VOLUME 54, NUMBER 5 PHYSICAL REVIEW LETTERS 4 FEBRUARY 1985 Collision-Strength Shift in Electron-Impact Single and Double Ionization of Sb and Bi Ions A. Muller, ' K. Tinschert, Ch. Achenbach, and E. Salzborn Institut fur Kernphysik, Universitat Giessen, D 630-0 Giessen, West Germany and R. Becker Institut fiir Angewandte Physik, Univeritat Frankfurt, D 6000 Fr-ankfurt, West Germany and M. $. Pindzola Atomic Theory Group, Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 (Received 13 November 1984) At energies greater than 200 eV the electron-impact single-ionization cross sections for Sb'+ (4d' 5s Sp) aud Bi + (Sd' 6s 6p) ions, o. 2 3 are larger than the corresponding cross sections, a & 2, for Sb'+ and Bi'+ ions, respectively. This extraordinary behavior is a consequence of the fact that the 4d (Sd) hole configurations goes from autoionization to bound when the charge state of the Sb (Bi) ions is increased from +1 to +2. Additionally, multiple ionization measurements (o. ~ 3 for Sb and o. ~ 3, o. 2 4, and a. 3 5 for Bi) are presented for electron energies up to about 1000 eV. PACS numbers: 34.80.Dp Recently, some basic understanding has been gained on multiple ionization of atomic ions via single col- lisions with electrons. ' 4 The large cross sections ob- served, especially for complex ions, may be attributed to single-ionization events ejecting an electron from one of the inner shells followed by the emission of one or more additional electrons via autoionization processes. However, no adequate attention has yet been given to the mutual interdependence of multiple and single electron-impact ionization of ions: Col- lision strength from a given subshell missing in the cross section tr~~+t for single ionization of a q-fold charged ion can be found in the cross section o-~~+z for double ionization and vice versa depending on the possibility of an inner-shell ionization-autoionization mechaism. In this Letter we discuss the shift of d electron col- lision strength between single and double ionization of Sbq+ (q =1, 2) and Bi~+(q =1, 2, 3) ions which are homologous with configurations 4d'o5s 5p3 't and 5dto6s26p3 q, respectively. The starting point of this investigation was the observation of an unusual behavior of cross sections crt 2 and o. 2 3 for Sb'+ (Bi'+) and S12+ (Bi +) ions, respectively. At elec- tron energies above about 200 eV we found tr2 3 ) trt 2 i.e. , for these species it is easier to remove an electron from the doubly charged ion (ionization potentials5 of 25. 3 eV for Sb + and 26. 6 eV for Bi2+) than to remove an electron from the singly charged ion (ionization potentialss of 16.24 eV for Sb'+ and 16.69 eV for Bi'+). A second important experimental result is the observation of the largest cross sections for double ionization ever found for an atomic ion with a-, 3 =6.5 X10 ' cm for Sb'+ and trt 3 =7.5 &&10 '7 cm2 for Bi"+, while the ions Bi2+ and Bi'+ have considerably smaller double-ionization cross sec- tions (a. z 4 =2.7 &10 ' cm and cr3 s =1.9 &&10 cm2 at maximum) and are almost equal above 500 eV. We will demonstrate below that this extraordinary behavior is due to the fact that the 4d (5d) hole con- figuration goes from autoionizing to bound as one goes from Sb'+ (Bi'+) to Sb2+ (Bi2+) ions. For the measurement of ionization cross sections we have employed a new experimental technique with crossed electron and ion beams which is based on the "animated-beams method " of Brouillard and co- workers. However, instead of sweeping the electron beam by electric deflection our high-perveance elec- tron gun (P = 10 A V t ) is moved mechanically at constant speed up and down across the well- collimated ion beam. The count rate of ionized ions and the digitized current of parent ions are registered by a multichannel analyzer as a function of the electron-beam displacement. In the extreme positions the electron beam does not overlap with the ion beam which yields an easy measurement of the background while both beams are "on. " The cross section is directly proportional to the number of ionization events accumulated during the movement of the elec- tron gun. With this method it is not necessary to make a separate measurement of the form factor which describes the overlap of both beams and, furth- ermore, each ionization cross section determined is in- dependently absolute. A remeasurement of the cross section a. t 2 for Ar'+ ions yielded very good absolute agreement with data published by Woodruff, Hublet, 414 1985 The American Physical Society