TIME-RESOLVED SPECTROSCOPY ON ULTRAFAST NONRADIATIVE RELAXATION OF TRANSITION METAL IONS IN SOLIDS

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Trivalent cerium impurities (Ce^<3+>) in YAG crystals were selected as a model system to investigate the de-excitation mechanism of d electrons in solids. The 4f electrons at the ground state were pumped to the second-lowest 5d state (5d_b) by UV laser pulses. Then, observed were the growths of the yellow-green (YG) emission from the lowest 5d state (5d_a) by a streak camera, as well as the growths of the excited state absorption (ESA) from the 5d_a to higher excited states by a pump & prove method.Each measurement gave consistent results. The growth times (tau) at temperatures below 80K were as long as 50ps. The tau decreased above 80K to be less than 5ps at 300K. The constant tau below 80K was interpreted from a tunneling process from 5d_b to 5d_a, while the decreasing tau above 80K was from a thermally-activated process. It follows that the nonradiative transition (NRT) between 5d states occurs not dynamically, but statically after completing a thermal equilibrium in the initial 5d state.The above results predict the existence of UV emission from the metastable 5d_b state to the 4f ground state. The UV emission was successfully detected at 350 to 400nm. Agreement of the decay time of the UV emission with the tau of the ESA and YG emission supports the static mechanism of the NRT for 5d electrons.To apply the time-resolved spectroscopy to a variety of d electron systems, extension of the pumping wavelengths and improvement of the pumping methods were required. As the former examination, F^+_ centers were shown to be available for an IR pulse source, while, as the latter, a method of time-delayed double excitation was demonstrated by making use of two pulse lasers.