Tetsusuke Hayashi

Edge Emission of Excitons in KI and RbI

The intensity of the edge emission in KI and RbI has been measured from 5 K to 100 K. The temperature dependence is interpreted in terms of a simple exciton decay model. Thermal quenching of the emission can be ascribed to the increase in the self-trapping probability of excitons. The edge emission is assumed to originate from a metastable free exciton state.

Time-resolved study of photoluminescence in anatase TiO2

Abstract The luminescence of self-trapped excitons (STE) in anatase phase of TiO 2 has been studied by time-resolved spectroscopy. It is found that the decay curves of the luminescence band consist of two components obeying exponential and power-law decay. Both components have the same spectral shape but the intensity of the power-law part is suppressed compared to that of the exponential part with increasing the excitation density. These facts suggest that two different processes exist for the formation of STE.

LUMINESCENCE PROCESS IN ANATASE TiO2 STUDIED BY TIME-RESOLVED SPECTROSCOPY

We have studied time response of self-trapped exciton (STE) luminescence in anatase phase of TiO2 as functions of excitation photon energy and temperature. The decay curves, which consist of two components obeying exponential and power-law decay, are found to depend significantly on the two parameters. The exponential component is stimulated efficiently under excitation near the absorption edge. This fact suggests that under such an excitation condition, photogenerated electron-hole pairs directly form STEs. It is found from temperature dependence of the decay curves that the exponential component is thermally unstable compared with the power-law one. It is inferred that the electron-hole pairs are thermally separated, and thus the direct formation of STEs is prevented.

Optical Absorption Studies on CdI2 Crystals in the Region near the Absorption Edge

Optical absorption measurements were made on single crystals of CdI 2 , in the region near the absorption edge. The intrinsic absorption edge of Cdl 2 is dominated by the well-defined structures due to indirect excitonic absorption below 30 K. With raising the temperature, these structures are smeared out into an exponential absorption tail with relatively large steepness. Another exponential component with small steepness appears on the low energy side above 100 K and grows up to surpass the former above 130 K. Absorption measurements were also made on very thin crystals. A prominent peak and a shoulder were found at 3.693 and 3.80 eV. The small steepness component is considered as the low energy tail of the 3.693 eV absorption band. Discussions are made on the excitonic states responsible for these absorption structures and tails as well as on the exciton relaxation in the CdI 2 crystal.

Indirect exciton luminescense and Raman scattering in CdI2

Abstract Intrinsic luminescence and Raman scattering in 4HCdI 2 have been investigated at 2 K. Weak emission bands observed near the absorption edge are attributed to the phonon-assistes indirect exciton luminescence. Several new Raman lines are observed under resonant excitation in addition to known lines. The symmetry of the phonon modes associated with the indirect transitions as well as with Raman scattering is discussed.

Self-trapped states and related luminescence in PbCl2 crystals

We have comprehensively investigated localized states of photoinduced electron-hole pairs with the electron-spin-resonance technique and photoluminescence (PL) in a wide temperature range of 5-200 K. At low temperatures below 70 K, holes localize on Pb 2 + ions and form self-trapping hole centers of Pb 3 + . The holes transfer to other trapping centers above 70 K. On the other hand, electrons localize on two Pb 2 + ions at higher than 50 K and form self-trapping electron centers of Pb 3 + 2 . From the thermal stability of the localized states and PL, we clarify that the blue-green PL band at 2.50 eV is closely related to the self-trapped holes.

Luminescence of (Tl + ) 2 Centers in KI

New emission bands have been observed at 450 nm and 341 nm when a crystal of KI doped heavily with Tl + was excited with ultraviolet light near 285 nm at 5 K. An absorption band has been found at 284.5 nm from the excitation spectrum. Dependence of the emission intensity on the concentration of Tl + shows that this luminescence is due to (Tl + ) 2 centers.

Edge Luminescence of KI and RbI under UV Excitation

Emission bands of the edge luminescence of KI and RbI measured under excitation with ultraviolet light are more sharp and closer to the absorption edge than those induced by X-ray excitation. It is suggested from the temperature dependence of the emission intensity that the origin of the uv-stimulated emission is same as that of X-ray induced one.

Self-trapped electrons and holes in PbBr2 crystals

We have directly observed self-trapped electrons and holes in PbBr 2 crystals with the electron-spin-resonance (ESR) technique. The self-trapped states are induced below 8 K by two-photon interband excitation with pulsed 120-fs-width laser light at 3.10 eV. Spin-Hamiltonian analyses of the ESR signals have revealed that the self-trapping electron centers are the dimer molecules of Ph 3 + 2 along the crystallographic a axis and the self-trapping hole centers are those of Br 2 - with two possible configurations in the unit cell of the crystal. Thermal stability of the self-trapped electrons and holes suggests that both of them are related to the blue-green luminescence band at 2.55 eV coming from recombination of spatially separated electron-hole pairs.

Luminescence of Lead Ion Centers in Cadmium Iodide Single Crystals

Emission and excitation spectra of the luminescence associated with Pb 2+ centers in CdI 2 layered crystals have been investigated. Excitation in the absorption band of the center induces an emission band with fine vibrational structures at the energy near the absorption band, and a broad emission band with large Stokes shift. Measurements of the decay time and time-resolved emission spectra reveal that the former band consists of two parts originating from ( 3 E u , 3 A 2u ) and 3 A 1u states of Pb 2+ with D 3d symmetry. The low energy broad band is ascribed to the emission from a relaxed state of excitons of CdI 2 perturbed by Pb 2+ . Thermally activated transfer of energy is found to take place from the excited states of Pb 2+ into the perturbed exciton state.