Ryuzi Katoh

Triplet Exciton Formation in a Benzophenone Single Crystal Studied by Picosecond Time-Resolved Absorption Spectroscopy

Abstract The formation of triplet excitons in a benzophenone single crystal has been studied by picosecond time-resolved absorption spectroscopy. The rate constant of the singlet-triplet intersystem crossing has been determined to be (28 ± 2 ps) −1 . With strong excitation the formation of the triplet exciton becomes faster. This is ascribed to the fission of a highly excited state, populated by a biphotonic process, into two triplet excitons.

Fission of a higher excited state generated by singlet exciton fusion in an anthracene crystal

Abstract It has been found that fluorescence from an anthracene crystal can be modulated by an external magnetic field under strong excitation (λ = 308 nm). Dependence on the magnetic field strength and its orientation with respect to the crystal axes have been measured. Magnetic field effects on the fluorescence provide direct evidence that a higher excited state, generated by singlet exciton fusion, undergoes fission into two triplet excitons.

Photoionization of a singlet exciton in an anthracene single crystal through two‐color, two‐step excitation

Photoionization of a singlet exciton has been studied in an anthracene single crystal. The photoionization spectrum and optical‐absorption spectrum have been measured with two‐color, two‐step excitation. By comparing the two spectra, the ionization efficiency (Φ) of higher excited states could be obtained as a function of the excitation energy. It was found that Φ=2×10−4 between 5.1 and 5.5 eV, independent of excitation energy. We interpret this result to indicate that autoionization occurs after a rapid relaxation from an initially populated higher excited state. Above 5.5 eV, Φ increases sharply. This is interpreted as the onset of another ionization channel, possibly the direct ionization. Polarization study of the photocarrier generation indicates that the intermediate state before the photoionization is the relaxed lowest singlet exciton state.

Photoionization and optical absorption of singlet excitons in a t-stilbene crystal: excitation energy dependence of the ionization efficiency

Abstract Photoionization and optical absorption of singlet excitons in a t -stilbene crystal have been studied. The ionization efficiency of the singlet exciton is a constant over the energy range of 5.4–6.5 eV. This is another example indicating that autoionization after a rapid relaxation is a common behaviour of this class of materials.

Generation of Charge Carriers Through Excited States

Abstract Charge carrier generation has been studied with two-color, two-step excitation. The complication associated with the penetration depth, which changes with the photon energy, can be eliminated with this technique. It has been found that in p-terphenyl and anthracene charge carriers are generated by photoionization of singlet excitons. Results of quantitative measurements of the photoionization cross section with additional information from transient absorption and fluorescence indicate that the initially created higher excited states relax rapidly to a lower-lying state which autoionizes. The capability of identifying the intermediate states with their lifetimes is demonstrated also with naphthalene and phenothiazine.

Observation of fluorescence from higher excited states in an anthracene crystal

Abstract Very weak fluorescence in the ultraviolet (210–380 nm) is observed when an anthracene crystal is excited at 397 nm. They are assigned as fluorescence from S 2 and S 3 states in the crystal. These higher excited states are populated by fusion of two S 1 excitons.

External photoemission by singlet-exciton photoionization in an anthracene single crystal

Abstract The external photoemission by singlet-exciton photoionization has been observed in anthracene single crystals with two-color, two-step excitation. One laser pulse (360 or 420 nm) generates singlet excitons and the photoemission occurs by the second laser pulse of longer wavelength (460–540 nm). The threshold energy of the photoemission has been found to be 5.5 eV and is insensitive to the energy of the first-step excitation. This indicates that the external photoemission occurs through the photoionization of a relaxed singlet exciton. The threshold energy of the photoemission via exciton photoionization (5.5 eV) is lower than that of the conventional photoemission with one-photon excitation (5.75 eV).

Observation of singlet exciton photoionization in anthracene single crystal at 2.95 eV

Abstract Photo-carrier generation with 420 nm light ( hv =2.95 eV) in an anthracene single crystal has been studied. The main experimental results are the following: (1) The photocurrent waveforms correspond to the penetration depth of the exciting light. (2) The number of carriers generated is proportional to I ex 2 and becomes proportional to I ex 3 4 under strong excitation. (3) The photocurrent has a temperature dependence different from that of the absorption coefficient of the ground state. These observations indicate that charge carriers are generated through photoionization of a singlet exciton, i.e. by sequential two-step excitation. With an applied electric field of 1.5×10 4 V cm −1 , the cross section of the singlet exciton photoionization (with λ=420 nm) is 7×10 −19 cm 2 . The Onsager length r 0 , determined from the temperature dependence of the photocurrent, is 36 A.

Fission and fusion of excitons in perylene crystal studied with VUV and x-ray excitation

Abstract Modulation of fluorescence from a perylene single crystal by a magnetic field has been studied with excitation in the VUV and with X-ray excitation. It has been concluded that fission of highly excited singlet state into a triplet pair dominates the modulation in the case of VUV excitation, whereas with X-ray excitation fusion of triplet excitons, generated by secondary electrons, plays an important role.

Transient photoabsorption by singlet excitons in p-terphenyl single crystals

Abstract The transient absorption spectrum of singlet excitons in a p -terphenyl crystal is reported. Using nanosecond laser pulses for excitation and monitoring, a spectrum has been obtained from 320 to 650 nm which shows two intense absorption peaks at 340 and 560 nm. The absolute cross sections of these absorptions are σ(337 nm) = 2.2 × 10 −15 cm 2 , σ(560 nm) = 7 × 10 −16 cm 2 , and the singlet-singlet annihilation rate constant is γ = 5 × 10 −8 cm 3 s −1 .