H. Tawara

K. Hosaka

National Institute for Fusion Science (NIFS-Repository)

§S. Contribution of Kinetic Energy to Secondary Electron Emission Yields from Clean Cu Surfaces at Low Energy Projectile Ion Impact Hosaka, K. t, Tawara, H. Studies of the secondary electron emission (SEE) . processes in collisions"· of low kinetic energy ions with solid surfaces are of great importance not only in basic atomic physics but also in applied fields such as plasma-wall interactions in fusion devices. In low energy projectile ion.impact on solid surfaces, the SEE yields 'Y. are given by 'Y = 'YKE + 'YPE' (1) where 'Y KE and 'Y PE are the kinetic emission and the potential emission, respectively. In the present study, we would like to focus our attention to the analysis of 'Y KE' The SEE yields 'Y from clean eu surfaces induced by low energy ion (II+' Ar+, Afl+, Kr+ and J(r2+) impact were measured using a cylindrical double-wall cup under an ultra high vacuum chamber (-- 10-10 Torr) [1]. The target surfaces were cleaned by Ar+ sputtering and the surface cleanness was examined by Auger electron spectroscopy. Figure 1 shows the present results of the SEE yields for Kr+ and J(r2+ ion impact on clean eu surfaces as a function of the impact energy. In this figure the experimental results at higher energies obtained by Baragiola et al. [2] are also shown. As seen in Fig. 1, it is noticed that our experimental results at low energies for Kr+ ion impact smoothly merge with their results at high energies. For all the projectile ion species investigated, as the impact energy dxreases, the SEE yields dxrease and finally reach constant values which can be we to the potential emission. The SEE yields are also found to increase with the ionic charge over the whole impact energy range investigated. The 'Y KE' which is OOcbced by subtracting 'Y PE from "total" SEE yields, is shown in Fig. 2. It seems that these kinetic emission parts, 'Y KE' for singly and ooubly charged ions lie on the single curve. It is clearly seen from Fig. 2 that the kinetic emission is in<kpenrent of the incirent ion charge, suggesting that the incirent ionic charge 00es not play any significant role for the kinetic electron emission from the metal targets as expected. It is also noted that the ion impact energy d!penrence varies at low and high energies and clear kinks are seen. At low energy region, 'Y KE for clean eu surfaces shows the apparent thresholdlike behavior and <h;reases roughly as V3.5 when the projectile ion velocity (v) dxreases. Because, in general, 'Y KE are proportional" to the electronic stopping power Se of the projectile ion in solids, these results indicate that Se also dxreases very rapidly as the projectile velocity dxreases. This su~n <h;rease of the Se at low energies is theoretically understood to be due to the non-negligible size of the excitation/ionization energy of the target elements, which is the lower limit of the integration of the energy transferred to the target, over the projectile kinetic energy. 10- 1 K~+ ••••• '" • • .. ••• • • • •• •• .. : 10-2 L----'---'-..L.I....LLI..&.L----I'--'-L...Ju..L&...u...~"'--'-'-..L..L.I...LU 10-3 10-2 10-1 10° Energy (keV/amu) Fig. 1. bnpact energy d!penrence of total SEE yields for Kr+ (circles) and J(r2+ (squares) ion impact on clean eu surface. Also shown are total SEE yields unrer Kr+ ion impact on clean eu surface investigated by Baragiola et al. (diamonds)[2] at high energies. 101 Ar+, Ar+ 0 ~ . 0 C- 10° . ",'" • '" '" '" s::: H+ 0 ...... tI.l tI.l 10-1 's ~ (,) 0 present work (Ar+) '+=l [J I3resent work (Ar++) CO <> aragiola (Ar+) s::: 10-2 • present work (Kr+) S2 • I3resent work (Kr++) • aragiola (Kr+) ... I3resent work (H+) A aragiola (H+) 10-3 L-L...L..I~I&I.-..L...L..I..I.I.I..uI.-...&....I-LU.WL.-'-&...I..LJw.u..--,-................... 10-3 10-2 10- 1 10° 101 102 Energy ( ke V I amu ) Fig. 2. 'Y KE unrer singly and ooubly charged ion impoct on clean eu target. And the kinetic emissions at higher energies taken from the <hta of Baragiola et al. [2] are also shown. Note that practically no difference is observed in the kinetic electron emission between singly and ooubly charged ion impact. References 1) Hosaka, K. et al. : Nucl. Instr. Meth. B 149 (1999) 414. 2) Baragiola, R.A. et al. : Sur. Sci. 90 (1979) 240. t JSPS Research Fellow 319 . 

Contribution of Kinetic Energy to Secondary Electron Emission Yields from Clean Cu Surfaces at Low Energy Projectile Ion Impact

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