Papers by Author: Yasuyuki Nagashima

Abstract: We have observed positron-stimulated ion desorption from a TiO₂(110) surface. H⁺ and O⁺ ions were desorbed at incident positron energies above the desorption thresholds for electron impact. However, only O⁺ ions were detected at energies below those thresholds. These results suggest that by surface ionization positron annihilation as well as by positron impact leads to the O⁺ ion desorption. By contrast, it is likely that the H⁺ ions are not desorbed by positron annihilation, but rather by impact ionization.

Abstract: We have studied the emission of ortho-positronium from alkali-metal coated polycrystalline tungsten surfaces. The positronium time-of-flight spectra show that the yield of the 5 eV positronium component increases by alkali-metal coating. In addition, a low energy positronium component appears by Cs or K coating. We suggest that this component is due to positron energy loss by inter-band transition or surface plasmon excitation.

Abstract: Recently, we have observed spontaneous emission of positronium negative ions from polycrystalline tungsten surface. In the present work, we have extended the study to the emission from tungsten single crystal. The emission from tungsten (100) surface plane has been confirmed. The fraction of positrons yielding the ions at the target surface was ()4102.02.1−×±.