吉田 弘幸

The electron affinity (EA) of an organic semiconductor is a measure of the electron transport level. Although reliable values of the EA are required for designing the device architecture of organic light-emitting diodes (OLED), there were no appropriate methods. Recently we have developed low-energy inverse photoemission spectroscopy which enables us to determine the EA of organic materials in solid with the precision required for research of OLED. Using this new technique, we precisely determined EA of typical OLED materials, TCTA, CBP, Ir(ppy)(3), BCP, Alq(3) and Liq as well as a newly developed dopant 4CzIPN. The obtained electron affinities are generally smaller by about 1 eV than the commonly believed values urging the reconsideration of the electron injection/transport mechanisms in OLED. We also compare EAs determined by various experimental and calculation methods for 29 materials. The results show that the reduction potential gives a reasonable estimate rather than the optical gap and ionization energy. (C) 2015 Elsevier B.V. All rights reserved.

高い電荷移動度を示すペンタセン単結晶は，有機固体内の電荷輸送メカニズムを解明するモデル物質の一つと注目されている。近年，有機半導体結晶の価電子バンド構造の測定が，角度分解型光電子分光法によって可能となった。しかし，これまでの研究では大気曝露を経た試料に対して測定が行われており，試料表面の酸化物の影響が懸念される。本研究では，ペンタセン単結晶を真空内で劈開し，得られた清浄表面のバンド分散を実測した。

The effect of thermal annealing on the energy levels of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) films was investigated using ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and low-energy inverse photoemission spectroscopy. We observed that thermal annealing at 150 degrees C induces reductions of both the ionization potential (IP) and the electron affinity (EA) with a narrowing of the band gap by 0.1 eV. These changes are associated with crystallization and a 2.54% reduction in the film thickness. Precise measurements of both the IP and EA enabled an evaluation of the effects of the electronic polarization energy in a model based on the charge localized in a single PCBM molecule.

The lowest unoccupied molecular orbital (LUMO) levels of six fullerene derivatives frequently used for organic photovoltaic cells are examined in the solid state. We employed a new experimental technique, low-energy inverse photoemission spectroscopy (LEIPS), which allows us to determine the electron affinities of solid samples within the uncertainties of 0.1 eV. Using the precisely determined electron affinities, the energy difference between the ionization energy of donor and the electron affinity of acceptor is compared with the open circuit voltage for various polymer/fullerene combinations taken from the literature. The result suggests that the well-known relation between the energy difference and the open circuit voltage should be reconsidered.

Electron affinity is a fundamental energy parameter of materials. In organic semiconductors, the electron affinity is closely related to electron conduction. It is not only important to understand fundamental electronic processes in organic solids, but it is also indispensable for research and development of organic semiconductor devices such as organic light-emitting diodes and organic photovoltaic cells. However, there has been no experimental technique for examining the electron affinity of organic materials that meets the requirements of such research. Recently, a new method, called low-energy inverse-photoemission spectroscopy, has been developed. A beam of low-energy electrons is focused onto the sample surface, and photons emitted owing to the radiative transition to unoccupied states are then detected. From the onset of the spectral intensity, the electron affinity is determined within an uncertainty of 0.1 eV. Unlike in conventional inverse-photoemission spectroscopy, sample damage is negligible and the resolution is improved by a factor of 2. The principle of the method and several applications are reported.

Electron-donor function of methanofullerenes (MFs) in bulk heterojunction systems is demonstrated by the combination of MFs with the electron-transporting pi-system that has a much higher electron affinity than MFs.

An apparatus for the low-energy inverse photoemission spectroscopy is described. In this technique, low energy electron having kinetic energy below 4 eV is incident to the sample and detect the emitted photons in the near ultraviolet range (below 5 eV, longer than 250 nm) to investigate the unoccupied states of the solid materials. Compared with the prototype apparatus reported previously [H. Yoshida, Chem. Phys. Lett. 539-540, 180-185 (2012)], the collection efficiency of photons is improved by a factor of four and practically any conductive substrates can be used. The overall resolution is 0.27 eV. (C) 2014 AIP Publishing LLC.

Molecular orientation dependences of the ionization energy (IE) and the electron affinity (EA) of diindenoperylene (DIP) films were studied by using ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. The molecular orientation was controlled by preparing the DIP films on graphite and SiO2 substrates. The threshold IE and EA of DIP thin films were determined to be 5.81 and 3.53 eV for the film of flat-lying DIP orientation, respectively, and 5.38 and 3.13 eV for the film of standing DIP orientation, respectively. The result indicates that the IE and EA for the flat-lying film are larger by 0.4 eV and the frontier orbital states shift away from the vacuum level compared to the standing film. This rigid energy shift is ascribed to a surface-electrostatic potential produced by the intramolecular polar bond (>C--H+) for standing orientation and pi-electron tailing to vacuum for flat-lying orientation. (C) 2013 AIP Publishing LLC.

An inverse photoemission spectroscopy (IPES) apparatus using a Czerny-Turner grating spectrometer is demonstrated. Previous IPES instruments based on grating spectrometers used a concave grating and operated in the vacuum ultraviolet range. The reflectance of such gratings is lower than 20% and the aberration cannot be finely corrected leading to an energy resolution of up to 0.1 eV. In the present study, employing the low energy IPES regime [H. Yoshida, Chem. Phys. Lett. 539-540, 180 (2012)], incident electrons with a kinetic energy below 5 eV are used, while photon emission in the range of between 250 and 370 nm is analyzed with a 10-cm Czerny-Turner grating spectrometer. The signal intensity is at least 30 times higher than the previous apparatus. The resolution of photon detection is set at 0.07 eV though the ultimate resolution is one order of magnitude higher. The experiment is performed both by sweeping the electron energy (isochromat mode) and by simultaneously analyzing the photon of whole wavelength range (tunable photon energy mode). (C) 2013 AIP Publishing LLC.

The electron affinity of pentacene thin films has been evaluated during the last decades, but it is still under controversial due to varieties of film quality and radiation damages of the films introduced during inverse photoemission spectroscopy (IPES) experiment together with insufficient energy resolution of the instruments. We employed the near-ultraviolet IPES with a better energy resolution 0.27 similar to 0.32 eV and using lower energy electron beams (0 eV <= Ei <= 4.9 eV) to study the unoccupied states of pentacene thin film. Due to a large mean-free-path of the electron in this energy region, the threshold electron affinity of the bulk of pentacene film was precisely determined to be 2.70 +/- 0.03 eV. Using the threshold ionization energy of 4.90 +/- 0.05 eV determined by ultraviolet photoemission spectroscopy, the band-gap energy of the pentacene film is obtained to be 2.2060.06 eV. (C) 2013 AIP Publishing LLC.

We demonstrate inverse-photoemission spectroscopy (IPES) in the near-ultraviolet range (hν < 5 eV) using electrons with kinetic energies less than 4 eV for the first time. The energy resolution of the instrument is attained to be 0.27 eV which is a factor of two better than the conventional IPES. From the spectra of a typical organic semiconductor, copper phthalocyanine, it is found that damage to the organic sample is significantly reduced, showing that this method is especially suitable to examine unoccupied states and precisely determine the electron affinities of organic semiconducting materials.

We investigated the role of metal/organic semiconductor interface morphology on the charge transport mechanisms and energy level alignment of the n-channel semiconductor poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5, 8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2ODT2)). Variable-temperature study of well-ordered edge-on-oriented P(NDI2OD-T2) monolayer and multilayer field-effect transistors fabricated via Langmuir-Schäfer (LS) method reveals a higher activation energy for the edge-on morphology when compared to that extracted for the face-on oriented P(NDI2OD-T2) spin-coated films, which showed a weaker temperature dependence. Near-ultraviolet inverse photoemission and low-energy electron transmission spectroscopies are utilized to study these microstructurally defined polymeric films. The cross correlations of these techniques with the device characterization reveals the role of the molecular orientation at the semiconductor/contact interface in shifting the charge injection barrier. Finally, we demonstrate that the injection barrier for electrons is higher for the LS/edge-on than in the spin-coated/face-on films. © 2013 American Chemical Society.

Polarization-dependent Raman microscopy is a powerful technique to perform both structural and chemical analyses with submicron spatial resolution. In conventional Raman microscopy, the polarization measurements are limited only in the direction parallel to the sample plane. In this work, we overcome the limit of conventional measurements by controlling the incident polarization by a spatially modulated waveplate. In this method, the polarization perpendicular to the sample surface (z-polarization) can be detected together with the parallel polarization (xy-polarization). Because of this unique polarization control, our Raman microscope has the ability to image the molecular orientation, together with the molecular analysis. Here, we have investigated thin films of pentacene molecules that are widely studied as an organic semiconductor material. The orientations of pentacene molecules are imaged with a spatial resolution of 300 nm. Our results clearly indicate that the lamellar grains show the lower tilt angles compared to the neighboring islands, which has not been proved in conventional methods. The substrate effects and the thickness dependence of the film are also studied. These results provide knowledge about the relationship between the devise performance and the film structures, which is indispensable for future device exploitations. Copyright (c) 2012 John Wiley & Sons, Ltd.

In previous inverse photoemission spectroscopy (IPES) experiments, either X-ray (h nu &gt; 1 keV) or vacuum ultraviolet (h nu approximate to 10 eV) photons were detected following the injection of electrons with energies of 10-1000 eV into solid materials. Here, we demonstrate IPES in the near-ultraviolet range (h nu &lt; 5 eV) using electrons with kinetic energies less than 4 eV. The energy resolution of the instrument is attained to be 0.27 eV. From the spectra of copper phthalocyanine films, it is found that damage to the organic sample is significantly reduced, demonstrating that this method is especially suitable for organic semiconducting materials. (C) 2012 Elsevier B.V. All rights reserved.

The C 1s core-level energy difference of surface and bulk regions of organic semiconductor films, Alq 3, bathocuproine, and Cu-phthalocyanine, are reported. For the analysis, we employed a novel method based on angle-resolved photoemission spectroscopy and target factor analysis, which we developed recently [Chem. Phys. Lett.2011, 511, 146-150]. In contrast to the earlier studies, the core energy differences between the surface and bulk layers were much more precisely determined together with the thickness of the surface layer. The results show that the core-level energy of the outermost layer is different by from 0.2 to 0.4 eV for all of the three compounds. The finding suggests that the electrostatic polarization energy plays an important role in screening charge carriers in organic solids. © 2012 American Chemical Society.

The electronic structures of buried interfaces between an organic semiconductor, N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) and metal surfaces of Au, Ag, Al and Ca were examined by the new experimental method that we have developed recently. In this method the energy levels at the organic/metal interface can be examined without changing the film thickness and related physical parameters e.g., the vacuum levels of the sample in contrast to the widely-used thickness-dependent photoemission experiments. The results were discussed in view of large interfacial dipole moment of the TPD and metal (Au and Ag) contacts.

The difference in the C1s core level energies between the surface and bulk regions of an organic semiconductor thin film is examined for pentacene and perfluoropentacene (F-pentacene). The experimental method we have recently developed allows us to determine precisely the differences as well as the thickness of the surface region. The C1s binding energies in the outermost layer turn out to be larger than those in the bulk by 0.32 eV (pentacene) and 0.25 eV (F-pentacene). The results are successfully explained by the interaction energy between the point charge and induced dipole moments suggesting the electrostatic polarization energy plays a main role in the difference of the core level energy. (C) 2011 Elsevier B.V. All rights reserved.

We have developed a new experimental method of X-ray photoemission spectroscopy (XPS) that can map out the core-energy levels as a function of depth from the surface of the film. A series of XPS data are recorded with different detection angles and expanded to the Taylor series of angle-averaged spectra using the target factor analysis. This procedure enables conversion of the measured angle variations in XPS to the core energy levels as a function of depth from the surface. This method has been applied to profiling the electronic levels of buried interface between organic semiconductor and metal surfaces. (C) 2011 Elsevier B. V. All rights reserved.

The spontaneously built-up surface potential in a vacuum-deposited thin film of a zwitterionic molecule, pyridinium 5,7-dihydro-5,7-dioxo-6H-cyclopenta[b]pyridin-6-ylide (4N-PI), was observed to decay upon illumination with a photon energy above the photoconduction threshold. The decay rate increased with increasing film crystallinity. The current-voltage relationship obtained from the surface potential decay can be explained by a combination of ohmic and space-charge-limited current conduction mechanisms, and the dependence of the fitting parameters on film crystallinity provided further strong evidence disappearance of the surface potential. that photogenerated charge carriers contribute to the

We successfully synthesized a room-temperature ionic liquid showing valence-tautomeric behavior, by combining a cobalt bis(dioxolene) complex dianion with a bulky trihexyl(tetradecyl)phosphonium cation. Its magnetic moment exhibits a gradual increase up to ca. 280 K through the glass transition (262 K) and then has an abrupt upturn at around 300 K. The valence-tautomeric behavior was confirmed by the temperature dependence of the electronic absorption spectra. This successful implementation allowed the "smart" ionic liquid, whose magnetic behavior responds evidently to temperature change.

Surface potentials were examined using the Kelvin method for thin films of zwitterionic molecules, pyridinium 1,3-dihydro-1,3-dioxo-2H-inden-2-ylide (PI or IPB) and two nitrogen-substituted derivatives. Spontaneous buildup of the surface potential on the film (5.5 V at a film thickness of 300 nm) was only observed for the compound with unidirectional orientation of the molecular dipole moments in the single crystal. The relationship between the alignment of the molecular dipole moments in the film and the measured surface potentials was investigated using grazing incidence x-ray diffraction, pole-figure measurements, atomic force microscopy, and Kelvin probe force microscopy.

We have developed a new method to calculate the static permittivity tensors of organic molecular crystals by applying the charge response kernel theory (Morita, A.; Kato, S. J. Am. Chem. Soc. 1997, 119, 4021) in which all the parameters were obtained with the density functional theory. The accuracy together with the requirements of the computation was discussed in terms of positions of the charge response sites and choice of a basis set. The calculated permittivities of typical organic compounds turned out to agree with the experimentally obtained values in the deviation of about 7% when a reasonable computational cost was maintained.

We examined the solvent effect on the electronic absorption and infrared absorption spectra of a push-pull molecule fabricated by connecting donor and acceptor segments linked with a quinoid bridge, {4-[4,5-bis(methylsulfanyl)-1,3-dithiol-2-ylidene]cyclohexa-2,5-dien-1-ylidene}malononitrile (BMDCM), in liquid Solutions. A BMDCM molecule represented by a resonance hybrid of the two canonical structures, the quinoid and benzenoid forms, showed remarkable solvatochromism. By analyzing the vibrational progressions in the electronic absorption spectra, we have grasped the behaviors of molecular structure and intramolecular charge distribution for the ground and excited states in reference to the solvent polarity.

Ultraviolet photoemission spectroscopic measurements of bis (benzo)pentathienoacene were carried out in gas and solid phase. For the measurements of solid phase, vacuum deposited films in both amorphous and crystalline phase were prepared on different substrates of HOPG and polycrystalline Au, respectively. The adiabatic ionization energies were determined to be 6.8(4), 5.3(2), and 5.0(8) eV, for gas, amorphous, and crystalline phases, respectively. The spectral lineshapes were interpreted with the aid of the density functional calculations for both isolated molecule and single-crystal structure. The calculated electronic structures were further analyzed in terms of the energy band dispersion and the transport properties of charge carriers.

A push-pull-type molecule {4-[4,5-bis(methylsulfanyl)-1,3-dithiol-2-ylidene]cyclohexa-2,5-dien-1-ylidene}malononitrile (BMDCM) turned out to show two different polymorphs depending on the phase in which the crystal growth was made. BMDCM molecules stack in the head-to-tail fashion in both polymorphs to cancel out their large dipole moments with each other. On the other hand, the relative arrangements of the molecule stacking columns are clearly different between the two polymorphs. A solution-grown crystal makes molecular long axes in all the columns align in the same direction. In contrast, a vapor-grown crystal exhibits alternate orientation of the molecular long axes among neighboring columns being almost perpendicular to each other. While there is such a large structural difference between the two polymorphs, we have found the polymorphic transition between them occurs essentially in a reversible manner. (C) 2009 Elsevier B.V. All rights reserved.

Crystal structures of pyridinium 1,3-dihydro-1,3-dioxo-2H-inden-2-ylide (PI) of a zwitterionic molecule and its nitrogen substituted compounds, 4N-PI and 5N-PI, were solved with X-ray diffraction analyses. Whereas PI and 5N-PI showed centrosymmetric crystal structures, 4N-PI demonstrated a noncentrosymmetric crystal structure where all the molecules orient to almost the same direction. To elucidate the stability of such a polarized structure, we examined interatomic close contacts among the nearest neighbor molecules in the crystals and calculated intermolecular interaction energies with relation to those contacts. As a result, the noncentrosymmetric crystal structure of 4N-PI turns out to be realized by a weak hydrogen bond in the C-H center dot center dot center dot N manner formed only in the case of this compound. (C) 2008 Elsevier B.V. All rights reserved.

{4-[4.5-Bis(methylsulfanyl)-1,3-dithiol-2-ylidene]cyclohexa-2,5-dien-1-ylidene}malononitrile (BMDCM) is designed toward a highly amphoteric and polar molecule (HAPM). Its electron donating and accepting abilities in the solid state were examined from the measurements of its electronic structure using ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES): The threshold ionization energy I(s)(th) and electron affinity A(s)(th) in the solid state were determined to be 5.1 +/- 0.1(5) and 3.5 +/- 0.4eV. respectively. These Values turn out to be comparable to the corresponding values of typical donor or acceptor compounds. indicating high abilities in both electron donating and accepting nature of BMDCM. The origin of such a nature of BMDCM is discussed in terms of aromatic stabilization in its ionic states. (C) 2008 Elsevier B.V. All rights reserved.

The electronic structure of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) was studied using ultraviolet photoelectron spectroscopy of vapor and thin film and inverse photoemission spectroscopy of thin film. The threshold ionization energy of PCBM was found to be 7.17 +/- 0.04 eV in gas phase and 5.96 +/- 0.02 eV in solid phase. The threshold electron affinity was 3.9 +/- 0.1 eV in solid phase. These values are 0.4-0.6 eV smaller than C-60. The density functional theory calculations gave consistent results with these trends and suggested that the electron donation from the side chain to C-60 backbone raised the C-60-backbone-derived pi orbitals of PCBM. The polarization energy of PCBM is 1.21 eV, which is almost the same as C-60 but is about 0.5 eV smaller than the value of typical aromatic hydrocarbons. (c) 2008 American Institute of Physics.

Calculations are presented for the energy bands of three polymorphs of pentacene: the thin-film, bulk, and single-crystal phases. In the calculation of the thin-film phase, the structural data from our recent results on the x-ray diffraction reciprocal space mapping were applied. The band structures are essentially two dimensional, i.e., only small dispersions are found along the c(*) direction. The energy dispersion of the thin-film phase is found to be larger and more isotropic than those of the other phases. The energy dispersions of the bands derived from highest occupied molecular orbital (HOMO), HOMO-1, lowest unoccupied molecular orbital (LUMO), and LUMO+1 levels are analyzed by comparing with the corresponding results on the basis of the tight-binding approximation; the dispersions are well described by transfer integrals involving only the nearest neighbor molecules. In accordance with this finding, a simple model is presented to explain the relation between the crystal structure and the energy dispersion. From the calculated bands, the effective masses are derived to discuss the transport properties of the hole and electrons. Angle-integrated photoemission spectra were also measured for the thin-film and bulk phases. Finally, spectral features of the HOMO-derived bands are interpreted by the calculated density of states.

The adsorption of 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) on a gold surface has been studied using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) under ultrahigh vacuum conditions. TTTA was deposited onto the gold substrate by gas-phase dosing. XPS and UPS spectra show that TTTA molecules strongly adsorb on the gold surface. In addition, the UPS spectrum of the TTTA monolayer is in good agreement with the gas-phase UPS spectrum of TTTA, which indicates that the molecules adsorb as a radical without electron transfer from the gold substrate. (C) 2007 Elsevier B.V. All rights reserved.

Diffusion and reaction of aluminum metal species (Al) vacuum deposited on perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) thin films were investigated using angle resolved x-ray photoemission spectroscopy. The acquired data were analyzed assuming that the diffusion of Al is described by a one-dimensional diffusion equation with a time-dependent diffusion coefficient. Depth profiles and diffusion coefficients are obtained for reacted and metallic Al separately. The results show that the metallic Al diffuses rapidly during the deposition while the metal diffusion continues at a lower rate even after the deposition at room temperature. On the other hand, the reacted Al does not diffuse further into the PTCDA layers. (C) 2007 American Institute of Physics.

The structure of the thin film phase of pentacene was investigated using x-ray diffraction reciprocal space mapping (RSM). The crystal structure was found to be triclinic with the following lattice parameters: a=0.593 nm, b=0.756 nm, c=1.565 nm, alpha=98.6 degrees, beta=93.3 degrees, and gamma=89.8 degrees. Atomic positions were determined by comparing the observed RSM diffraction intensities with theoretical calculations. (C) 2007 American Institute of Physics.

The structure of pentacene thin films in the bulk phase having interplane spacing of d(001)=1.44 nm grown on SiO2 was investigated using grazing-incidence x-ray diffraction. The films were prepared with two different methods: one treated with an organic solvent after vacuum deposition and the other thermally treated. Both films are similar in structure to the single crystal reported by Campbell [Acta Crystallogr. 15, 289 (1962)]. (c) 2006 American Institute of Physics.

Cluster anions of acrylonitrile (AN), known to give intracluster anionic polymerization products, were deposited on solid substrates. The obtained films were examined by using infrared absorption spectroscopy, X-ray photoemission spectroscopy, and gel permeation chromatography with the aid of quantum chemical calculations. The acquired spectroscopic data are similar to those of polyacrylonitrile (PAN), while the normal polymerization of AN or reactions related to PAN seemed not to occur noticeably. On the contrary, the product analysis shows that most of the constituent molecules of the films are formed via cyclohexane-1,3,5-tricarbonitrile (CHTCN), a dominant product of the intracluster polymerization of AN, accompanied by fragmentation and dimerization.

Electronic structures of the unoccupied states in lithium phthalocyanine (LiPc) thin films of the x- and alpha-polymorphs were directly observed with inverse photoemission spectroscopy (IPES). The energy position of the lowest energy band and the lineshape of the second lowest energy band were different between the obtained IPE spectra for the two forms of the films. By comparing with energy levels in a LiPc molecule and its dimers calculated using the density functional method, we interpreted such differences as follows: the first band placed in lower energy for the alpha-form is due to more stabilized singly occupied molecular orbital (SOMO) and the observed splitting of the second feature for the x-form is lead from the removed degeneracy of 6e(g) levels caused by a large intermolecular interaction along the molecular stacking axis. (C) 2003 Elsevier Science B.V. All rights reserved.

inverse photoemission (IPE) spectra of thin films of metal-free phthalocyanine (H2Pc), 3d-metal phthalocyanines (MPcs: M = Mn, Fe, Co, Ni, Cu and Zn) and lithium phthalocyanine (LiPc) have been measured to directly observe the electronic structures of their unoccupied states. In particular, IPE spectra of MPcs obtained systematically for 3d metals have been studied with the contribution from the 3d-metal orbitals to the electronic structures of the lower unoccupied states focused on. The difference spectra between MPcs (M = Mn, Fe, Co, Ni and Cu) and ZnPc as the reference are compared with the reported results from X-ray absorption spectroscopy as well as extended Huckel and density functional calculations to show that they reflect the density of unoccupied states derived from the central metals with permitting tentative assignments of their features. (C) 2002 Elsevier Science B.V. All rights reserved.

The electronic structure of low-lying unoccupied electronic states in a hexatriacontane (n-C36H74) thin film was directly observed using inverse photoemission spectroscopy (IPES). In the IPE measurements it was confirmed that the hexatriacontane thin film is easily degraded by electron irradiation. Therefore by reducing the current density of the incident electrons gradually we finally obtained a reliable IPE spectrum free from radiation damage and/or surface charging. As a result, we conclude the IPE spectrum reported by Dudde and Reihl was influenced by radiation damage. The reliable spectrum shows two features near the vacuum level: they were not resolved in the reported spectrum. Further, it is compared with the reported results from other electron spectroscopies and theoretical calculations concerning the unoccupied electronic states in long chain alkanes. (C) 2002 Elsevier Science B.V. All rights reserved.

Unoccupied electronic states of thin films of 3d-metal phthalocyanines (MPc: M=Mn, Fe, Co, Ni, Cu and Zn) and metal-free phthalocyanine (H2Pc) were investigated by inverse photoemission spectroscopy. The experimental results were analyzed with emphasis on the contribution of the 3d-metal orbitals to the lower unoccupied electronic states of MPc. The difference spectra between the sample MPc (M=Mn, Fe, Ni and Cu) and the reference ZnPc were found to be in fair agreement with the previously reported X-ray absorption spectra in the energy regions corresponding to the core excitation of the central metals, This shows that the difference spectra exhibit the partial density of states attributed to the central metals. A comparison of the 3d-metal orbital contributions was also made between the experimental difference spectra and reported results from extended Huckel and density functional calculations. (C) 2001 Elsevier Science B.V. All rights reserved.

Semiempirical molecular orbital calculations of 2-(4-dicyanomethylenecyclohexa-2,5-dienylidene)-4,5-ethylenedithio-1,3-dithiole and its analogues, as test molecules towards a highly amphoteric and polar molecule (HAPM) designed on the basis of combination of electron-donating and accepting molecular segments with a pseudo-delocalized electron system were carried out to estimate several molecular parameters, in particular, those regarding the second-order optical nonlinearity, in order to examine a difference between the proposed molecular design and the conventional one for the 'push-pull' type of molecules. Large values of the first molecular hyperpolarizability are obtained for the present molecules, however, it is turned out that the trade-off relation between hyperpolarizability and transparency of a material is also concerned with these materials. This indicates that the proposed molecular design toward HAPM is unique and can be useful to bring about novel materials with remarkable properties.

Photoabsorption spectroscopy has been performed on singly charged strontium clusters Sr-n(+) for the sizes n = 61, 127, and 200, in the 1.8 to 5.2 eV energy range. Superimposed on the giant resonance, related to the Mie plasma resonance of a metallic sphere, the spectra reveal an extended wing in the high-energy range attributed to interband transitions. Evidence of striking correspondence with the absorption spectrum of a macroscopic sphere obtained from the measured real and imaginary parts of the bulk strontium dielectric function is shown.

Inverse photoemission spectroscopy (IPES) in the vacuum ultraviolet region was applied to directly observe unoccupied electronic structures in a few kinds of organic thin films. The genuine IPE spectrum for the lowest unoccupied states in a long-chain alkane film was first obtained with a fairly low level of electron irradiation. The electronic structure around the energy gap in a perylene-3,4:9,10-tetracarboxylic dianhydride (PTCDA) film was examined by combining the observed IPE spectrum with previous ultraviolet photoemission spectra, to discuss its charge carrier characteristics. IPE measurements of PTCDA films were further extended to study electron injection behaviours due to alkali metal doping into those films, and by analysing the observed results the amount of injected electrons per molecule was evaluated with relation to the dopant concentration. The derived relationship has been explained with the aid of DV-X alpha calculations of the energy levels concerned.

Ultraviolet photoemission (PE) and inverse photoemission (IPE) spectra were measured for thin films of N,N'-dimethylperylene-3,4,9,10-bis(dicarboximide) (DM-PBDCI) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), the former known as an n-type organic semiconductor and the semiconducting nature of the latter still unidentified. The electronic structure around the energy gap, obtained from the combination of PE and IPE spectra observed, is found to be almost the same for the two compounds, indicating that PTCDA could be assigned as an n-type semiconductor. (C) 1998 Elsevier Science B.V.

Photoelectron spectra of vanadium cluster anions, V-n(-) (3 less than or equal to n less than or equal to 100), were measured at a photon energy of 3.49 eV (355 nm) by using a magnetic-bottle-type photoelectron spectrometer. The electronic density-of-state profiles and the photoelectron spectra of V-4(-) and V-3(-) were calculated by the spin-polarized DV (discrete variational)-X alpha method for several plausible geometrical structures, The most plausible structure was determined so that the calculated photoelectron spectrum based on this structure well reproduces the observed one. The calculation showed that V-4(-) has a square planar geometrical structure and a charge distribution of D-4h symmetry. On the other hand, V-3(-) was found to possess an equilateral triangle geometrical structure, but C-2v symmetry for the charge distribution. The electronic states in the vicinity of the Fermi energy were found to consist of 3d atomic orbitals. The population differences between the minority and the majority spins per atom turned out to be 4.6 and 1.7 for V-4(-) and V-3(-), respectively. The result implies that the d-electron spins are coupled ferromagnetically in these cluster anions. In addition, the size-dependence of the electron affinity of V-n was explained by a spherical conducting droplet model in the n greater than or similar to 9 range. (C) 1997 American Institute of Physics.

The photoelectron spectra of Co-n(-) (3 less than or equal to n less than or equal to 70), ComN- and ComO- (3 less than or equal to m less than or equal to 6) were measured by a XeCl excimer laser (4.025 eV). Also the reactivities of Co-n(-) with O-2 and CO were measured. A clear distinction between the cluster size of n less than or equal to 6 and that of n greater than or equal to 7 was observed both in the photoelectron spectra and in the reactivities. (i) In the size range above n = 7, the electron affinities of the cobalt clusters determined by the photoelectron spectra are linearly dependent on the reciprocal of the cluster radius, but below this size they deviate from this relation. (ii) The reactivity of Co-n(-) with O-2 in the size range below n = 6 is higher than that above n = 7. However, the reactivity with CO in the size range below n = 6 is extremely lower than that above n = 7. These findings suggest that a transition of electronic structure in size dependence, which is correlated with the reactivity, occurs at n similar or equal to 7.