Strong Relativistic Effects and Natural Linewidths Observed in Dielectronic Recombination of Lithiumlike Carbon

Abstract

Few-electron systems of light elements are usually very well described in terms of LS coupling. Selection rules for atomic processes like radiative transitions, photoionization, and autoionization can be expressed in terms of L and S. For instance, in an electric dipole transition DS › 0, DL › 61 or 0 while parity is changed, and for Coulombic autoionization S, L, and parity are all conserved. It is well known that intercombination transitions, i.e., spinforbidden radiative transitions sDS fi 0d, are extremely weak for light elements. The extent to which such selection rules are obeyed indicates in some sense the validity of LS coupling. Dielectronic recombination is a process in which an electron is recombined with an ion through interaction with the electrons of the ion. The first step of the process is the reversal of autoionization; i.e., a free electron interacts with the bound electrons to form a doubly excited state. This state can decay by autoionization, but it can also relax by a radiative transition to a bound state. In the latter case, dielectronic recombination is completed. In the present work, dielectronic recombination (DR) has been used as a tool to gain spectroscopic information on autoionizing states of the displaced system of berylliumlike carbon. For a Be-like system there are two ionization limits present: 1s 2 2s 2 S and 1s 2 2p 2 P. In Be-like carbon (i.e., C 21 ), these two thresholds are separated by 8 eV. Separate Rydberg series run toward these limits. Consequently there will exist levels of the type 1s 2 2pn, 1,3 L which are energetically degenerate with continuum states 1s 2 2s«, 1,3 L. If the states of the former type have