IEEE Transactions on Nuclear Science, Vol. NS-30, No. 2, April 1983 ELECTRON CAPTURE TO THE CONTINUUM AT ASYMPTOTICALLY HIGH VELOCITIES S. D. Berry, I. A. Sellin, K.-O. Groeneveld*, D. Hofmann*, L. H. Andersen, M. Breinig, S. B. Elston and M. M. Schauer University of Tennessee, Knoxville, Tennessee 37996-1200 and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 and N. Stolterfoht, H. Schmidt-B6cking, G. Nolte and G. Schiwietz Hahn-Meitner-Institut fur Kernforschung, D 1000 Berlin 39, F. R. Germany Summary The velocity distrubutions of electrons ejected from gaseous He targets into the forward direction by 15-18 au velocity bare 0, Ne and Ar nuclei are measured. The shape of the resulting ECC (electron capture to the continuum) cusps is studied in detail by three fitting methods and compared to theory. The results favor theoretical treatments which include higher-order ejected electron partial waves to describe the observed asymmetry. Introduction In ion-atom collisions a sharp cusp in the velo- city spectrum of ejected electrons is observed when the clectron velocity ve matches the emergent ion velocity v in both direction and speedl. Two possible origins for these electrons are excitation of a projectile elec- tron to a low-lying continuum state (electron loss to the continuum, ELC), and transfer of a target electron to a low-lying (projectile) continuum state (electron capture to the continuum, ECC). When the incident ion has several electrons available, ELC is the dominant process, producing a symmetric cusp peak with width essentially independent of projectile velocity2. When the projectile is fully stripped, only ECC can occur; the cusp is asymmetric with higher intensity on the low- velocity side of the peak and the width is observed to be roughly proportional to the ion velocity3. The work presented here concerns only bare projectiles with velocities much greater than characteristic target electron velocities, so only ECC in this velocity range will be further considered. ECC experiments have been in considerable disagree- ment with early theories of the cusp shape4. With charge transfer to the continuum the dominant produc- tion process for Ve v electrons, the electron wave functions are represented by projectile-centered Cou- lomb waves. Within the first Born-Brinkman-Kramers approximation the form of the differential cross sec- tion, for continuum capture from a ls state into a cone centered at 00, describes a symmetric, sharply peaked cusp centered at Ve = v. At the asymptotically high velocities where the calculation is thought to be valid, first Born theories in general predict s states to domi- nate, because of a predicted X v-2' dependence for ejected electron partial waves. More recently, several theories have been proposed which predict an asymmetry in the ECC cusp. Shakeshaft and Spruch5 suggested that the observed asymmetry might indicate the importance of including the second Born term in Born expansion calculations. In the second Born term all partial waves are thought to be important, and an asymmetry results from odd-Q partial waves (p, f, . . . ) for which dc/dv is asymmetric under the * Permanent address: Institut fUr Kernphysik, Uni- versitat Frankfurt, August-Euler-Strasse, D 6 Frankfurt am Main 90, F. R. Germany transformation ('e - v) + (Ve - t). Their numerical calculation for C6+ incident on H is characterized by a sheer drop in intensity on the high-velocity side of the peak (Figure la). A counterproposal by Chan and Eichler6 noted that retaining terms linear in (Ice - v|)/v = ye/v beyond that first calculated in the first Born-Brinkman-Kra- mers approximation produces an asymmetry favoring the low-velocity side of the cusp (Figure lb). The cusp shape resulting from the approach of Chan and Eichler (CE) lacks the sharp drop of the Shakeshaft and Spruch (SS) result and becomes less asymmetric at high v as % 1/v. In contrast, the second Born-induced asymmetry is expected to persist at high v. Further study of the cusp shape by Miraglia and Ponce7'8 addresses details of the leading anisotropic terms within the second Born approximation. Garibotti and Miraglia9 present results derived from the first- order term of a multiple scattering theory which leads to a cusp shape qualitatively similar to that of Shakeshaft and Spruch. In recent experiments our group has sought to test the second Born asymmetry conjecture by using high-velo- city bare heavy ions (0, Ne and Ar) in collision with He gas. Comparison of results for Ar18+ with models of SS and CE shapes has been previously reported10. It must be cautioned that because the theories mentioned above consider hydrogenic targets, this comparison is only partially appropriate. In this paper we extend our comparison to recent experiments at the Hahn-Meit- ner-Institut (October 1981) and Oak Ridge National Laboratory (March 1982), and also use a partial wave expansion fitting method (Meckbach, Nemirovsky and Garibottill and Lucas et al.12) to better characterize the measured ECC cusp shape. ( a) (b) ELECTRON VELOCITY Figure 1. (a) Shakeshaft and Spruch cusp shape, as modelled for data-fitting purposes (see text). (b) Equivalent Chan and Eichler cusp shape. 0018-9499/83/0400- 902$01.00( 1983 IEEE 902