Takeshi Miyanaga

Decay time measurements of intrinsic luminescence in alkali halides using single-bunched light pulses from UVSOR

The combination of single-bunched light pulses (duration: 0.5 ns, interval: 177.6 ns) from UVSOR (Okazaki, Japan) with a method of time-correlated single-photon counting was applied to study decay behaviors of intrinsic luminescence in nine typical alkali halide crystals (Na, K, Rb) × (Cl, Br, I). Under excitation into the band-to-band transition, two emission bands, so far called σ emission (fluorescence with a short lifetime) and π emission (phosphorescence with a much longer lifetime), were observed in general at LHeT. The decay behaviors observed were essentially the same as those reported in previous studies where pulsed particles were used as an excitation source. A novel situation has, however, been recognized in the π emission in both NaBr and NaI: A fluorescent component was found clearly to coexist with the long-lived phosphorescent component. This suggests that the initial state of the π emission in NaBr and NaI is a singlet-triplet pair slightly split by the exchange interaction.

Optical absorption of Cu-centers in CdCl2 and CdBr2 single crystals

Abstract The absorption spectra of CdCl 2 and CdBr 2 single crystals doped with cuprous or cupric halides were measured. Optical bleaching of the absorption bands due to Cu + gives rise to new absorption bands which suggest the conversion of Cu + →Cu 2+ . This was confirmed also by detecting the growth in ESR signals characteristic of Cu 2+ centers.

Optical Spectra of Cadmium Halide Crystals in 3–30 eV Region

Reflection spectra of single crystals of CdCl 2 , CdBr 2 and CdI 2 at 35 K were measured in the region up to 30 eV using synchrotron radiation. Photoemission experiments were also performed on in-situ evaporated films of cadmium halides to determine binding energies of the outermost s -core levels of halogen ions. Optical constants were derived through Kramers-Kronig analysis. In the region up to 15 eV were found a number of peaks which are attributed to the transitions from the halogen valence band to the cadmium 5 s and 5 p conduction bands. Around 16 eV were observed the peaks due to the Cd 4 d core excitons. Weak structures in the 19-24 eV region are assigned to the transitions from the halogen s -core levels. In CdI 2 , it was found that the total oscillator strength for the transition from the valence band is enhanced due to the presence of shallow core levels.

EPR Studies of Strong Jahn-Teller Coupling Systems CdCl2: Ag2+ and CdBr2: Ag2+

The EPR spectra of Ag 2+ ions in CdCl 2 and CdBr 2 are investigated in the temperature range from 4.2 K to 306 K. Low temperature spectra, exhibiting static tetragonal Jahn-Teller distortions, are observed below 77 K. The transition from the low temperature spectra to the motionally averaged high temperature ones is observed between 97 K and 170 K for CdCl 2 : Ag 2+ and between 98 K and 213 K for CdBr 2 : Ag 2+ . The transition temperature for CdCl 2 : Ag 2+ depends on the orientation of the crystal and on the position of the super hyperfine line in question, which is explained by the motional averaging model. The anisotropy of the g -value of the low temperature spectrum of CdBr 2 : Ag 2+ is found to be extraordinarily small ( g // =2.072, g ⊥ =2.067 at 77 K). The reasons for this small anisotropy of the g -value are discussed.

Electron Paramagnetic Resonance Study of Ag++Ions in CdCl2

The EPR spectra of a Ag ++ ion in CdCl 2 are studied at liquid nitrogen temperature and at room temperature. The Ag ++ ion is found at the center of the octahedron of six Cl - ions. The octahedron of Cl - ions is elongated tetragonally due to a Jahn-Teller effect at liquid nitrogen temperature. It is also observed that the principal axis of the g -tensor deviates from a line joining the Ag ++ ion and one of the nearest neighbor Cl - ions towards the crystal c -axis. This deviation is interpreted to be due to the influence of the axial symmetry field parallel to the c -axis on the Jahn-Teller effect. At room temperature a single broad EPR line with axial symmetry is observed.

Fine Structures of Cd2+ 4d Core Excitons in CdCl2-CdBr2 Mixed Crystals and CdI2 Crystal

Reflection spectra of CdCl 2 -CdBr 2 mixed crystals and pure CdI 2 crystal were measured in detail in order to investigate Cd 2+ 4 d core exciton states. The second-energy-derivative reflectance spectra calculated from measured spectra reveal that the core exciton absorption consists of eight fine structures in CdCl 2 and nine fine structures in CdBr 2 and CdI 2 . The fine structures in mixed crystals shift continuously with the change in composition, and the correspondence between the fine structures in CdCl 2 and CdBr 2 has been clarified. Peak positions and relative intensities of the fine structures were obtained by a line shape analysis. Based on a one-electron excitation picture, the observed fine structures are explained in terms of spin-orbit splitting of the 4 d core level and crystal field splittings of the 4 d and 5 p levels of the Cd 2+ ion.

Photochemical Reactions of Molecules on Alkali Halide Surfaces Induced by Undulator Radiation

Two photochemical reactions of molecules on alkali halide surfaces induced by undulator light are described. The first investigation involves the formation of CN- -centers. When some alkali halides were irradiated in the atmosphere of N2 gas and gases containing C, luminescence attributed to CN- ions was observed, the intensity of which increased with the increase in irradiation dose. It is proposed that sources of the CN- ions are generated in the gas phase and the CN- ions, with the aid of undulator light, stick on the surface or are adsorbed in the bulk near the surface, resulting in the formation of CN- -centers. These centers are stable up to 400 K. The second investigation involves dissociation of H2O molecules. The OH band and Lyman α line of H were observed from a KBr surface, when it is irradiated with undulator light in the atmosphere of H2O vapor. It is proposed that the H2O molecules are physisorbed on the KBr surface and dissociate due to the presence of undulator light.

Decay Time Studies on UV-Luminescence in CdBr2CdCl2 Mixed Crystals

The decay time measurements have been carried out on the self-trapped exciton (STE) luminescence in CdBr2, CdCl2 and CdCl2CdBr2 mixed crystals. The strong STE luminescence is observed at low temperatures below 50 K in the near ultra-violet region (UV-luminescence) having the peak at 3.26 eV in CdBr2 and 3.76 eV in CdCl2. In the decay curve of the UV-luminescence appear two components, the fast one with the ns life time and the slow one with the μs life time which are related to the spin-singlet and -triplet STEs, respectively. The time integrated intensity ratio of the singlet to the triplet component is much smaller in CdBr2 (0.064) than in CdCl2 (32) reflecting the difference in strength of spin-orbit interaction between Br− and Cl−. In the mixed crystals of CdCl2(1−x)CdBr2(x), the value of the time integrated intensity decreases rapidly with small increase of the CdBr2 content, x, from the value in CdCl2 to that in CdBr2. This means that the STE state strongly depends on the Br− content. The experimental results are explained by means of a structural model for the STE in cadmium halide crystals, namely, the excited states of [Cd2+X6−]4− complex molecular ions with D3d symmetry.

Luminescence from surface CN− centers created photochemically on alkali halide crystals

Abstract Photo-chemical production of surface CN− centers were investigated on NaCl, KCl and RbCl by using undulator radiation. Luminescence spectra were measured on the crystals irradiated with undulator radiation in the ambient gas of N2 and Co2. About ten regularly spaced molecular emission bands were observed in the region from 3.3 to 5.8 eV. They are connected to the surface CN− centers created through photo-induced processes. Temperature dependences of the luminescence intensity and decay time indicate that the produced CN− ions replace the Cl− ions to form thermally stable centers in the surface region of the crystals. A new decay component of the CN− luminescence was found for each crystal and is attributed to the surface CN− centers. Excitation processes of the surface CN− centers are discussed in terms of energy transfer from the bulk to the surface.