Yozo Kaifu

Optical Properties of Bismuth Tri-Iodide Single Crystals. II. Intrinsic Absorption Edge

The optical absorption spectra near the fundamental edge have been measured on high quality single crystals of BiI 3 . Above 50 K, the absorption coefficient near the edge is well described by the Urbachs rule. At lower temperatures, the corresponding transitions are of the allowed indirect type involving the creation of free excitons, and absorption components associated with three phonon energies have been resolved. The energies of the phonons are found to be 2.6±0.1, 7.1±0.2 and 13.4±0.4 meV. The absorption cross-section of the indirect transition is very large compared with that observed in indirect semiconductors and ionic crystals. These results are discussed in view of band nature of BiI 3 . Preliminary results on sharp absorption lines near the edge are also given.

Optical Properties of Bismuth Tri-Iodide Single Crystals. I Interband Transitions

The reflection spectra of BiI s single crystals were measured between 1.5 and 5.5 eV at 6, 77 and 286 K. A Kramers-Kronig dispersion analysis of the data was made, and the intrinsic absorption coefficient and the real and imaginary parts of the dielectric constants were calculated. The large peak at 2.0 eV is ascribed to direct excitonic transition near the fundamental absorption edge. The main features observed in the absorption spectra are discussed in terms of two proposed models for anionic and cationic excitons in heavy metal halides. Measurements have also been made on the ordinary refractive index near the absorption edge. The data are well described by a two-oscillator model.

Optical Properties of Extrinsic Two-Dimensional Excitons in BiI3 Single Crystals

Interface excitons caused by stacking faults in BiI 3 , crystals have been investigated on absorption, photoluminescence and its excitation spectra. Characteristic three sharp absorption lines (called R , S and T ), appear close to the fundamental absorption edge. The temperature dependence of the line shape shows a typical feature of exciton-phonon interaction in the case that K =0 at the bottom of the exciton band. Resonant luminescences of these lines have also very narrow line widths, and no Stokes shifts can be detected. The luminescence line shape of phonon side band shows Maxwellian-like having the line width proportional to thermal energy but accompanied by a fine structure which reflects a quasi two-dimensional band nature of this exciton system. The high density excitation effects of these states reveal a specific interaction among the R , S and T excitons.

Hydrogen-Bonding of Excited Carbazole and 2-Naphthylamine with Various Proton Acceptors in Non-Polar Solvent

or n-butylacetate(B4) in cyclohexane becomes appreciable at remarkably smaller concentrations of proton acceptors (A) compared with that in the ground electronic state. When A is pyridine strong fluorescence quenching occurs. The concentration of pyridine where this fluorescence quenching becomes appreciable is far smaller than that where the change in the absorption spectrum becomes measurable. This circumstance holds also when /S-naphthylamine is used as fluorescer (proton donor [D]). The equilibrium constant of hydrogen bonding in the ground state was evaluated from the absorption spectral change and that in the excited state was estimated from the spectral shift for various -D pairs. The rate constants of hydrogen bond formation and decomposition, respectively, in the excited state, were evaluated in the case of several D-A pairs from the fluorescence intensity measurements of deaerated solutions. In addition, the mechanism of fluorescence quenching when A is pyridine was discussed on the basis of charge transfer interaction. Also, some discussions

Some Optical Properties of CuCl Single Crystals

The fluorescence emission was investigated in single crystal specimens of CuCl at low temperatures. Strong emission lines of instrumentally limitted width are observed at wavelengths in the range of exciton absorption below 20°K. These lines are associated with recombination radiations from free and bound excitons with or without phonon cooperation, and they shift toward higher energy with increasing temperature as are observed in the exciton absorption bands. The average temperature coefficient of all the peak energy at low temperature is +2.5×10 -4 eV/°K. From the temperature dependence of the emission intensity, the activation energies of the various processes are estimated; the ionization energy of the exciton is 0.16 eV and the binding energies of the bound exciton complexes ranged between 0.03 and 0.007 eV.

Secondary emission of indirect exciton in BiI3

Resonant secondary emissions of BiI3 crystals in indirect exciton edge are investigated at liquid helium temperature by using a tunable dye laser. The emission spectra depend critically on the excitation energy, and consist of 1st-, 2nd- and 3rd-order resonant Raman lines and hot-luminescence bands of LO phonon assisted exciton recombinations. From an analysis of Raman lines, the associated phonon energies and the exciton effective mass are estimated. Preliminary discussion on the hot-luminescence is also given.

Some Optical Properties of Lead Activated Potassium Halide Phosphors

Absorption, emission and excitation spectra of KCl:Pb and KBr:Pb have been measured at various temperatures. These observed spectra are similar to those of KCl:Tl, and the absorption bands seemed to be due to transitions in Pb ++ ions which occupies the normal positive ion sites. The two principal excited states are 1 P 1 and 3 P 1 of Pb ++ ion. 273 mµ absorption, 345 mµ emission in KCl:Pb and 302 mµ absorption, 365 mµ emission in KBr:Pb are attributed to 3 P 1 – 1 S 0 transitions perturbed by surrounding host lattice ions. The excitation to 1 P 1 state does not manifest any dstinct emission which is different from the case of Tl activated phosphors. The near ultra-violet emission suffers violent thermal quenching at room temperature, and the activation energies of these nonradiative process are found to be 0.28 eV and 0.18 eV for KCl:Pb and KBr:Pb, respectively. Several discussions using a single configuration coordinate model are given.

Exciton-Induced Luminescence in KI/Tl.

In a single crystal of KI/Tl, emissions from Tl + ion and a certain impurity were observed under excitation of light in the first characteristic absorption band of KI. These exciton-induced luminescences were measured at various temperatures for specimens of several concentrations of Tl + ion. The intensities of the exciton-induced luminescences attained saturated values at about liquid oxygen temperature. In consideration of the amount of energy transferred to the impurity, the saturated intensities of the exciton-induced luminescence from Tl + ion are almost independent of the concentration of Tl + ion in the range of 10 15 ∼10 19 /cm 3 and showed a quantum efficiency as high as 30%. Therefore the migration of an exciton through the crystal was confirmed.

Secondary emission due to recombination of excitons near indirect band bottom. II. Experimental and theoretical analysis in BiI3

For pt.I see ibid., vol.16, p.4719 (1983). Experimental and theoretical studies are presented on the secondary emission (SE) due to recombination of excitons near the indirect band bottom in BiI3. The authors have made systematic measurements to observe the excitation energy, temperature and sample dependences of SE spectra, and obtain a consistent interpretation on these experimental results applying the theory developed in preceding work. The observed SE spectra consist of the Raman scattering (RS) lines, involving resonant higher-order Raman processes, and the hot luminescence (HL) bands in the scheme. Under the excitation near the indirect absorption edge, the spectral structure of SE reflects the relaxation processes of excitons which are governed by the intraband scattering by acoustic phonons and a non-radiative decay due to crystal imperfections. It is shown that the two parameters characterising these two processes are uniquely determined by fitting the calculated intensities of RS lines to the observed ones, and the computed spectral lineshape of HL using these parameter values reproduces experimental results satisfactorily. The relationship between RS and HL intensities is also discussed by comparing the observed and calculated intensity ratios.

Zone-folding effects on phonons and excitons in polytypic BiI3 single crystals

Abstract Resonant Raman scattering (RRS) of new mode phonons in BiI3 crystals containing stacking faults is reported. Under excitation with laser light tuned to a characteristic sharp absorption line, new mode Raman lines show the resonance behavior. The new phonon modes and the origin of the sharp absorption line can be interpreted in terms of folding back effects of the Brillouin zone for phonons and excitons in a polytypic structure