Ionization of Rydberg atoms

Ion formation in sodium vapor (∼${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}3}$) containing Rydberg atoms (18≤n≤35) was investigated. Both atomic ions and diatomic molecular ions resulting from collisions between highly excited sodium, ${\mathrm{Na}}^{\mathrm{*}\mathrm{*}}$, and Na(3s) were observed. However, the predominant production mechanism was found to be photoionization of ${\mathrm{Na}}^{\mathrm{*}\mathrm{*}}$ by blackbody radiation, the rate of which was measured at 500 K and found to be ≃5.7×${10}^3$ ${\sec }^{\mathrm{-}1}$ for n=18, in good agreement with theory. The ${\mathrm{Na}}^{+}$ signal from blackbody photoionization is observed to be independent of n, which we interpret as implying rapid and nearly complete self-l-mixing, mixing by collisions with Na(3s) atoms, for both s and d states. It is probable, however, that the s states are mixed to the adjacent, n-1, manifold of l states. It was also found that the heavy-body collision cross sections for ${\mathrm{Na}}^{+}$ and ${\mathrm{Na}}_2$ ${\mathrm{}}^{+}$ formation, while roughly three orders of magnitude lower than our estimate of the self-l-mixing cross section, are appreciable, of order 100 A${̊}^2$.