Masayuki Ohtani

Electrochemical oxidation of reduced nicotinamide coenzymes at Au electrodes modified with phenothiazine derivative monolayers

Au wire electrodes coated with phenothiazine derivatives such as thionine, azure C, and azure A covalently bound to a self-assembled monolayer of 3,3′-dithiobis(succineimidylpropionate) on an Au substrate showed two redox waves due to monomeric and dimeric forms of the phenothiazine derivatives. It was found that the dimeric forms have high activities for electrocatalytic oxidation of NADH. The second-order reaction rate constant for NADH oxidation was evaluated by digital simulation. The rate constants obtained at the thionine-modified electrodes were about 104 times greater than that obtained for the oxidized phenothiazine derivatives dissolved in solution, and were greater than those reported previously with use of other electrocatalysts.

Generation of picosecond hard x rays by tera watt laser focusing on a copper target

Hard x-ray generation from tera-watt laser irradiation (4 TW, 42 fs at 780 nm) on a solid copper target has been studied in a range of the laser-power density between 3×1016 and 2×1017 W/cm2. Short-pulsed x rays are generated within 6.4 ps and include Cu Kα and Kβ line emissions and a continuum between 3 and 6 keV. The maximum intensity of Cu Kα radiation has been estimated to be 6.5×1010 photons (4π sr pulse)−1. The mechanism of hard x-ray generation has been discussed.

How Big Are Charge Clouds inside the Charge-Coupled Device Produced by X-Ray Photons?

We report here the charge cloud shape produced by an X-ray photon inside the charge-coupled device (CCD) as well as a method to measure it. The measurement is carried out by using a multi-pitch mesh which enables us to specify the interaction position of X-rays with subpixel resolution not only for single events but also for split events. Split events are generated when the X-ray interaction position is close to the pixel boundary. The width of this area depends on the apparent charge size. Finally, we measured the signal output from the pixel according to the interaction position of X-rays. By differentiating this function, we obtain, in detail, the charge cloud shape which can be well represented by an asymmetric Gaussian function. The charge cloud size for Al-K X-rays is 0.7×1.4 µm2 while that for Mo-L X-rays is 0.8×1.4 µm2. The size of the photoelectron in Si produced by these X-rays is about 0.04 µm. Taking into account the mean absorption length for these X-rays in Si, diffusion process in the depletion region cannot explain the charge cloud size. The asymmetry of the charge cloud probably arises from the asymmetry of the electric field in the CCD.

Spectroscopy of Hard X-Rays (2-15 keV) Generated by Focusing Femtosecond Laser on Metal Targets

Spectroscopy of hard X-rays generated by focusing a femtosecond laser (42 fs at 780 nm) onto metal targets consisting elements of various atomic number (Z) is carried out in the energy range between 2 and 15 keV using a direct-detection charge-coupled-device camera.Sharp K-shell line emissions are observed for X-rays generated from medium-Z targets (Ti, Fe, Ni, Cu and Zn), which can be used for X-ray diffraction without further monochromization. A broad continuum and a weak trace of L-shell line emissions are observed for X-rays from high-Z targets (Mo, Nb and W). The energies of the characteristic X-rays agree well with that of X-rays from neutral atoms or slightly ionized ions.

Measurement of quantum efficiency of a charge coupled device

We report a measurement of the quantum efficiency of a charge coupled device (CCD) with a simple method and report a resultant quantum efficiency in the energy range from 0.47 to 9 keV. The CCD is a front side illuminated device and is fabricated by Hamamatsu Photonics K.K. The quantum efficiency for 4.5 keV x rays is 11% for the single pixel events, 33% for the spectroscopic events, and 60% for flux mode operation. We present the quantum efficiency as a function of the energy.

Preparation of a microelectrode array by photo-induced elimination of a self-assembled monolayer of hexadecylthiolate on a gold electrode

Abstract An electrode characteristic of a microelectrode array was fabricated by irradiating with UV light, through a photomask, a self-assembled monolayer of hexadecylthiolate formed on an Au electrode in air. Further, it was discovered that irradiation of the monolayer electrode in a K4Fe(CN)6 solution allowed removal of the monolayer depending on the irradiation time. An electrochemical response characteristic of a microelectrode array appeared before complete removal of the monolayer took place. A microelectrode array was also fabricated by irradiating the monolayer electrode at selected places in a K4Fe(CN)6 solution with a focused laser beam of the second harmonic of an Nd: YAG laser (532 nm). Cyclic voltammograms of the photo-fabricated electrodes are compared with those predicted by voltammetric theories.

Development of a Schwarzschild-type x-ray microscope

A Schwarzschild-type x-ray microscope has been designed, constructed, and tested. Ni/C multilayers were used as the x-ray mirrors, with a thickness (2d) of 7 nm and 30 layer pairs. The microscope has attained a spatial resolution of 0.5 microm at a magnification of 15. By using bright laser-produced plasmas as an x-ray source [R. Kodama, K. Okada, N. Ikeda, M. Mineo, K. A. Tanaka, T. Mochizuki, and C. Yamanaka, J. Appl. Phys. 59, 3050 (1986)], images could be recorded during the 400-psec laser pulse.

X‐ray optical properties of molybdenum‐carbon, molybdenum‐silicon, and nickel‐carbon multilayers

We have fabricated multilayer reflectors of molybdenum‐carbon, molybdenum‐silicon, and nickel‐carbon for x rays of 1–200 A with an electron beam evaporation method in ultrahigh vacuum. The multilayers were deposited on flat and figured substrates of float glass and superpolished glass with a surface roughness of 2–3 A (rms). The thickness of a layer pair and the number of layer pairs were 30–100 A and 10–20, respectively. The x‐ray characteristics of these multilayer reflectors were evaluated in the 1.5–200 A region with characteristic x rays (Cu‐Kα, Si‐Kα, Al‐Kα, and C‐Kα) and monochromatized synchrotron radiation. Peak reflectivities of molybdenum‐carbon were found to be 70% at Cu‐Kα and 40% at Al‐Kα in grazing incidence and those of molybdenum‐silicon to be more than 30% in the 150–170 A band in near‐normal incidence. On the basis of these experimental results, the x‐ray optical properties of multilayers are discussed.

Comparison of methods of measuring the primary charge-cloud shape produced by an X-ray photon inside the CCD

Abstract We report here the comparison of two methods of directly measuring the charge-cloud shapes produced by X-ray photons inside a CCD. The measurements are performed using a mesh technique in which we can confine the X-ray interaction location with subpixel resolution. There are two methods: a DD (double differential) method employs all X-ray events and a CG (center of gravity) method employs only split events. The DD method reveals the mean charge-cloud shape generated in a relatively shallow region, while the CG method reveals one generated in a relatively deep region. We performed the measurement using Al–K X-rays and Mo–L X-rays. The charge-cloud sizes generated by these X-rays are 0.7∼1.7 μ m (standard deviation). The charge-clouds clearly show asymmetric shape, elongated perpendicular to the charge transfer direction. This is probably due to the nonuniformity of the electric field inside the CCD.

Fabrication of 0.1 µm Line-and-Space Patterns using Soft X-Ray Reduction Lithography

Soft X-ray projection lithography with a reduction rate of 32 was examined using Mo/Si-multilayer-coated Schwarzschild optics. The optics were designed to function at 13 nm, and were aligned with the synchrotron radiation light source. The patterns on a transmission mask were imaged in a 0.18-µm-thick polymethyl methacrylate resist. Line-and-space patterns down to 0.1 µ m were fabricated.