Masahiro Kotani

Three-dimensional orientation of single molecules observed by far- and near-field fluorescence microscopy

We present a simple and straightforward method for determining absolute spatial orientations of transition dipole moments of single fluorescent molecules. Far-field polarization microscopy provides angles of the dipole moments projected in the plane of the sample. Optical field near total internal reflection surfaces has a strong component perpendicular to the sample and, for a given in-plane angle, provides unambiguous orientation of the molecular dipole moment. Experimentally, both excitation modes are alternated to monitor real-time conformational dynamics of tetramethylrhodamine molecules covalently attached to a quartz substrate.

Spontaneous formation of an ordered structure during dip-coating of methylene blue on fused quartz

Molecular orientation in thin films of methylene blue, prepared by dip-coating, has been studied by UV-visible absorption spectroscopy and X-ray diffraction. In multilayered films the molecules are essentially standing normal to the surface of a fused quartz substrate and form a layered structure with a periodicity that corresponds to the molecular length. Due to this arrangement the film is only faintly colored, since, with normal incidence, the electric field of the incident light is orthogonal to the transition moment of the molecules. This structure can be formed by self-organization in the course of drying, not by epitaxy.

Reactivity, absorption and emission of dimethylgermylene photo-generated from dodecamethylcyclohexagermane

Abstract The UV photolysis of dodecamethylcyclohexagermane ( 1 ) in cyclohexane at room temperature proceeds readily with loss of dimethylgermylene species to give two of the lower homologs, decamethylcyclopentagermane ( 3 ). The photochemically generated dimethylgermylene species reacts with 2,3-dimethylbuta-1,3-diene or CCl 4 to give 1,1,3,4-tetramethyl-1-germacyclopent-3-ene or trichloromethyldimethylchlorogermane, respectively. The transient absorption of 1 in cyclohexane at 450 nm obtained by laser flash-photolysis is due to dimethylgermylene. The reaction rates of dimethylgermylene with some substances are examined. In 3-methylpentane glass dimethylgermylene shows an absorption band at 430 nm and a fluorescence peak at 650 nm. The photo-generated germylene appears to be the singlet ground state.

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.

Photon Stimulated Desorption of Positive Ions from LiF

Photon stimulated desorption (PSD) of positive ions from LiF is observed in the photon energy region between 30 and 70 eV. The creation of the Li+(1s→2s) core exciton leads to the rapid rise of both Li+ and F+ desorption yields. It is found that PSD of positive ions from LiF is due to the interatomic Auger transition. The yield spectrum of F+ ions is different from that of Li+ ions above 60 eV. The F+ yield spectrum has a deep valley at around 63 eV, while the Li+ spectrum shows a plateau in the same region. However the difference in the yield spectra of Li+ and F+ ions can not be explained at present.

Molecular alignment in organic thin films

Abstract Molecules can be oriented when they are adsorbed onto a solid surface with an appropriate surface structure. Examples are shown which illustrate a unidirectional alignment. 4-Dimethylamino-4′-nitrostilbene (DMANS) exhibits an oriented adsorption when evaporated onto a cleaved face of a glycine single crystal. The same compound exhibits also a high degree of orientational order when an evaporated film of the compound is mechanically brushed in a direction. The mechanism of the molecular alignment is discussed based on the results of polarized optical absorption, optical second-harmonic generation, X-ray diffraction and scanning force microscopy.

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.

Charge carrier generation in p‐terphenyl crystal. Autoionization of higher excited states

Photoionization of singlet excitons in p‐terphenyl crystal is studied with nanosecond laser excitation. The photoionization spectrum and the photoabsorption spectrum of the lowest excited state are measured. The ratio of the two spectra gives the efficiency with which higher excited states, generated by the two‐step excitation, ionize. The observed efficiency is constant below 7.3 eV and is remarkably high (∼10−2 ). The results do not support the view that the excess energy is converted directly to the kinetic energy of the generated electrons in the crystal and indicate that the initially prepared higher excited states relax rapidly and the succeeding autoionization yields electrons with a fixed kinetic energy.

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.