Ataru Kobayashi

Dissociative photoionization of CF4 from 23 to 120 eV

Using synchrotron radiation as a continuum light source, dissociative photoionization of CF4 has been studied in the photon-energy region of 23–120 eV. Ion branching ratios were obtained by analyzing time-of-flight mass spectra and were converted to the absolute partial cross sections for the production of singly charged CF3+, CF2+, CF+, F+, and C+ ions, as well as doubly charged CF32+ and CF22+ ions by using the reported total absorption cross sections of CF4. Ion branching ratios were differentiated with respect to the incident photon energy. The results obtained by this analytical photoion spectroscopy clearly show dissociation pathways of the CF4+ and CF42+ ions, many of which are observed for the first time in the present study. These pathways are discussed by comparing with the reported electronic states of the ions.

Ionization cross section ratios of rare-gas atoms (Ne, Ar, Kr and Xe) by electron impact from threshold to 1 keV

Electron impact multiple ionization relative cross sections of rare gas atoms (Ne, Ar, Kr and Xe) have been measured using a pulsed electron beam and a pulsed ion extraction combined with a time-of-flight analysis of the charge. The measurements cover a larger range of energies and charges than in earlier experiments, from threshold to 1 keV and from charge n = 1 to 4 (Ne), 5 (Ar), 7 (Kr) and 6 (Xe). The average number (γ) of electrons ejected from the atom per ionization event is calculated and used to evaluate the contributions of the inner-shell processes. The results are also compared with those in the case of photon impact. As regards the major ions produced, relative uncertainty as low as 1.3% for the ionization cross section ratios is achieved for all the sample gases, except near threshold. As for the γ-values, relative uncertainty is estimated to be as low as 1.2%.

Characterization of frequency-tripled nanosecond pulsed Ti:sapphire laser injection seeded by a frequency-scanning cw Ti:sapphire laser by use of optogalvanic spectroscopy of silicon atoms

The performance of a narrow-linewidth nanosecond pulsed deep-UV coherent light source consisting of a frequency-tripled nanosecond pulsed Ti:sapphire laser injection seeded by a frequency-scanning cw Ti:sapphire laser has been characterized by using optogalvanic spectroscopy of silicon atoms as a diagnostic. The envelope of the optogalvanic spectrum indicates the pure Doppler broadening of silicon atoms, which was estimated to be as broad as 4 GHz, without the broadening effect from the laser linewidth itself.

Angular distribution of ionic fragments in the dissociation of SO22

The photoion–photoion coincidence technique has been used to measure the angular distributions of the O++S+ and O++SO+ fragments in the dissociative double photoionization of SO2 in the 37–120 eV range with linearly polarized synchrotron radiation. The present work shows that the obtained values of the asymmetry parameter β are close to zero (−0.2⩽β⩽0.2) over the whole energy range of photoexcitation, indicating that the fragmentation is almost isotropic in both cases of O++S+ and O++SO+ channels. It is found that the symmetry analysis which has been partially successful for explaining the behavior of β values observed for linear molecules or for molecules with cylindrical symmetry is unsatisfactory to account for the observed energy dependence of β values in the dissociation processes of SO22+. For more quantitative understanding of β values, the dynamical effects such as asymmetric bending or stretching motion of the SO2 molecule prior to the ionic fragmentation, rather than the static effects such as e...

Single-frequency 389-nm CW coherent light source for optical pumping of metastable 3He atoms

A hyperpolarized helium-3 (3He) has been researched extensively for application in fields such as polarization analysis, 3He spin filtration for n (p, d) γ, and medical imaging. In the medical field, nuclear magnetic resonant imaging (MRI) using hyperpolarized gas is attracted recently because air space in human lungs can be monitored in detail by aspirating polarized gas. There are two methods of producing hyperpolarized 3He gas: spin-exchange optical pumping (SEOP) and metastability exchange optical pumping (MEOP). It is well known that the way of making polarization by the MEOP method is more shortly and more directly. In this study, however, the metastable 3He atoms provided by hollow cathode discharge are polarized by circularly polarized light. When 3He atoms pumped optically, the 23S-23P transition at the wavelength of 1083nm is leveraged typically. The 3He atom actually has the optical transition 23S→ 23P at 389nm, but the transition has never been used. Therefore, we developed the 389-nm coherent light source to polarize 3He gas by MEOP method. This light source utilized the second harmonic generation (SHG) of continuous-wave Ti:sapphire laser operating at a wavelength of 778nm based on BiB3O6 (BiBO) in an external cavity.

Comparative study on THz time-domain spectroscopy using 780-nm 1.3-ps laser pulses with different detections of LT-GaAs photoconductive antenna and ZnTe electro-optic sampling

We have demonstrated the comparison of terahertz (THz) time-domain spectroscopy (THz-TDS): a low-temperature grown GaAs photoconductive antenna (PCA) and ZnTe electro-optic (EO) sampling with 780-nm 1.3-ps laser pulses. As a result, the different detection limits up to approximately 0.8, 1.0, and 1.3 THz are obtained with a bow-tie, dipole antenna, and ZnTe crystal, respectively. In the PCA sampling, the frequency at main peak of the spectrum measured with the dipole antenna is higher than the one with the bow-tie antenna. The dynamic range of the power spectrum measured with the bow-tie antenna is higher than any other detection methods. In addition, we compare the PCA sampling with femtosecond laser pulses to the one with picosecond laser pulses in terms of the response of the PCAs.

Tunable picosecond THz-wave generation based on trapezoidal MgO:LiNbO3 crystal in novel pentagram-shaped pump-enhancement cavity

It is well known that lithium niobate (LiNbO3) has excellent characteristics for efficient tunable/broadband THz-wave generation. Over the last few years, we have investigated novel THz-wave sources based on MgO-doped LiNbO3 (MgO:LiNbO3), and have succeeded in developing a tunable picosecond THz-wave source by using a novel pentagramshaped pump-enhancement cavity. One of the limiting factors in efficient THz-wave generation is the strong absorption by MgO:LiNbO3 in THz-wave region. To overcome this problem, we employ a surface-emitted configuration which consists of a trapezoidal MgO:LiNbO3 crystal and a pump-enhancement cavity folded in the shape of a pentagram. The pentagram-shaped cavity is designed for the noncollinear dual resonance of both pump and one of the down-converted waves. As a result, 1.5-ps pump pulses from a mode-locked Ti:sapphire laser operating at 780 nm allow tunable THzwave generation via parametric down-conversion resulting from stimulated phonon-polariton scattering in the MgO:LiNbO3 crystal. By slightly translating the position of one of the cavity mirrors, we experimentally find that the THz-wave frequency is tunable in the range from 0.1 to 3.5 THz with the average output power of dozens of nanowatts. The maximum THz-wave average power is up to 40 nW around 2 THz at the pump power of 800 mW, which is several times higher than the THz-wave output generated by using rectangular MgO:LiNbO3 crystals for Si-prism-coupled configuration under the same pump condition.

Fabrication of Al2O3/TiO2 multilayer mirrors for water-window attosecond pulses

Novel metal-oxide multilayer mirrors for water-window wavelengths have been already studied and then fabricated by atomic layer deposition (ALD) or atomic layer epitaxy (ALE) methods which have the self-limiting nature of the surface reactions and can control thickness on an atomic scale over large areas. The reason why metal-oxide multilayer mirrors are effective in the water-window wavelength is that they can prevent the formation of various alloys at the interface resulting in scattering loss, and the absorption of oxygen in oxides is negligible at the wavelength. In this study, high and low refractive materials were chosen to be TiO2 and Al2O3 respectively, because they can be fabricated by ALD or ALE methods and Ti L-absorption edge is located at 2.73nm. We investigated the atomic-scale growth of these films and then found that the growth rates could be constant. Moreover, Al2O3/TiO2 multilayer mirrors were fabricated by the ALE method. As a result, the soft x-ray reflectivity of the 10-bilayer mirror was 1.54%, approximately.

Nuclear spin polarization of 3He atoms with a frequency doubled Ti:sapphire laser toward nuclear magnetic resonance of porous media

NMR based on laser-polarized 3He gases has been attracted as a powerful tool for characterizing physical parameters of porous media and then imaging human lungs. In this paper, the feasibility study of nuclear polarization of 3He atoms utilizing the 23S-33P transition at 389 nm is reported in comparison with the conventional 23S-23P transition at 1083 nm. The 389-nm light has been available readily with the development of various indium gallium nitride light-emitting diodes (InGaN LEDs). In this work, the frequency-doubled light of a 778-nm CW Ti:sapphire laser with the nonlinear crystal (BiB3O6) was used as the optical pumping light at 389 nm. The other light from a Littrow external cavity diode laser was also used for optical pumping at the 1083-nm wavelength and then measurement of the nuclear polarization. The nuclear polarization of 1.8% with optical pumping at the 23S-33P transition was demonstrated and then it was found that the (23S1, F=1/2)-(33P0, F=1/2) transition was the most efficient transition of 23S-33P lines for the magnetic field of 1.6 mT and the gas pressure of 0.5 Torr.

Generation of quasi-continuous wave 389-nm coherent light by frequency doubling of a Ti:sapphire laser for nuclear spin polarization of 3He atoms

Magnetic Resonance Imaging based on the hyperpolarized helium-3 (3He) gas has been attracted as a non-destructive testing technique for the porous media and the medical imaging. In order to produce nuclear spin polarization of 3He, optical pumping is the efficient way using a resonant line. However, there is no resonant light source to the line from the ground state of 3He. Then, we have been focusing on the nuclear spin polarization in a discharge cell using the metastability exchange optical pumping (MEOP) technique. We aim at the optical transition 23S1→23P0 at λ=389nm that has never been investigated for the polarization. Therefore, at first, we developed a single-frequency 389-nm coherent light source based on the second harmonic generation of a single-frequency 778-nm continuous-wave Ti:sapphire laser light with a BiB3O6 (BiBO) nonlinear crystal in an external cavity for the enhancement. As a result, we obtained the 389- nm output radiation with the high conversion efficiency of 56%. Additionally, we also demonstrated the frequency doubling of a quasi-continuous wave Ti:sapphire laser for the optical pumping of multiple optical transitions.