研究者詳細 - 秋元　郁子

論文 - 秋元　郁子

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Vibrational, electronic, and vibronic excitations of polar C60F18 molecules: Experimental and theoretical study

A. A. Popov, V. M. Senyavin, V. I. Korepanov, I. V. Goldt, A. M. Lebedev, V. G. Stankevich, K. A. Menshikov, N. Yu. Svechnikov, O. V. Boltalina, I. E. Kareev, S. Kimura, O. Sidorova, K. Kanno, I. Akimoto

PHYSICAL REVIEW B ( AMER PHYSICAL SOC ) 79 ( 4 ) 2009年01月 [査読有り]

Polarized optical conductivity spectra in the IR and visible range and variable-temperature photoluminescence spectra of the C60F18 single crystals in the 4-300 K temperature range are reported. Density-functional theory (DFT) calculations of vibrational spectra and time-dependent DFT calculations of the excitation energies of the C60F18 molecule are performed to interpret experimentally observed phenomena. Orientation of the C-3v-symmetric C60F18 molecules in the single crystal is revealed by comparison of the experimental polarized IR spectra and vibrational DFT calculations. A(2) symmetry is assigned to the lowest energy singlet excited state of C60F18, and hence pure electronic S-0 -> S-1 excitation is found to be dipole forbidden. Fine vibronic structure found at low temperature is interpreted in terms of Herzberg-Teller and Franck-Condon mechanisms. Considerable similarity with vibronic spectra of C-60 is found and explained by presumable localization of the lowest energy electronic excitation of C60F18 on the fullerenelike part of the molecule.

DOI

Up to 30 times enhancement of deep UV emission at room temperature by prolonged excitation of localized exciton in NaCl: I crystal

Ikuko Akimoto, Masahiro Shimozato, Ken-Ichi Kan'No

Physica Status Solidi (C) Current Topics in Solid State Physics 6 ( 1 ) 342 - 345 2009年 [査読有り]

A material emitting DUV light at room temperature is promising for an application to a new laser action. We report the remarkable phenomena that deep ultra-violet (DUV) emission at 220 nm, so-called NE emission, in NaCl:I crystal is enhanced up to 30 times at room temperature by prolonged excitation of the localized exciton. Such an accumulated effect is kept even if the excitation is interuped for a few minutes but is completely reset by white light irradiation at room temperature. The phenomena are discussed considering thermal ionization of iodine anion by the VUV light excitation, trapping of dissociated electrons in hole centers and recombination of electrons and iodine hole centers. © 2009 WILEY-VCH Verlag GmbH &amp
Co. KGaA.

DOI

Up to 30 times enhancement of deep UV emission at room temperature by prolonged excitation of localized exciton in NaCl:I crystal

Ikuko Akimoto, Masahiro Shimozato, Ken-ichi Kan'no

PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 6, NO 1 ( WILEY-V C H VERLAG GMBH ) 6 ( 1 ) 342 - 345 2009年 [査読有り]

A material emitting DUV light at room temperature is promising for an application to a new laser action. We report the remarkable phenomena that deep ultra-violet (DUV) emission at 220 urn, so-called NE emission, in NaCl:I crystal is enhanced up to 30 times at room temperature by prolonged excitation of the localized exciton. Such an accumulated effect is kept even if the excitation is interuped for a few minutes but is completely reset by white light irradiation at room temperature. The phenomena are discussed considering thermal ionization of iodine anion by the VUV light excitation, trapping of dissociated electrons in hole centers and recombination of electrons and iodine hole centers. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI

Multifrequency EPR analysis of the positive polaron in I-2-doped poly(3-hexylthiophene) and in poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene

Aranzazu Aguirre, Peter Gast, Sergey Orlinskii, Ikuko Akimoto, Edgar J. J. Groenen, Hassane El Mkami, Etienne Goovaerts, Sabine Van Doorslaer

PHYSICAL CHEMISTRY CHEMICAL PHYSICS ( ROYAL SOC CHEMISTRY ) 10 ( 47 ) 7129 - 7138 2008年 [査読有り]

The W-band continuous-wave electron paramagnetic resonance (EPR) analysis of chemically induced polarons in drop-cast and spin-coated polyphenylenevinylene-type and polythiophene-type polymer films reveals rhombic g tensors in both cases. The dependence of the W-band EPR signals on the orientation of the spin-coated films with respect to the magnetic field indicates a high degree of backbone alignment with the substrate and allows a partial assignment of the g tensor orientation. The derived molecular orientations of the polymer chains in the spin-coated films show clear differences between the two types of polymers. The proton hyperfine interactions obtained from X-band HYSCORE (hyperfine sublevel correlation) and Q- and W-band pulsed ENDOR (electron-nuclear double resonance) experiments are interpreted in terms of earlier theoretical studies on the extension of the polarons.

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Investigation of optical properties of fluorofullerene C60F18 single crystals

A. M. Lebedev, K. A. Menshikov, N. Yu. Svechnikov, V. G. Stankevich, O. V. Boltalina, I. V. Goldt, S. Kimura, T. Nishi, I. Akimoto, K. Kan'no

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT ( ELSEVIER SCIENCE BV ) 575 ( 1-2 ) 96 - 98 2007年05月

For the first time optical investigations of C60F18 single crystals were carried out in the temperature region (300-4) K. Intensity of the obtained photoluminescence (PL) spectra has a strong brake near 100 K. At low temperature the clear fine structure of luminescence spectra is observed. The reflectance spectra have shown main changes for the energy about 4 eV (typical for pi-pi* transitions), and for E > 8 eV for deeper electronic levels. (C) 2007 Elsevier B.V. All rights reserved.

DOI

Decay dynamics of photo-luminescence in tris(2-phenylpyridine)iridium phosphor

I. Akimoto, K. Nagao, T. Tashiro, H. Osuga, K. Tanaka, K. Kan'no

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 122 ( 1-4 ) 440 - 443 2007年01月 [査読有り]

The phosphor material, tris(2-phenylpyridine)iridium (Ir(ppy)(3)), has attracted much attention as an emitter of highly efficient electrophosphorescent devices. In the present work, decay dynamics of photo-luminescence and its temperature dependence have been investigated in the Ir(ppy)(3) emitting center. From the analyses of luminescent decay profiles and time-resolved luminescent spectra, thermally activated population transfer from every triplet sub-level to a singlet state is suggested. (c) 2006 Elsevier B.V. All rights reserved.

DOI

Magnetic field dependence of triplet state in iridium complex phosphor

Ikuko Akimoto, Kazuhiro Nagao, Ken-ichi Kan'no

PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 4, NO 3 ( WILEY-V C H VERLAG GMBH ) 4 ( 3 ) 813 - + 2007年 [査読有り]

The magnetic field dependence of photo-luminescent spectrum in tris (2-phenylpyridine) iridium complex is reported. Upon applying magnetic field from 0 to 6 T, remarkable change of spectral band shape was observed at 4.2 K. Comparing the spectral variation under magnetic field with the time resolved spectra, it is clarified that the luminescence involves distinctive contributions from, at least, two spectral sets of zero-field-split triplet sub-levels, and an additional fast decay component which is presumably associated with the nearly singlet manifold. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Photoluminescence and relaxation processes in the optically excited states of fluorinated fullerenes

I. Akimoto, M. Mori, K. Kan'No

AIP Conference Proceedings 786 33 - 36 2005年09月

Origin of the photoluminescence and relaxation processes in the optically excited states of fluorinated fullerenes has been investigated by site selective excitation and time resolved spectroscopy. The intrinsic luminescence consists of at least two components with the different kind of decay kinetics. One of which is of single exponential with τd ∼ 6 ns and the other is of non-exponential but proportional to t-1. The latter component seems to be recombination luminescence due to an intermolecular polaron, in which an electron and a hole are separately localized with accompanying some structural relaxation. © 2005 American Institute of Physics.

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Photo-luminescence properties and exciton-carrier interaction in 1,2-diarylethylene derivatives films

Akimoto, I, KI Kan'no, H Osuga, K Tanaka

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 112 ( 1-4 ) 341 - 344 2005年04月

New organic materials, 1,2-diarylethylene derivatives consisting of ortho-fused heterocyclic system like benzodithiophene (BDT) have been reported as candidates of thermally stable fluorescent materials, promisingly used in organic electro-lummescence devices. Optical properties of the compound possessing methyl or phenyl substituents in ethylene moiety are investigated. Photo-luminescence (PL) is observed at blue-green region accompanied by molecular vibrational structures. The quantum efficiency of the fluorescence in H-BDT toluene solution is estimated as 0.32 at room temperature. On the basis of quantum chemical calculation, the molecular structure and electronic structure are discussed. We also investigated the electronic and optical properties of H-BDT thin films prepared by sublimation method. For a preliminary single-layered device (fTO/H-BDT/Al:Mg), the I-V property and the effect of applied electric field on PL intensity were investigated and a possibility of the exciton-carrier interaction in the organic layer was pointed out. (c) 2004 Elsevier B.V. All rights reserved.

DOI

Luminescent decay kinetics in a quasi-one-dimensional crystal, C5H10NH2PbI3

Akimoto, I, M Sakai, K Kan'no

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 108 ( 1-4 ) 31 - 35 2004年06月 [査読有り]

Photoluminescence and its decay kinetics for a single crystal of piperidinium triiodoplumbate C5H10NHPbI3 have been investigated. The luminescent spectrum exhibits a single emission band at 2.1 eV with FWHM 0.3 eV, which has been attributed to a self-trapped exciton. The luminescent kinetics is discussed for the temperature range below 70K, where the integrated luminescent intensity is almost constant. The exponential decay time tau(d) apparently decreases from 44 mus at 5 K to similar to2 mus at 70 K while the initial intensity I-0 increases by one order of magnitude, so that the product I-0 x tau(d) is almost constant. Such a phenomenon is explained by assuming that the initial state of the luminescence consists of at least two sub-levels, between which population transfer is thermally achieved with an activation energy of 6 meV. (C) 2004 Elsevier B.V. All rights reserved.

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Photoluminescence and Near-Edge Optical Absorption in the Low-Temperature Phase of Pristine C60 Single Crystals

Ikuko Akimoto, Ken-Ich Kan'no

Journal of the Physical Society of Japan 71 ( 2 ) 630 - 643 2002年02月 [査読有り]

Photoluminescence and near-band-edge electronic structure of pristine C60 single crystals have been investigated in the low temperature phase below 90 K. By applying two-photon excitation spectroscopy, a monomorphic spectrum is extracted from the several crystals which exhibit apparent diversity for ordinary one-photon excitation. Also, a localized absorption band with discrete vibronic sub-peaks is clearly confirmed at 1.685 eV below the fundamental absorption edge (1.815 eV). Overall features of the luminescence-line-narrowing spectrum obtained by site-selective excitation into the localized absorption band are assigned to the vibronic progressions coupled with the Raman-active gerade-symmetry modes of individual C60 molecules (i.e., eight h g, two ag, and their combinations), accompanied by phonon-sidebands due to cooperative librations and intermolecular translations. The origin of photoluminescence and its relaxation kinetics are fully discussed in connection with the inhomogeneously broadened localized states which are presumably due to perturbed Frenkel excitons stabilized with symmetry-lowering by adjacent intrinsic defect or some lattice disorder.

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Dynamics of the photoinduced ionic-to-neutral phase transition in tetrathiafulvalene-p-chloranil studied by femtosecond time-resolved reflection spectroscopy

I. Akimoto, K. Tanimura, S. Koshihara

Phase Transitions 75 ( 7-8 ) 815 - 822 2002年01月 [査読有り]

DOI

Femtosecond time-resolved spectroscopy of photoinduced ionic-to-neutral phase transition in tetrathiafulvalen-p-chloranil crystals

K Tanimura, Akimoto, I

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 94 483 - 488 2001年12月 [査読有り]

Recent experimental results are reviewed of the photoinduced phase transformation between ionic and neutral phases in tetrathiafulvalen-p-chloranil crystals. Spectroscopic studies have revealed different features of Frenkel-type and charge transfer (CT)-type excited states in inducing photo-induced structural phase transition: the transition can be induced only above threshold-excitation intensities in the case of CT excitation. The thresholds imply that non-linear processes of CT excited states are crucial in the transition. On the other hand, time-resolved studies have demonstrated a dynamical aspect of the transition. optical reflectance signals that probe the evolution of the transition show dumped-oscillatory changes before establishing N-phase domains at around 100 ps after excitation. Mechanism of the transition has been discussed based on these results. (C) 2001 Elsevier Science B.V. All rights reserved.

Origin of photoluminescence and spectral analysis of vibronic structure resolved in C-60 single crystals

Akimoto, I, K Kan'no

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 87-9 788 - 790 2000年05月 [査読有り]

Origin of the photoluminescence and band-edge electronic structure of pristine C-60 single crystals have been studied in the temperature range from 4 to 300 K by applying several spectroscopic techniques such as absorption, photoluminescence under one-photon and two-photon excitation, and site-selective luminescence line narrowing spectroscopy. Photo-absorption characterized by the prominent vibrational structures is found around 1.688 eV below the fundamental absorption edge (1.814 eV), which apparently exhibits mirror images of the well-known spectrum of photoluminescence called type A. Upon selective excitation into the band, luminescence line narrowing is observed: The spectra consists of sharp vibrational lines attributed to the progressions of eight H-g and two A(g) modes and libron sidebands. These results indicate that type A luminescence originates from the inhomogeneously broadened localized state which may be stabilized by symmetry lowering and local rotational disorder spread over more than one molecule. (C) 2000 Elsevier Science B.V. All rights reserved.

Luminescence from C-60 single crystals in glassy phase under site-selective excitation

Akimoto, I, M Ashida, K Kan'no

CHEMICAL PHYSICS LETTERS ( ELSEVIER SCIENCE BV ) 292 ( 4-6 ) 561 - 566 1998年08月 [査読有り]

Luminescence from C-60 single crystals has been investigated at liquid He temperature under selective excitation into the tail part below the fundamental absorption edge. Luminescence line narrowing is observed for resonant excitation, in which a characteristic vibrational quantum is found similar to 420 cm(-1) in the excited state. It is revealed that the spectrum which has been known as photo-luminescence from C-60 single crystals is an superposition of site-selective luminescence from several localized states. Such inhomogeneity is most likely to arise from fluctuation of the intermolecular orientational correlation in a pair of C-60 molecules over which the excitation is delocalized. (C) 1998 Elsevier Science B.V. All rights reserved.

Relaxation dynamics into two types of luminescence states in C-60 single crystals below 80 K

Akimoto, I, J Azuma, M Ashida, K Kan'no

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 76-7 206 - 210 1998年02月 [査読有り]

Luminescence spectra from pure C-60 Single crystals and their time behavior have been studied in the temperature range below 80 K under the excitation into two distinctive energy ranges. In case of the excitation into the higher-lying band slates, two kinds of fluorescence bands, called types A and B, were observed with their onsets at 733 and 684 nm around LHeT. These bands exhibit the anti-correlated temperature dependence, indicating thermally activated population transfer from the initial state of type B to that of type A. On the other hands, the selective excitation into the tail part (similar to 733 nm) below the fundamental absorption edge stimulates not only a series of Raman lines but also the resonant luminescence which evolves into the type A luminescence with inhomogeneous behavior. It is noteworthy that for just resonance a novel vibronic structure with Delta similar to 120 cm(-1) is clearly resolved. The present study strongly suggests that fluctuation in the local intermolecular orientational correlation of molecules is essential to the relaxation processes of photo-excited states in C-60 Single crystal. (C) 1998 Elsevier Science B.V. All rights reserved.

Pulse-annealing study of photo-stimulated luminescence and trapped-hole states in BaFBr:Eu2+

A Ohnishi, Akimoto, I, K Kanno, Y Iwabuchi, N Mori

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 72-4 775 - 777 1997年06月 [査読有り]

Optically detected magnetic resonance and zero-field splitting energy of the self-trapped exciton in NaBr

K. Kan'no, M. Shirai, T. Matsumoto, I. Akimoto

Materials Science Forum ( Trans Tech Publications Ltd ) 239-241 569 - 572 1997年

The triplet states of the type I self-trapped exciton (STE) in NaBr have been studied using a pulsed X-band (9.4 GHz) ODMR technique for the magnetic field H//[110] under the two-photon excitation with KrF laser pulses: An EPR signal from the Mz=0 sublevel in the [110]-oriented STE was time-resolved at 5.4 K, by monitoring a transient change induced in the decay curve of triplet emission ("π band" at 4.6 eV) when a microwave pulse (10 μs duration) was applied at 20 μs after the laser excitation. The low-field transition of (Mz=0 ⇒ Mz=-1) was confirmed to appear around 1.44T as a broad EPR band with partially resolved seven lines. The Zero-field splitting was unambiguously determined to be D=0.2 meV, in good agreement with our previous study of magnetic field effect on luminescence lifetimes. The D value thus determined is one order of magnitude larger than that of the type III STE in KBr (0.03 meV). Such a large D value comes most likely from a geometry of on-center type, i.e. [VK + e] configuration, though it is still open problem whether the type I STE in NaBr strictly keeps the D2h symmetry.

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Emission spectra and decay characteristics in photo-stimulated (CnH2n+1NH3)2CdCl4 : n=1, 2, 3

A. Ohnishi, T. Yamada, T. Yoshinari, I. Akimoto, K. Kan'no, T. Kamikawa

Journal of Electron Spectroscopy and Related Phenomena ( Elsevier ) 79 163 - 166 1996年 [査読有り]

Recombination luminescence from (CnH2n+1NH3)2CdCl4 : n=1, 2, 3 has been studied using synchrotron radiation pulses. A largely stokes-shifted emission band was found to appear at 2.50eV for the n=2 crystal when excited into the excitonic absorption range, while at 2.25eV when excited into the band-to-band transition range. Decay curves of both bands are composed of fast and slow components. Non-exponential decay of slow components suggests that the 2.25eV band is induced by tunneling recombination between self-trapped electrons and self-trapped holes. As for the 2.50eV band, recombination of correlated electron-hole pairs with strong lattice relaxation is supposed.

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