Stark-induced x-ray emission from high Rydberg states of H-like and He-like silicon ions

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Highly excited Rydberg states of H-like or He-like silicon are formed when ${\mathrm{Si}}^{14+}$ or ${\mathrm{Si}}^{13+}$ ions capture a single electron from an $n=10$ Rydberg target. Previous studies of the capture process indicate that a few $n$'s near $n=76$ or $n=72,$ respectively, are formed in the two cases. When these products are subjected to a weak external electric field $(E\ensuremath{\approx}20\mathrm{V}/\mathrm{c}\mathrm{m}),$ an enhanced rate of decay directly to the $1S$ ground state results as long-lived high-$L$ states are mixed with the rapidly decaying P states. The yield of these $\mathrm{nP}\ensuremath{-}1S$ x rays approaches a limit when the external field is sufficient to fully Stark mix the Rydberg manifold and this limiting yield can be used to deduce the fraction of Rydberg ions which are formed in states of low m $(m=\ensuremath{-}1,0,1).$ Measured values of this yield indicate a low-$m$ fraction of about $40%.$