Author(s): Kanno, K; Kawabata, T; Ishikawa, Y; Katayama, J; Hirao, M; Tabaru, Y; Watanabe, M; Kumon, K; Tsuji, H; Tanaka, K; Saito, K; Yoshimura, T; Forschler, B; Lewis, V; Lee, CY

Photo-excited states of piperidinium tribromoplumbate (C 5 H 10 NH 2 PbBr 3 ) single crystals were investigated by spectroscopic methods. The strong anisotropy was observed in polarized reflection spectra. A peak with large oscillator strength appears at 3.9 eV only for the light polarized parallel to the [PbBr 3 ] n 1- chain, which is assigned as the lowest exciton transition. These polarized reflection spectra indicate that electrons and holes are strongly confined within the chain, suggesting that the system can be considered as natural multiple quantum wires. Two luminescence bands appear around 3.0 eV (V emission) and 1.9 eV (R emission) under excitation into the intrinsic absorption region. The large Stokes-shifts of the luminescence bands indicate strong electron-phonon coupling in the excited states. Temperature dependence of luminescence intensities and decay lifetimes reveal that population transfer from the initial state of the V emission to that of the R emission is thermally activated via a potential barrier of about 35 meV. The excitation spectra and the decay kinetics of the luminescence bands strongly suggest that the initial states of luminescence bands are qualitatively different kinds of self-trapped excitons.

X-ray and ultra-violet photoemission spectra have been measured to study the valence-band structures of piperidinium trihaloplumbates C5H10NH2PbX3 [X = 1, Br], which have natural nano-scale one-dimensional structures. The valence-band spectra of lead halides, which have the three-dimensional structures, are also reported for comparison. The partial densities of states related to the constituent ions and molecules were obtained with the comparison between the XPS and UPS. Empirical LCAO calculations were also carried out to analyze the experimental results. The quantum-well-like electronic structure in C5H10NH2PbX3 [X = I, Br] is proposed.

Origin of the photoluminescence and band-edge electronic structure of pristine C60 single crystals have been studied in the temperature range from 4 to 300K 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 1.688eV below the fundamental absorption edge (1.814eV), 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 Hg and two Ag 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.

Optical properties of pristine C-60 single crystals have been studied in the temperature range from 4 K 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. Localized absorption characterized by the prominent vibrational structures is found around 1.69 eV below the fundamental absorption edge (1.81eV), 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 consist of sharp vibrational lines attributed to the progressions of eight Hg and two Ag modes and libron sidebands. These results indicate that the type A luminescence originates from the inhomogeneously broadened localized state which may be stabilized by the Symmetry lowering and local rotational disorder spread over more than one molecule.

Temporal behaviors of the radiative recombination of excitons have been studied in β-ZnP 2 by using picosecond laser pulses at 2 K. Under interband excitation with E // c , the singlet exciton lumi...

Temporal behaviors of the radiative recombination of excitons have been studied in beta-ZnP2 by using picosecond laser pulses at 2K. Under interband excitation with E//c, the singlet exciton luminescence decays with two exponential components, 250ps and 1.25 ns. The latter value is reasonable for the allowed exciton transition. The triplet exciton luminescence decays also with two exponential components, 150ps and about 2.05ns. These decay times of the triplet luminescence are surprisingly short, because the triplet exciton is only weakly allowed and expected to have a long lifetime. The relative intensity of the fast component of the triplet luminescence decreases, as the excitation density decreases. This component is attributed to the formation of the excitonic molecule via collision of two triplet excitons. The decay time of the slow component does not change largely with the excitation power, suggesting effective nonradiative decay processes. When the exciton is created in a bulk of the crystal under excitation into the triplet exciton band with E//b, the triplet luminescence shows a build-up component, for which exciton diffusion mechanism is suggested. The de-excitation processes of the triplet exciton are briefly discussed.

The lowest triplet state of the type II relaxed exciton associated with a guest iodine monomer in KBr:I has been investigated at 3.7 K by a time-resolved optically detected magnetic resonance (ODMR) technique using a combination of pulsed microwave and ArF laser pulses for the magnetic field H//[110]. ESR spectra show both fine structure and partially resolved hyperfine structures of a V-K core (IBr)(-). The zero-field splitting is determined to be D = 11.29 kG and the hyperfine constants are estimated to be A(//)(I) 262 G and A(//)(79Br =) 302 G. These hyperfine constants, which are much greater than 1/2 of those of the corresponding hetero-nuclear V-K center, reveal large overlap of the electron on the V-K Core, especially on the bromine nucleus. This suggests that the bromine anion shifts toward the iodine anion and the electron is bound to the resulting half-vacancy. Time evolution of ODMR spectra and magnetic field dependence of the triplet lifetimes are also explained through the hyperfine interactions.

The results of optical investigations of C-60 fluorine derivatives using synchrotron, laser and traditional light sources are presented and discussed. Optical absorption and luminescence spectra were studied. Luminescence and luminescence excitation spectra enable us to assume that the luminescent state is derived from the excitation of the pi double bonds of the C-60 skeleton and that the excitation of the C-F bonds of the C60F2x molecule relaxes through the luminescent state with high probability.

This paper is devoted to investigation of luminescence and some electric properties of C-60 single crystal and their connection with X-traps model.Luminescence properties for C-60 single crystal, the temperature dependence of the luminescence spectra and decay kinetics were measured for wide range of excitation energies. At low temperatures the emission spectra with well resolved structure were detected. Opposite to the case of C-60 films, the single crystal shape of the luminescence spectra also change with temperature. This behavior can be described in terms of X-trap model as proposed in [1].The luminescence kinetics under 36 eV excitation as a function of temperature was investigated in nanosecond range. At temperature T < 300 K, the decay curve is non-exponential and it can be fitted formally by two exponents with characteristic times in the 1-100 ns range. We suppose the explanation of this relaxation behavior requires more particular information about electron-hole recombination dynamics which can be affected by X-traps.Following X-traps concept we present preliminary results of thermostimulated currents spectroscopy experiments. It reveals the availability of two thermorelaxation processes in C-60 single crystal and therefore indeed the presence at least of two types of traps for charges in this crystals.

Luminescence from C60 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 ∼420 cm−1 in the excited state. It is revealed that the spectrum which has been known as photo-luminescence from C60 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 C60 molecules over which the excitation is delocalized.

Luminescence spectra from pure C60 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 states, 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 ( ~ 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 Δ ~ 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 C60 single crystal.

Fluorinated fullerenes C60Fx (x<=48) are attractive compounds of a novel wide-band-gap insulator with high symmetric carbon-based molecules. Various compositions x of fluorine atoms with semi-covalent or semi-ionic C-F bondings can be prepared by reacting C-60 powder with F-2 gas. Very recently single crystals with an excellent crystal habit have become available, and an X-ray structural analysis of the single crystal has been successfully carried out. After reviewing the fundamental knowledge on the sample preparation and the molecular and crystal structures, we present of preliminary results of absorption, luminescence and Raman spectra, decay times and photo-induced desorption obtained for films, powders and single crystals of highly fluorinated fullerenes C60Fx (x=42-48). These results suggest that the atomic and/or molecular relaxation following the electronic excitation of C-F bondings results in the decomposition of C-60 skeletons.

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=O 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 (''x band'' at 4.6 eV) when a microwave pulse (10 mu s duration) was applied at 20 mu s after the laser excitation. The low-field transition of (Mz=0 double right arrow Mt=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 ill STE in KBr (0.03 meV). Such a large D value comes most likely from a geometry of on-center type, i.e. [V-K + e] configuration, though it is still open problem whether the type I STE in NaBr strictly keeps the D-2h symmetry.

Photo-stimulated luminescence (PSL) with F(Br−)-band light was studied with a pulse annealing technique in the X-rayed BaFBr and BaFBr:Eu2+ crystals. It was found that the PSL band due to Eu3+ centers in BaFBr:Eu2+ is kept unchanged for pulse annealing up to 280 K, though the one due to O− centers in BaFBr comes to disappear after pulse annealing around 200 K. This clearly indicates that Eu2+ ion plays an important role as the hole-trap in the image storage.

We review updated understandings of the electronic and atomic structures, relaxation dynamics and spin-multiplicity of self-trapped excitons (STEs) in alkali halide crystals, on the bases of recent experimental results of the STE luminescence. It is pointed out that seemingly complicated diversity in the features of STE luminescence and F-H pair formation can be explained from an unified viewpoint. That is, depending upon the kinds of crystals there can arise the ''adiabatic instability'', which results in multiple local minima on the adiabatic potential energy surfaces (APES's) for the singlet and triplet STE states of the lowest orbital energy. A simple phenomenological model is proposed, which indicates that the electron-hole spin interaction plays an essential role in correlation with the electron-lattice and hole-lattice interactions. The shape of the APES's, the configuration dependence of the electron-hole exchange energy, relaxation dynamics and the structure of STEs are discussed in relation to the bi-stability of parity-broken and -unbroken STEs.

Fluorinated fullerenes C60Fx (x ⩽ 48) are attractive compounds of a novel wide-band-gap insulator with high symmetric carbon-based molecules. We present results of absorption, luminescence and excitation spectra obtained for films of highly fluorinated fullerenes C60Fx (x = 42). We confirm that the absorption is maximized around 6.4 eV and broad luminescence bands appear around 2.4 and 3.4 eV.

Photoluminescent (PL) properties of pure C60 thin solid films were investigated in the emission range of 1.2–2.4eV over a wide temperature range, under excitation with 36eV photon from the undulator beamline of UVSOR. Degradation of PL intensity of the 1.67eV band was discovered under long-time irradiation in ultra-high vacuum condition and its dose dependence was examined at various ambient conditions. Temperature dependence of PL intensity from 10 K to 300 K exhibited an anomaly at the vicinity of the structural phase transition Tc ∼ 250K. Time-resolved decay measurements exhibited the decay components of 1.2±0.1ns and of about 400μs. From the influence of oxygen on degradation of PL intensity, we suppose that the triplet state is highly populated and non-radiative decay process mainly occur.

Auger-decay-free core luminescence in RbF, CsF, CsCl, CsBr and BaF 2 has been studied under monochromatic vacuum ultraviolet light excitation using a time-resolved technique. Luminescence spectra associated with core holes are carefully separated from those originating in the valence-band excitation. Based on the spectral shape and energy range of the luminescence bands, a lattice relaxation effect following the core hole creation is discussed.

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.