides can relax into several structures, we suggested a new model of several types of relaxed configurations of excitons in alkali halides.
3) It has been examined whether the concept obtained for alkali halides can be used in other solids, such as fluorides and silicon dioxide and these materials are found to behave similarly not only for relaxation of exciton but also for evolution of defects in the process of exciton relaxation. In particular the results for amorphous silicon dioxide indicate clearly that the model for the luminescence suggested by Mott and Street does not hold in this material.
4) Creation of another exciton in the proximity of a self-trapped exciton was shown to result in formation of defects in a condition where single excitation does not lead to defect formation. This high density effects are found to occur in NaCl, MgF_2, CaF_2 and are interpreted in terms of formation of deformable lattice by localization of two holes.
5) Using high-sensitivity measurements of Ga atoms emitted from GaP and GaAs (110) surfaces by laser irradiation, defectinitiated emission of Ga atoms is found. The emission from some types of defects is found to be most efficient for excitation at the surface states and that the yield vs. fluence relations can be fitted to power functions with powers ranging from 1 to 6, depending, on the type of defects. The higher power is attributed to the necessity of multi-hole localization for emission to be induced.
In conclusion the results of the present investigation have revealed details of static and dynamic properties of exciton relaxation in alkali halides and similarity of the processes in several solids including amorphous SiO_2. Furthermore localization of multi-holes can cause drastic structural changes in the system where localization of a hole cannot cause any effects. Less
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