Absolute cross sections for electron-impact single ionization of Si+ and Si + have been measured using crossed beams of ions and electrons and calculated using a configuration-average distorted-wave method. Corrections have been made for metastable components and small fractions of nitrogen impurities in the incident ion beams. Excitation-autoionization measurably enhances the cross sections of both Si+ and Si +. Ionization rate coef5cients and fitting parameters are presented for the experimental data.PACS number(s): 34.80.Kw

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Absolute total cross sections for electron-impact single ionization of Kr, Kr'+, and Kr' ions have been measured using a crossed-beams technique from below threshold to 500, 400, and 500 eV, respectively, with absolute uncertainties of 12%%uo or less. The measured cross sections are in good agreement with distorted-wave calculations and show significant contributions from excitation autoionization.Nonzero cross sections below threshold for the Kr and Kr'+ ions suggest the presence of metastable ions in those two species that were extracted from an electron-cyclotron-resonance ion source. Such evidence of metastable ions was not found in the case of the Kr'+ ion measurment. PACS number(s): 34.80.Kw I. INTRODUCTION Electron-impact ionization of ions is an important atomic process in laboratory and astrophysical plasmas. Accurate cross sections and rate coefficients are required for modeling and diagnosing these plasmas. Much work has been done on electron-impact ionization of ions [1], but cross sections are lacking for several key ions of interest to fusion researchers [2]. In particular, cross sections for ionization and excitation of krypton ions by electrons are critical for understanding tokamak plasmas into which krypton has been injected to facilitate core ion temperature measurements [3]. Highly ionized krypton ions will be dominant in the core plasma, but moderate to low charge states will be abundant in the edge plasma. In this paper, we report absolute total cross sections for electron-impact single ionization of Kr +, Kr +, and Kr + ions, with outer shell ground configurations of 4s 4p, 4z 4p, and 4z, respectively. The only previously published cross sections for ionization of multicharged krypton ions were for single ionization of Kr + [4 -7], Kr + [4,5,7,8], and Kr + [9], for double ionization of Krq+ (q =2, 3,4) [10,11], and for triple ionization of Kr + [11]. Cross sections for single ionization of Zn+, isoelectronic with Kr +, have also been reported [12]. Since it is not practical to measure the ionization cross sections for all 36 charge states of krypton ions, we haveconcentrated on a few ions for which indirect ionization processes should be significant [13]. Measurement of the ionization cross sections for Kr + was also attempted but found not to be possible because of extremely high ion