Kaveenga Rasika Koswattage

Full Picture of Valence Band Structure of Rubrene Single Crystals Probed by Angle-Resolved and Excitation-Energy-Dependent Photoelectron Spectroscopy

The valence band structure of rubrene single crystals was experimentally determined by high-resolution angle-resolved and excitation-energy-dependent photoelectron spectroscopy at room temperature. The energy position of the peak derived from the highest occupied molecular orbital did not depend on the excitation energy, reflecting an absence of energy dispersion along the surface normal direction. A two-dimensional valence band dispersion relation over the surface Brillouin zone obtained by angle-resolved photoemission to three critical points was reproduced excellently by a two-dimensional tight binding approximation. Highly anisotropic values of intermolecular transfer integrals to four adjacent molecules were obtained from the present results.

Selective adsorption of atomic hydrogen on a h-BN thin film

The adsorption of atomic hydrogen on hexagonal boron nitride (h-BN) is studied using two element-specific spectroscopies, i.e., near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray photoelectron spectroscopy (XPS). B K-edge NEXAFS spectra show a clear change in the energy region of the π* band before and after reaction with atomic deuterium. On the other hand, N K-edge NEXAFS spectra show only a little change. B 1s XPS spectra show a distinct component at the low binding energy side of a main component, while N 1s XPS spectra show peak broadening at the high binding energy side. These experimental results are analyzed by the discrete variational Xα method with a core-hole effect and are explained by a model in which hydrogen atoms are preferentially adsorbed on the B sites of h-BN. Based on the experimental and theoretical results, we propose a site-selective property of BN material on adsorption of atomic hydrogen.

Determination of the highest occupied molecular orbital energy of pentacene single crystals by ultraviolet photoelectron and photoelectron yield spectroscopies

The electronic structures of pentacene single crystals (SCs) were elucidated by ultraviolet photoelectron spectroscopy (UPS) and photoelectron yield spectroscopy (PYS). An asymmetric HOMO peak profile of the pentacene SCs obtained by UPS exhibits a close similarity to the k-projected density-of-states of the valence band that has been predicted by a theoretical calculation [H. Yoshida and N. Sato, Phys. Rev. B 77, 235205 (2008)]. The ionization energy of the pentacene SCs is successfully determined to be 4.95 (± 0.03) eV which is evidently greater than that of the bulk films of pentacene [4.90 (± 0.02) eV].

Photoabsorption cross section of C70 thin films from visible to vacuum ultraviolet

Absolute photoabsorption cross sections of C(70) thin films were determined for hv values from 1.3 to 42 eV using photon attenuation. The spectrum showed a prominent peak of 1320 Mb at 21.4 eV with several fine structures mostly due to sigma-->sigma(*) single-electron excitation. The complex refractive index and complex dielectric function were calculated up to 42 eV with Kramers-Kronig analyses. From the present data of C(70) thin films, the cross section curve of molecular C(70) was calculated using the standard Clausius-Mossotti relation dealing with correction of the local electromagnetic field, with a plausible assumption that the anisotropy in molecular structure of C(70) was smeared out by molecular rotation at room temperature.

Density of states evaluation of an insulating polymer by high-sensitivity ultraviolet photoemission spectroscopy

Although the contact electrification of insulating polymers has been widely used in various technologies, the mechanism of electrification is still not well understood and several models have been proposed to explain the mechanism. Some of the models assume the existence of bandgap states that can store or release electrons to charge the polymer; however, the density of states in the bandgap region is not well examined. In this study, an approach to directly measure the density of state of insulating polymers using hν-dependent high-sensitivity ultraviolet photoemission spectroscopy is proposed. Demonstration of the approach to a representative insulating polymer, nylon-6,6, is reported with the estimation of the charge density and charge penetration depth as a function of the work function difference.

Electronic Structure of Chlorophyll a Solution Investigated by Photoelectron Yield Spectroscopy

Various bio-related processes are driven by electron transfer reactions. Therefore the electronic structures of bio-molecules in their living environment are keys of their functionalities. One significant example photosynthesis which has attracted much attention due to urgent necessity of clean energy source. In this study, we carried out photoelectron yield spectroscopy (PYS) measurements to demonstrate the electronic structures of oligomerizedChl-a molecules, which is known as an essential reaction center of the photosystem in general green plants, under the atmospheric environment. The ionization energies of the Chl-a aggregates are successfully derived.