Kenichi Tomioka

Exciton band structure of crystalline anthracene

Abstract The density of states in the first exciton band of crystalline anthracene has been obtained at room, dry-ice-alcohol, and liquid nitrogen temperatures by applying a thermal modulation technique to observe the hot band-first exciton band optical transition (1-0 transition). The density of states function obtained has been interpreted in terms of the exciton band structure and the Davydov splitting. The K dependent selection rule for the 1-0 transition is discussed. The bandwidth of the first exciton band is 0.09 eV.

Exciton‐phonon interaction in crystalline phenanthrene

The polarized optical absorption spectrum of the first exciton band (the first progression member of 1A1 exciton band) of crystalline phenanthrene has been measured in the temperature range 81 to 293 °K. The exciton‐phonon interaction is discussed using Toyozawas theory. The lower energy tail of the first exciton absorption band is confirmed to be expressed by Urbachs rule with the modified steepness parameter σ′a = 2.39 and σ′b = 2.35. The half‐width of the first exciton band is also confirmed to change linearly with temperature. The value of the steepness parameter and the temperature dependence of the half‐width provide experimental evidences that the exciton‐phonon interaction in crystalline phenanthrene is weak. The degree of the interaction is compared with that in anthracene. The assignment of the shoulder absorption band located at the first exciton absorption tail is discussed.

Structural phase transition in pyrene - how does it occur?

Abstract The phase transition in pyrene crystal has been studied by a continuous differential optical transmittance measurement. The result suggests that the transition around 120 K is of the first order structural transition and occurs domain by domain which are formed temporarily in the crystal. In some crystals the phase transition does not occur around 120 K but only a variation of the lattice constants occurs.

Phase transition measurement of pyrene and phenanthrene based on a rotary polarization method

The phase transition temperature in pyrene and phenanthrene crystals has been measured by a polarization modulation method, which enables one to detect the variation in the refractive index in the transparent region of a crystal. The phase transition occurs at 120K in pyrene and between 210 and 260K in phenanthrene.

Reflectance Spectra and Optical Constants of Pyrene Crystals

Reflectance spectra of pyrene have been measured on the a b plane of single crystals at 300 and 77 K. Refractive indices, complex dielectric constants and absorption coefficients are calculated for the a - and b -polarized components by Kramers-Kronig analysis of the reflectance spectra. Brief discussions on the Davydov splitting, vibrational levels, polarization ratios and the spectral change due to the phase transition are given.

Growth of Single Crystal Film of One-Dimensional 9, 10-Dichloroanthracene

Single crystal film of 9, 10-dichloroanthracene is grown on a glass or fused quartz plate placed at the bottom of a vessel filled initially to 80% of its volume with benzene solution saturated with 9, 10-dichloroanthracene. By slow evaporation of the benzene at room temperature, 9, 10-dichloroanthracene deposits on both surfaces of the plate from its upper end. The largest surface area is about 5×7 mm2. The normal to the film surface is oriented parallel to the crystallographic b-axis. The gross features of the reflectance and absorption spectra (the peak position and the relative peak height) are consistent with published data measured on small single crystals.

Thermomodulation Study of 0–0 Exciton Absorption Bands in Anthracene

Thermoreflectance spectra in the lowest singlet-singlet transition region of anthracene have been measured at liquid nitrogen temperature. The derivative optical constants are obtained by the Kramers-Kronig analysis of the spectra. The asymmetric broadening of the b -polarized 0-0 exciton absorption band with temperature is suggested. The frequency of the K =0 polariton for the 0-0 bsorpton band has been confirmed to be located at 25484 cm -1 and at 25497 cm -1 in the a - and the b -polarized spectra respectively. The K =0 polariton-exciton splittings for the a - and b -polarization are respectively 77 cm -1 and 378 cm -1 , whose magnitudes are proportional to the oscillator strength of the respective 0-0 absorption band. The b -polarized thermoreflectance spectrum at liquid helium temperature has also been measured and discussed briefly.

Recombination Luminescence in BaO

The photoluminescence of BaO powders has been studied at temperatures between 103 and 423 K. A luminescence band peaking at 2.92 eV is observed by exciting the samples with light in the region of exciton bands. A bimole-cular decay process of this 2.92 eV luminescence is observed at room temperature. This process is interpreted in terms of the thermal dissociation of excitons.