VOLUME 82, NUMBER 19 PHYSICAL REVIEW LETTERS 10 MAY 1999 Experimental Evidence for Magnetic Field Effects on Dielectronic Recombination via High Rydberg States T. Bartsch, S. Schippers, A. Müller, and C. Brandau Institut f ür Kernphysik, Universität Giessen, 35392 Giessen, Germany G. Gwinner, A. A. Saghiri, M. Beutelspacher, M. Grieser, D. Schwalm, and A. Wolf Max-Planck-Institut f ür Kernphysik and Physikalisches Institut der Universität Heidelberg, 69117 Heidelberg, Germany H. Danared Manne Siegbahn Laboratory, Stockholm University, 10405 Stockholm, Sweden G. H. Dunn JILA, University of Colorado, Boulder, Colorado 80309-0440 (Received 11 December 1998) We report the first experimental observation of magnetic field effects on dielectronic recombination ( DR) via highly excited Rydberg levels. Crossed static electric and magnetic fields E y and B z were imposed on the collision region in high resolution DR measurements with Li-like Cl 141 ions at the heavy ion storage ring TSR in Heidelberg. Enhancement of DR rate coefficients a for the group of high Rydberg states attached to the 2p 1y2 and 2p 3y2 series limits was observed when motional electric fields E y up to 380 Vycm were introduced. The associated enhancement rate daydE y which we found to be constant at least for E y # 100 Vycm decreased by almost a factor of 2 when the longitudinal field B z was increased from 20 to 69 mT. [S0031-9007(99)09107-3] PACS numbers: 34.80.Lx, 31.50. + w, 31.70. – f, 34.80.My This work for the first time experimentally demon- strates that a magnetic field crossed with an electric field substantially affects the cross sections and rates for dielec- tronic recombination ( DR), in particular through the effect it has on the expected electric field enhancement of this process. DR is a fundamental electron-ion reaction well known to be important in plasma environments. One can view it as a two step process beginning with the excitation of an ion by an electron which is simultaneously captured into a level n, (n and , denoting the principal and the angular momentum quantum numbers); this first step is the time inverse of autoionization. In the second step the new ion is stabilized by photon emission from the inter- mediate doubly excited state. Realizing that high Ryd- berg levels are particularly relevant, Burgess found DR to be the dominating recombination mechanism in the solar corona [1]. Since then DR has received much theoretical attention, and calculated DR rate coefficients constitute an essential ingredient of plasma modeling codes. First measurements of rate coefficients were conducted [2,3] employing well characterized plasmas, but most of the ex- perimental progress in understanding the numerous facets of DR has been achieved since then by using colliding beams techniques [4,5]. It is well known by now that external electric fields can strongly influence cross sections and rates of DR. This was recognized early by Burgess and Summers [6] and Jacobs et al. [7]. Electric field enhancement of DR was subsequently found in numerous theoretical calculations [8]. Huber and Bottcher investigated theoretically pos- sible effects of pure static magnetic fields on DR [9]. Up to 5 T no effects were found. For higher fields up to 100 T, however, DR rates were calculated to fall off slowly with increasing magnetic field. Experiments on DR employing controlled and measurable external mo- tional electric $ y3 $ B fields in the electron-ion collision region have been scarce but have clearly established the enhancement of DR by electric fields [10–12]. Also, measurements in which the external fields had not been controlled [13–16] appeared to be understandable on the basis of the theoretical concept of electric field enhance- ment of DR, although the comparisons left ambiguities (see, e.g., Ref. [8]). Electric field enhancement of DR arises from the Stark mixing of , states and the resulting influence on the autoionization rates which, by detailed balancing, determine the capture of the free electron. Autoionization rates strongly decrease with increasing , and, therefore, only low , states significantly contribute to DR. Electric fields mix the low , and high , states, increasing the autoionization rates of the higher-, states, and therefore the contributions of Rydberg states to the DR process. Our previous storage-ring experiment on field enhanced DR of Si 111 ions [12] provided detailed results for a wide range of external electric fields. Although the theoretical calculations were in reasonable agreement with the ex- periment as to the magnitude of the observed enhance- ment effect, significant disagreement was found regarding 0031-9007y 99 y 82(19) y 3779(4)$15.00 © 1999 The American Physical Society 3779