FOURIER ANALYSIS OF TEMPORAL AND SPATIAL OSCILLATIONS OF TUNNELING CURRENT IN SCANNING TUNNELING MICROSCOPY

Partially oxidized Si(111) surfaces and surfaces of highly oriented pyrolytic graphite (HOPG) were studied by two different ultrahigh vacuum scanning tunneling microscope (UHV-STM) systems and by an STM system working under ambient conditions, respectively. The STM current images of partially oxidized Si(111) surfaces and HOPG surfaces were analyzed by one/two-dimensional fast Fourier transformation (1D-FFT/2D-FFT). The phenomenon of temporal oscillations of tunneling current on the partially oxidized Si(111) surfaces was detected with both UHV-STM systems. Temporal as well as spatial oscillations of tunneling current appeared in highly resolved STM current images of the Si(111) surfaces simultaneously, but both kinds of oscillations could be discriminated according to their different influence on the 2D-FFT spectra of the current images, while varying the scanning range and rate. On clean HOPG surfaces only spatial oscillations of tunneling current induced by the surface structure were observed.     

High order harmonic generation from solid targets: Towards intense attosecond pulses

Experiments and computer simulations on high order harmonic generation (HOHG) from steep plasma gradients using intense femtosecond laser pulses are presented. The importance of the plasma scale length for the efficiency of HOHG is demonstrated. By changing the pump intensity and the target parameters the regime of HOHG can be switched from non-relativistic to relativistic. Simulations show that attosecond pulses should already be present in our experiments in both interaction regimes. Two-color driven harmonic generation is shown to provide a highly efficient way of attosecond pulse production.     

Determination of the k 0-values for 75Se, 110mAg, 115Cd–115mIn, 131Ba, and 153Sm by irradiation in highly thermalized neutron flux

The k ₀-values were determined for five high Q ₀(n,γ) reactions, including ⁷⁴Se(n,γ) ⁷⁵Se, ¹⁰⁹Ag(n,γ) ^110mAg, ¹¹⁴Cd(n,γ) ¹¹⁵Cd–^115mIn, ¹³⁰Ba(n,γ) ¹³¹Ba, and ¹⁵²Sm(n,γ) ¹⁵³Sm. These determinations were carried out under favorable experiment conditions: the irradiations were performed in a highly thermalized neutron flux, the irradiated target samples were counted at a far distance from HPGe detector with an efficiency carefully calibrated, and the k ₀-values were calculated against an internal comparator. When compared to the new values from this work, the 2003 recommended ^110mAg k ₀-values are confirmed. The other confirmed recommended k ₀-value is that of ⁷⁵Se 400.7 keV line. However, for the other ⁷⁵Se γ-lines, the new k ₀-values are 4–10 % higher. It is assumed that an inaccurate efficiency calibration was used when the recommended k ₀-values were measured. For the other three nuclides, the new k ₀-values are higher by 4 % for the ¹¹⁵Cd–^115mIn γ-lines, lower by 6–8 % for the ¹³¹Ba γ-lines, and lower by 8.8 % for the ¹⁵³Sm 103.2 keV γ-line.     

Gamow-Teller strength in the exotic odd-odd nuclei 138La and 180Ta and its relevance for neutrino nucleosynthesis

The Gamow-Teller strength distributions below the particle threshold in 138La and 180Ta, deduced from high-resolution measurements of the (3He,t) reaction at 0 degrees, allow us to evaluate the role of charged-current reactions for the production of these extremely rare nuclides in neutrino-nucleosynthesis models. The analysis suggests that essentially all 138La in the Universe can be made that way. Neutrino nucleosynthesis also contributes significantly to the abundance of 180Ta but the magnitude depends on the unknown branching ratio for population of the long-lived isomer.