STRONG RADIATIVE DECAY OF DOUBLE RYDBERG STATES IN HIGH-Q IONS

We have measured cross sections for true double-electron capture (DC) and transfer ionization (TI) in slow Xe(q+)-(Ar, He, Xe) collisions in the charge-state regime 15 less-than-or-equal-to q less-than-or-equal-to 44. Following the extended classical over-the-barrier model we assume that the first step in the population mechanism is the same for both processes and that the two transferred electrons initially go to a double Rydberg state. We define total probabilities for radiative stabilization as P(rad) = sigma(DC)/(sigma(DC) + sigma(TI)). These quantities represent products of the branching ratios for radiative decay of all the individual intermediate states in the decay processes averaged over the possible cascade paths. Within the experimental uncertainties there is no difference for P(rad) measured for two-electron transfer from He, Ar, or Xe. The common q-behaviour of P(rad) consists of a strong increase in the region 26 < q < 36 to a level of about 0.35, a ''flat top'' for 37 < q < 42, and a rather steep decrease for q = 43 and 44. Apparently, a substantial fraction of the intermediate states have high branching ratios for radiative decay. Electron emission seems to occur late in the cascade when one of the electrons is close to the core. We offer a qualitative explanation for these observations and discuss the possible influence from configuration mixing with strongly asymmetric doubly excited states at high q.