LASER-EXCITATION EFFECT ON OPTICAL ABSORPTION BAND DUE TO METAL-CLUSTERS IN IONIC CRYSTAL AFTER ELECTRON IRRADIATION

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This work is mainly focussed on sodium clusters (colloids) formed in NaCl single crystal after intense electron pulse irradiation and subsequent heat treatment. The Na cluster in NaCl gives a characteristic optical absorption band peaking at 2.10eV, which is believed to be composed of a number of component due to clusters of various sizes. The purpose of this work is to examine the 2.10eV band after intense excitation with laserlight in order to get an information of the components of the band. The work has been carried out according to the following program:(1) Production of Na clusters in NaCl with electron irradiation and heat treatment.(2) Analysis of Na cluster formation through optical absorption measurement.(3) Setup of optical excitation system and spectrum measuring one.(4) Investigation of the 2.10eV band after laser excitation.As for terms (1) and (2), electron irradiation was carried out using a LINAC at ISIR, Osaka university and optical absorption spectrum was measured at … More 100K after each treatment of the sample. Detailed analysis on the basis of Gaussian peak of each component of the spectrum has revealed new facts that the Na clusters may be formed from a complex of four F centers aggregation in a (111) plane (so called N_1 center), and the 2.10eV band is composed of two bands at the initial stage of the formation.As for terms (3) and (4), setup of intense excitation system was made with a Nd:YAG and DYE laser. Optical absorption spectrum was observed with time resolved method using a monochromator coupled with a CCD linear sensor. Measurements were carried out by changing the conditions; the exciting light intensity, the delay time from the excitation to the measurement, etc. The results were always negative. These results lead us to the conclusion that no effect of intense optical excition can be observed on the absorption band due to the Na clusters. This may indicate a remarkable result that the intense excitation effect on the absorption band due to the free electron is completely different from that to the localized ones, since saturation effect is observed with the same system for electron irradiated NaCl single crystal without heat treatment. Less