Shigeo Shionoya

Mechanism of photostimulated luminescence in BaFX:Eu2+ (X=Cl,Br) phosphors

Abstract In BaFX:Eu 2+ , the excitation by X-ray or ultraviolet light and the photostimulation of luminescence by visible light are caused by the processes of Eu 2+ ⇌Eu 3+ and F + -centers⇌F.

Energy Transfer between Trivalent Rare‐Earth Ions in Inorganic Solids

The purpose of the paper is to clarify the mechanism of excitation energy transfer between unlike trivalent rare‐earth ions in inorganic solids. We have measured the manner in which the intensities of the luminescences of Tb3+ (5D4→7F5) and Eu3+ (5D0→7F2) are quenched and the decay rates are made faster by the coexistence of other kinds of trivalent rare‐earth ions as a result of energy transfer. As the host material, calcium metaphosphate glass was used. The combination of Tb3+ donor and Nd3+ acceptor was studied most in detail. The results are discussed in terms of the resonance theory of energy transfer, and the experimental data are analyzed using the numerical calculations of the theory recently made by Inokuti and Hirayama. The overlap integrals of donor emission spectra with acceptor absorption spectra were estimated from the data of spectral measurements. Among the resonance mechanisms due to various electrostatic multipole interactions, the dipole—quadrupole interaction gave the best fit between ...

Mechanism of broad-band luminescences in ZnS phosphors—I. Spectrum shift during decay and with excitation intensity

Abstract The measurements of spectrum shift during decay and with excitation intensity are made for the broad-band luminescences in ZnS phosphors due to Ib activator and IIIb or VIIb coactivator, in order to investigate the luminescence mechanism. The Green-Cu Blue-Ag luminescences show remarkable spectrum shifts; namely, the emission peak shifts to lower energies during decay and also with decreasing excitation intensity. It is concluded that these luminescences are due to the so-called donor-acceptor pair emission mechanism, assuming that the donor and acceptor states involved are created, respectively, by the coactivator and activator. The self-activated luminescence shows a gradual spectrum shift at liquid helium temperature, although it shows a somewhat irregular shift at liquid nitrogen temperature. It seems that one has to assume the pair emission mechanism also for this luminescence. The Blue-Cu, Red-Cu and Red-Cu.In luminescences show no spectrum shifts. It is concluded, together with the knowledge already obtained from other measurements, that the transitions for these luminescences occur between two localized levels confined within the associated centers.

Anisotropic excitonic molecules in CdS and CdSe

Abstract Luminescence lines due to excitonic molecules are observed in CdS and CdSe. The line shape is analyzed by taking account of the broadening due to elastic collision. It is pointed out that the anisotropy of the effective hole masses makes the molecular binding energies much larger than evaluated by using the geometrical means of the hole masses.

Infrared absorption due to donor states in ZnS crystals

Abstract Single crystals of ZnS: Cu,Al and ZnS: Ag,Al were grown from the melt under high pressure by the Bridgman method. It was found that a remarkable broad band absorption appears in an infrared region of 3 to 14 μ under simultaneous ultraviolet illumination. This absorption is ascribed to the transition of electrons trapped at shallow donor states due to aluminum to the conduction band. The observed spectra are analyzed in terms of the theories of the effective mass approximation and the k·p perturbation. A good agreement is obtained between experiment and theory. The optical and thermal ionization energies for donor electrons are found to be 0·100 and 0·074 eV, respectively, and the electron effective mass is determined to be (0·34 ± 0·02)m0. This type of infrared absorption was found to be associated only with the green-Cu and blue-Ag luminescences. Some discussions are presented as to the correlation of this infrared absorption-with the visible luminescence.

Mechanism of broad-band luminescences in ZnS phosphors—II. Characteristics of pair emission type luminescences

Abstract As to the Green-Cu, Blue-Ag and self-activated luminescences in ZnS phosphors, which have been elucidated in Part I to be interpreted by the donor-acceptor pair emission mechanism, decay characteristics are investigated in detail. The manners of decay are measured at 4.2°K for several photon energies of the entire emission band. The decay persists for up to 1000 sec. All the decay curves obey the relation of power law, and the decay rate becomes faster with increasing photon energy in the emission band. From some arguments, it is shown that these facts lead one to definitely conclude the pair emission mechanism. The features of these pair emission bands in ZnS are discussed. In case of the Green-Cu luminescence, the correlation of the long persistent decay with the infrared absorption due to aluminum coactivator as well as that due to the ionized state of copper is discussed, and is pointed out as being very strong evidence for the conclusion. Further, some discussions and interpretations are presented about the polarization properties of these luminescences and also about the manners of the temperature-dependence of the emission spectra.

Excitation migration among inhomogeneously broadened levels of Eu3+ ions

Abstract Excitation migration between 5 D 0 states of Eu 3+ ions in Ca(PO 3 ) 2 glass is studied by using the technique of the selective excitation of the inhomogeneously broadened 5 D 0 levels. In time-resolved emission spectra for 5 D 0 → 7 F 1 transition after the pulse excitation due to 7 F 0 → 5 D 0 transition by the use of a tunable dye laser at 77°K, a subpeak is observed to appear and to grow in intensity with increasing delay time, besides the main peak excited directly by the excitation light. The observed facts are explained by considering the phonon-assisted energy transfer between 5 D 0 states involving an acoustic phonon. From the analysis of the results, the probability of the excitation migration due to the phonon-assisted process is estimated to be ≌ 10 A ms -1 . From the theoretical consideration, the rate of the linewidth change and the probability of the excitation migration due to the resonant transfer process are obtained. These results indicate that the phonon-assisted process is more dominant than the resonant process for the excitation migration in the system studied.

New luminescence line associated with the inelastic collision of excitonic molecules in CdS and CdSe

Abstract A new luminescence line associated with the inelastic collision process of two excitonic molecules, in which one of the four excitons involved is radiatively annihilated leaving the other three as single excitons, is observed in CdS and CdSe. The line shape is analyzed by taking account of both elastic and inelastic collisions.

Time-resolved spectra of spontaneous luminescence from high density electron-hole plasma in CdS

Abstract Pico-second time-resolved spectra of the spontaneous luminescence from high density electron-hole plasma in CdS are measured at 4.2 K suppressing the stimulation effect. It is found that after the pulse excitation hot carriers are cooled rapidly for the first 100 psec, and that therafter up to the 400 psec delay time the shape of spectra hardly changes but the spectral width depends on excitation density. Although it seems as if some kind of state like electron-hole liquid is formed, one cannot easily regard that state as the condensed electron-hole drop state.

Effect of pressure on the luminescence of zinc sulphide phosphors

Abstract The effect of hydrostatic pressure of up to about 52 kb has been investigated on the six representative luminescence bands in zinc sulphide phosphors. The Cu-blue and -green, Ag-blue and self-activated blue emissions show blue shifts, while the Mn 2+ -orange emission exhibits a red shift. The green emission in ZnS: Pb is rather insensitive to pressure. From the observed pressure shift of the self-activated luminescence in ZnS: Cl, the one dimensional configuration coordinate diagram has been constructed for this centre with the aid of the other data on the luminescence. Peak shifts found in the Cu-blue and -green and Ag-blue emissions are nearly parallel to the pressure change of the band gap energy of ZnS. These behaviours under isothermal compression are interpreted with the existing models of the centres in connection with their characteristics under isobar temperature change. The red shift observed in the orange emission of ZnS: Mn is explained in terms of the crystal field theory for the Mn 2+ ion. The effect of pressure has also been investigated on the excitation spectrum for the Mn 2+ emission to examine the pressure shifts of the higher energy transitions. The results were analysed by using the crystal field theory for the Mn 2+ ion in cubic ZnS, and the crystal field strength was estimated to be Dq = 410±10 cm −1 .