Masao Tomura

Resonance Luminescence Lines of Free Excitons in Alkali Iodide Single Crystals

Narrow luminescence lines and their excitation spectra in RbI, NaI, KI and CsI crystals are studied under the excitation in the first exciton absorption band at a temperature range 5∼100 K. The most remarkable results of the luminescence are as follows: (1) This luminescence line is located quite closely to the peak position of the first exciton absorption band. (2) The half-width is smaller than 5 meV. (3) The quantum efficiencies are estimated to be 0.01, 0.001, 0.0002 and 0.0006 for RbI, NaI, KI and CsI, respectively, under an assumption that, the quantum efficiency of the total luminescence including the broad intrinsic luminescence with large Stokes shift, is to be unity. (4) Temperatures above which the intensities of these lines decrease are 20, 25, 30 and 20 K for RbI, NaI, KI and CsI, respectively. On the basis of these results, a radiative annihilation process of a free exciton is discussed.

Fluorescence decay times of naphthalene and naphthalene excimers

Fluorescence spectra and fluorescence decay times of naphthalene and methylnaphthalenes in solution have been measured under various conditions, i.e. in several different solvents and at different temperatures from 300°k to 100°k. Furthermore, the concentration dependence of the fluorescence decay time was examined in detail in order to determine the fluorescence lifetime of the excimer. The mechanism of the excimer formation and decomposition reaction is discussed on the basis of observed results.

Diffusion of Excitons in KI Crystals

Under the excitations in the fundamental absorption bands, luminescence and excitation spectra for doped impurities as well as a self trapped exciton in a KI crystal are studied in detail at a temperature range of 5∼100 K. In doped crystals, impurity luminescences are induced instead of the intrinsic luminescence at 3.01 eV which comes from the self trapped exciton under the excitation in the first exciton absorption band. The induced impurity luminescence are confirmed to be due to collision of free excitons with doped impurities. The diffusion coefficient of the free exciton is found to be proportional to T -1/2 below 40 K and above that temperature to \(T^{1/2}[\exp(\hbar\omega/kT)-1]\). The value of the diffusion coefficient is estimated to be about 0.1 cm 2 sec -1 at 5 K.

Diffussion of Singlet Excitons in Anthracene Crystals

The temperature dependence of the decay time of singlet excitons in anthracene crystals was measured under no suffering of reabsorption effect. It was clarified that the excitons were populated on the two Davydov states in quasi-thermal equilibrium before emission process. Furthermore, probabilities of energy transfer from the exciton to doped acceptors were measured and the values of the diffusion coefficient D of the exciton was estimated as 10 -1 ∼1 cm 2 /sec which was much larger than that obtained by many researchs. A similar measurement to that by Simpson was carried out by using a pure perylene film instead of the tetracene doped film in the earlier measurement, giving D as 1.5 cm 2 sec -1 . Furthermore, the drift time due to the diffusion of the exciton through a thin anthracene layer was directly observed, giving D as 1.3 cm 2 sec -1 .

Luminescence Lines Due to the n=2 Free Exciton State of Wannier Series in KI Crystals

A resonance luminescence line and its LO phonon side bands due to n =2 free excitons in KI are studied. Main results of measurements; (1) the resonance luminescence line is located at 6.186 eV which corresponds closely to the peak position of the zero phonon absorplion line due to n =2 excitons, (2) the shape of the line is symmetric Lorentzian with 1.6 meV width, (3) the quantum yield is 1 ×10 -5 , (4) the height of the potential barrier between the n =2 free exciton state and self-trapped exciton states is 3.0 meV, (5) the self-trapping probability by a tunnel process through the potential barrier is 2 ×10 11 sec -1 and (6) the scattering probability from the n =2 free exciton state to the n =1 free exciton state is 2 ×10 11 sec -1 . Furthermore, it is concluded that so called σ(4.12 eV) luminescence and π(3.31 eV) luminescence are emitted from the self-trapped exciton states due to the n =2 exciton state.

Migration of Excimer Energy in Pyrene Crystal

The decay times of excimer emission of pure and perylene doped pyrene crystals were measured by a pulse method and, thereby, the probability of energy transfer from the excimer to doped perylene molecules was obtained. Furthermore, its temperature dependence was obtained. By the analysis of these results the diffusion coefficient D of the excimer migration in the pyrene crystal was obtained as D = D 0 exp (- E / k T ) ( D 0 =1.0×10 -2 cm 2 sec -1 and E =0.071 eV). It was confirmed that the excimer could migrate like an exciton in the pyrene crystal.

Intrinsic Luminescences and Exciton Diffusion in NaI Crystals

Luminescence and excitation spectra of intrinsic and extrinsic luminescences of RbI crystals are studied in a temperature range of 2.5∼80 K. Under the excitation in the first exciton absorption band, only one luminescence at 3.03 eV is observed in an undoped crystal. This luminescence is ascribed to the self-trapping of a free exciton to a lattice site. In Tl + doped crystals under the excitation in the exciton absorption band, the 3.03 eV luminescence decreases and conversely the Tl + luminescence increases with the increase of the Tl + concentration, while the total yield of both luminescences remains constant. It is concluded that the induced Tl + luminescence is the result of the exciton diffusion. The diffusion coefficient of the free exciton is found to be proportional to \(T^{-\frac{1}{2}}\) below 20 K and above that temperature to \(T^{\frac{1}{2}}\ [\exp(\hbar\omega/kT)-1]\) in which \(\hbar\omega\) is the energy of the longitudinal optical phonon.

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.

Excimer emission of crystalline naphthalene

Abstract Excimer emission of naphthalene has been observed in microcrystals obtained by grinding single crystals mechanically and in evaporated films. The emission spectrum of the excimer in the microcrystal was equal to that in the evaporated film. The decay times of both excimer emissions were about 140 ns at 77 K. The temperature dependence of these decay times has been interpreted by the distribution of the excimers in thermal equilibrium between two vibrational states.

Energy Transfer by Resonance between TI^+ Ions in KI Single Crystals

Energy transfer by dipole-dipole interaction between Tl + ions in KI single crystals was studied with several kinds of crystals different in Tl + concentration at low temperatures. Since two emission bands (2845 A and 3044 A) which appear under excitation at the B absorption band of Tl + ions overlap with the A absorption band of the ions, there is a possibility of energy transfer by resonance from excited Tl + ions to normal Tl + ions. The absorption coefficient of the A band and the emission bands produced by excitation at the B band were measured; and, thereby, the critical transfer distance R 0 was obtained from Dexters formula. Furthermore, the decay curves of the donor emissions were measured by a pulse method and their shapes were analyzed with Eisenthal and Siegelss theory. The values of R 0 thus obtained for 2845 A and 3044 A emissions were 35 A and 26 A, respectively. They were in good agreement with those obtained from Dexters formula, that is, 33 A and 24 A respectively.