Atsuo Matsui

Luminescence of Free and Self Trapped Excitons in Pyrene

The resonance luminescence due to radiative annihilation of free excitons in Pyrene has been observed. The luminescence band is located at 376 nm (F) at room temperature and is quite distinctive from so-called excimer luminescence band located at 450 nm. There is a potential barrier between the free exciton state and the excimer state which is located 540 cm -1 (66 meV) lower in energy with respect to the free exciton state. The luminescence decays exponentially with a life time of 180 nsec for both free excitons and excimers at 282 K. Discussion is given to explain free exciton creation and luminescence processes in terms of thermal equilibrium established between free excitons and excimers. The excimer is recognized as the self trapped exciton.

Time-resolved excitonic luminescence processes in poly(phenylenevinylene)

Integrated luminescence spectra, time-resolved luminescence spectra, and luminescence decay times of stretch-oriented poly(phenylenevinylene) (PPV) are studied in the temperature range 300∼12 K. The reflectance spectrum at room temperature is also studied. The luminescence spectrum is composed of vibronic bands and a broad band, which are interpreted as caused by radiative annihilation of free and selftrapped excitons, respectively. Temperature dependence of the intensity of free-exciton luminescence and temperature dependence of the decay time are interpreted in terms of relaxation of excitons. The height of a potential barrier, which separates the free exciton state and the self-trapped state, is found to be 350 cm -1 . At 20 K, exciton relaxation toward the self-trapped state occurs by a quantum-mechanical tunneling process. The tunneling rate obtained is (90 ps) -1 .

Luminescence of Free and Self-Trapped Excitons in α- and β-Perylene Crystals

Luminescence spectra and decaytimes have been measured over a wide temperature range. Thermal equilibrium population between free and self-trapped excitons has been confirmed in both dimeric (α) and monomeric (β) forms of perylene crystals. Two-center type self-trapped exciton in α-perylene, and one-center type and two-center type self-trapped excitons in β-perylene are stable and play prominent roles in exciton radiative decay. The decaytimes of free and self-trapped exciton luminescence are discussed in terms of exciton-lattice interaction. The exciton band width is briefly discussed.

Optical Properties of Anthracene Single Crystals

Reflection spectra of anthracene are analysed by Kramers-Kronig method in the wavelength region from 192 to 600 mµ. The oscillator strength of the first electronic transition A 1 g → B 2 u is obtained with Lorentz field correction. The oscillator strength for individual vibronic band at room temperature is in excellent agreement with that obtained in the solution spectrum. The total oscillator strength f a + f b is 0.100 and 0.107 at room and at liquid nitrogen temperatures respectively. The quantity of the ratio of the oscillator strength as well as the absolute value of the oscillator strength is discussed in connection with the ratio of absorption intensities. Result suggests that the first vibronic band involves a contribution which arises from a thermally induced transition. Lorentz field in the anthracene crystal shows large anisotropy with respect to the direction of electric field applied. The oscillator strength obtained by simplified method is compared with that reported by several investigators.

Wavelength-Dependent Decay Times and Time-Dependent Spectra of the Singlet-Exciton Luminescence in Anthracene Crystals

Decay curves and spectral shapes of the singlet-exciton luminescence in anthracene crystals have been studied at room temperature and found to depend on the experimental geometry and on the penetration depth of the exciting light. The well-known wavelength-dependent decay time and time-dependent spectrum occur only when the luminescence is excited in the process of the one-photon absorption and is observed in the backward-scattering geometry. In the forward-scattering geometry, the decay time is independent of the luminescence wavelength. When the excitons are uniformly generated in the crystal in the process of the two-photon absorption, the wavelength-dependent decay time and time-dependent spectrum do not occur even in the backward-scattering geometry. These results are interpreted in terms of diffusion of the singlet excitons and reabsorption of the short wavelength photons of the exciton luminescence.

Optical Absorption and Exciton Lattice Interaction in α- and β-Perylene

The shape of the lowest exciton absorption band in α- and β-perylene due to the B 2u ←A g transition, the halfwidth of the absorption band, and the steepness parameter of the Urbach tail are investigated. From linear temperature dependence of the halfwidth and the Lorentzian line-shape, the transition A u ←A g is well explained by the exciton band picture. The lattice relaxation behavior of excitons after they are formed is described by the strong exciton-phonon coupling scheme. Based on the steepness parameter obtained, 0.93 in α-perylene and 1.38 in β-perylene, and the amount of the Davydov splitting obtained, 2040 cm -1 in α-perylene and 960 cm -1 in β-perylene, the lattice relaxation energy is estimated to be 1530 cm -1 and 523 cm -1 respectively. Also the energy difference between the free and self-trapped exciton states is expected to be 620 cm -1 in α-perylene and 43 cm -1 in β-perylene.

Lineshape Analysis of the 0–0 Exciton Absorption Band in Pyrene Crystals

The lineshape and the line-width of the 0–0 exciton absorption band associated with the transition 1 B 3u → 1 A 1g are investigated over the temperature range from 300 K to 126 K. The line-width changes linearly with temperature in the b -polarized 0–0 band ( b -band), indicating that the exciton state in pyrene is described by a weak scattering scheme. The high-energy side of the b -band is found to have a Lorentzian lineshape which is also an indication of weak scattering. Contrary to the b -band, the high-energy side of the a -polarized 0–0 band ( a -band), which is located above the b -band, has a Gaussian lineshape suggesting that excitons in this band are scattered to the lower b -band. The lineshape on the low-energy side of the a - and b -bands exhibit a tendency to be a Gaussian at low temperatures. The exciton-phonon coupling constant and the exciton band half-width are discussed.

Self Trapping of Excitons in Monomeric Perylene Crystals

Based on the observation of free exciton luminescence at room temperature, self trapping of excitons is recognized in β-perylene crystals which have a monomeric structure. Temperature dependence of the luminescence spectra suggests the existence of two sorts of self trapped states. The result obtained are consistent with those anticipated by the analysis of the Urbach rule in β-perylene.

Frenkel Exciton Dynamics in Anthracene under High Pressure and Quasi-Free Exciton State

The polarized absorption and luminescence spectra of anthracene crystals have been investigated under high pressures and at room temperature. A satellite absorption band which shifts to lower energy with increasing pressure is found to be related with the vibronic luminescence series. As the origin of the vibronic luminescence a model of a quasi-free exciton is proposed. The vibronic luminescence bands loose their intensity with pressure and are replaced by intrinsic self-trapped exciton luminescence at pressures higher than about 18 kbar. Self-trapping of excitons seems to be enhanced by the second order phase transition which occurs at 24 kbar. The exciton-phonon coupling constant at 22 kbar is estimated to be at least 1.6 which should be compared to 0.85 at ambient pressure.

Gas-Flow Cryostat

A cryostat designed to cool specimens with cold gas has been constructed. Its working temperature range is from room temperature to 4.38 K, although it is normally operated down to 5.1 K. Three specimens can be mounted in the cryostat simultaneously, and their temperature can be reduced at as slow a rate as 0.1°C/min. The luminescence emitted from the specimens over a solid angle of about 0.26 sr is collected and guided to a monochromator.