204 Nuclear Instruments and Methods in Physics Research B10/11(1985) 204-206 North-Holland, Amsterdam zyxwvutsrq A NEW TECHNIQUE FOR THE MEASUREMENT OF IONIZATION CROSS SECTIONS WITH CROSSED ELECTRON AND ION BEAMS A. MULLER, K. TINSCHERT, C. ACHENBACH and E. SALZBORN Institut fti Kemphysik, Universitiit Giessen, D - 6300 Giessen, FRG R. BECKER Institut fiir Angewandte Physik, Universitiit Frankfurt, D -6ooO Frankfurt, FRG A new experimental technique has been developed and used to measure cross sections for electron impact ionization of ions. Each single cross section is determined independently absolute by moving the electron gun mechanically across the ion beam with simultaneous registration of the ionization signal and the actual beam current. Different from the method of Brouillard et al. it is not necessary then to provide constant speed of the motion. Typical relative uncertainties of the measured cross sections are of the order of f 2% and absolute total errors are usually within f 6.5%. Very good agreement of ionization cross sections IJ, r measured with our new technique for Ar+ ions is found with data from Harrison’s group. Cross sections for single and multiple ionization of singly and multiply charged ions of Ar, Kr, Sb and Bi have been measured from threshold to 1000 eV. 1. Introduction The experimental investigation of electron impact ionization of ions has revealed significant new aspects of electron-ion interaction. Important contributions of excitation-autoionization to the ejection of a single electron [l] and of inner-shell ionization-autoionization to multiple ionization [2-41 have been observed. With increasing charge state of the ions the indirect mecha- nisms often tend to dominate the direct ionization processes [5,6]. Since electron-impact ionization of ions is of funda- mental importance in understanding plasmas and is difficult to predict from first principles there is an immediate practical interest to accumulate experimental knowledge about features and trends in the cross sec- tions. An absolute accuracy within flO% would be desirable for data used in plasma modelling calcula- tions. Because of the general interest in the field and the inherent data needs we have considerably improved the experimental method used for our previous investiga- tions of electron impact ionization. With our new tech- nique it is possible to measure each ionization cross section independently absolute. The new tecbntque is based in principle on a method first described by Defiance et al. [7]. This alternative method of cross section measurements has several ad- vantages compared to the ordinary crossed beams tech- nique using modulation of beam intensities [8]. It does not require a separate measurement of the form factor and since both beams are always “on” pressure modula- 0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) tion effects cannot occur. We have modified the animated beams principle [9] in the following way: An intense ribbon electron beam is moved mechanically up and down through an ion beam at a constant speed. The amplitude of the motion is large enough to provide zero-overlap of both beams in the extreme positions which gives an easy possibility to measure the back- ground. The number of accumulated signal counts ob- tained in the overlap region is directly proportional to the cross section and the total intensities of both beams no matter how these intensities are distributed across the sections of the beams. Simultaneous with the rate of ionized ions, the current of the parent ion beam and the actual speed of the motion are registered as a function of electron beam position. It is thus possible to do correct normalization of accumulated signal counts to accumulated parent ion charge. 2. Method When electron and ion beams parallel in themselves collide under an angle of 90° the rate R of ionized ions is given by R = oI&M/F, (1) with M = (u,’ + Df)l”/( vr,oiqez), and the form factor (2) jrn i,(z)i,(z)dz zyxwvutsrqponmlkjihgfedcbaZYXWV , -cc 1 (3)