Hiroshi Kanzaki

Optical Absorption and Luminescence of Excitons in Silver Halides Containing Isoelectronic Impurities. Part I. AgBr:I-

Various correlations are observed between absorption and luminescence spectra of silver halides with isoelectronic impurities at low temperature. In Part I, the bound exciton transitions are described mainly on AgBr:I - together with related cases of AgCl:Br - and AgCl:I - . In AgBr:I - , mirror-image like relations are observed between absorption and emission spectra with zero phonon line at the center. Multiphonon structures are observed in both spectra and the energy of coupled phonon is substantially smaller in absorption that in emission. The difference in phonon energies reflects strong binding of excitons with phonons discussed by Toyozawa and Hermanson. In absorption spectra, additional structures are observed and interpreted as creating excitons in higher excited states. In emission, one phonon sidebands are observed reflecting phonon spectrum in pure AgBr. Discussions are also made on temperature variation of absorption spectrum and on asymmetric intensity distribution between absorption and emi...

Optical Absorption and Luminescence of Excitons is Silver Halides Containing Isoelectronic Impurities. Part II. AgBr:Cl- and AgBr

Low temperature optical spectra are studied on AgBr:Cl - in comparison with AgBr. Contrary to AgBr:I - , no bound excitons at chlorine are formed in AgBr:Cl - . Absorption spectra of AgBr:Cl - are similar to AgBr, except for existence of exciton absorption without phonon assistance. Photoluminescence spectra of AgBr and AgBr:Cl - contain emission band due to decay of free exciton. This observation leads one to conclude that positive holes in AgBr are not self-trapped even at 2°K. Comparison of free exciton transition energies between absorption and emission spectra reveals existence of two kinds of phonons assisting the indirect transition. Various extrinsic emission bands are also observed. One of them corresponds to shallow-bound exciton at Cd 2+ -type impurity and the other to deep-bound exciton at residual iodine. Transition energies of free and bound excitons, except iodine-bound exciton, shift to higher energy in proportion to chlorine concentration. These results can be explained by virtual crystal...

Optically Detected ESR of the Excited States in Silver Halides

We have observed ESR spectra of the excited states related to the prompt and delayed luminescences in silver halides. The localized state responsible for the prompt luminescence in AgCl is identified as the self-trapped exciton localized at a chloride ion. The self-trapped hole stage of Ag 2+ is found to participate in the delayed luminescence in AgCl which is due to the recombination of the distant electron-hole pair. Some information is obtained on the trapped electrons in volved in the delayed luminescence.

Radiative Decay-Time of Bound Excitons in Silver Bromide

Luminescence due to decay of iodine-bound exciton in AgBr:I - is observed under irradiation of a Q -switched ruby laser and can be concluded as originated from interband two-photon transition process. Radiative decay-tome of the luminescence is studied as a function of temperature (2∼120°K), emission wavelength (460∼550 nm) and iodine concentration (2.1×10 18 ∼5.3×10 19 cm -3 ). The decay kinetics and its temperature dependence are found to be different between the two emission components in the emission spectrum; high energy component (∼495 nm) and low energy one (∼520 nm). The decay time at 2°K are (25±4)×10 -6 and (16±3)×10 -6 s for high and low energy components, respectively, at iodine concentration of 2.1×10 18 cm -3 . The concentration dependence of relative intensity of the two components can be understood by assuming that the high energy component corresponds to exciton bound to isolated iodide ion and the low energy component to that bound to iodine pair.

Low Temperature Photoconductivity of F Center in KCl

Transient Photoconductivity of additively colored KCl is studied for excitation in the F , K , L 1 and L 2 bands at low temperatures down to 1.3°K. Electron quantum yield is found near unity in L bands and in high energy tail of K band in the whole temperature range. K band can be divided into two components; K 1 band with low electron yield and K 2 band with unit yield. It is concluded K 2 band corresponds to transition to the conduction band minimum and L bands to the states in the conduction band. Electron yield for K 1 and F bands is studied as a function of temperature and applied electric field. Thermal binding energy of excited state is found the same for K 1 and F states. Field ionization from K 1 state starts at far lower field than from F state. Transport of electrons at low temperature is characterized by non-ohmic behavior above critical field, where electron Schubweg is proportional to E 0.5∼0.7 . The high field behavior can be understood as hot electron phenomena.

Observation of Dislocations in Silver Halides Part I. Annealed Crystals

The appearance of sub-boundaries are revealed in the crystals of AgCl and AgBr deformed at room temperature, by the technique of printing-out and etching. The origin of sub-boundaries can be explained by the progress of polygonization during the deformation.

On the Plastic Deformation of Copper Single Crystals. Part 2. Recovery Process and Mean Length of Dislocations.

The anomalous specific heat due to recovery from strain hardening has been measured on the single crystals of copper deformed at various temperatures. The apparatus for specific heat measurement is that of Nagasaki and Takagi. (1) The maximum value of the energy stored during deformation at room temperature is found to be 0.1 cal per gram. (2) The energy released in the second valley of the specific heat anomaly (the energy accompanied with stopped dislocations) shows rapid increase when the resolved strain exceeds 25 percent. (3) The recoverying temperature takes the maximum value when the resolved strain is about 25 percent. (4) The crystals deformed at -180°C and -80°C recover sometimes below the room temperature. Results of (3) and (4) may be explained by taking account of the length of dislocation lines. (5) The density of dislocations is calculated from the dependency of recoverying rate upon the degree of hardening. The obtained value of the separation distance of dislocations is about 200 A and sh...

Exchange Effects in Optical Detection ESR and Dynamics of the Optical Pumping Cycle of F -Centers in KCl

ESR spectra of F -center pairs or clusters in which each electron is either in the ground state or in the relaxed excited state have been observed at 34 GHz and 1.6K in KCl using the optical detection technique. They are analyzed in terms of an appropriate spin-Hamiltonian including the exchange interaction between the electron spins. Also, the change in the intensity of the F -center luminescence due to ESR is calculated by considering an optical pumping cycle in the presence of external static- and microwave- magnetic field. The result is reasonably compared with the experimental results.

Photoconductivity in Thallous Chloride at Low Temperatures

Primary photoconductivity has been investigated in single crystals of TlCl. Measurements are made mainly in a wavelength region of the first exciton band and near liquid nitrogen temperature. The quantum efficiency for producing free electrons is found to be somewhat dependent on wavelength in the region of the exciton band. This dependence suggests that the life time of an exciton is very short and the exciton decomposes into an electron and a hole before it achieves equilibrium with the lattice. The Schubweg of a hole is obtained to be about one hundredth of that of an electron at 103°K. Properties of electron-trapping centers effective above 100°K are also investigated.

Low Temperature Hall Mobility and Magnetoresistance in KCl at High Electric Field

Hall mobility of photoelectrons in zone refined and additively colored KCl single crystals is found to be so high as 2×10 4 cm 2 /volt·sec at 4.2°K and to decrease in proportion to E -0.5 beyond a critical electric field. Corresponding to the mobility decrease, photocurrent is proportional to E 0.5 beyond the critical field. Dominant scattering centers at low electric field are concluded to be charged impurities rather than F centers by measurements of Hall effect and magnetoresistance. Polaron transport in KCl crystal is interpreted fairly well by the usual transport theory including hot electron theory. However, two important difficulties become noticeable in applying the usual theory. First, the trapping lifetime of electron is shorter than the scattering time. Secondly, photocurrent saturation is not observed even under very high electric field at which drift velocity of electron is much higher than the thermal velocity and emission of optical phonon is expected to occur appreciably.