Field ionization of highly excited states of sodium

We have observed the selective ionization of the degenerate $\left|m_j\right|$ levels of $s$, $p$, and $d$ states of sodium in the range of $n$ from 15 to 20 and have shown that atoms produced in these low-field (∼ 10 V/cm) states pass adiabatically from low-field $\left|m_j\right|$ to intermediate-field $\left|m_l\right|$ states. Experiments on the Stark manifold states in intermediate fields (∼ 1 kV/cm) and high fields (∼ 4 kV/cm) show that the $\left|m_l\right|=0\mathrm{} \mathrm{and} 1$ states pass adiabatically from intermediate- to high-field states and that the passage of the $\left|m_l\right|=2\mathrm{}$ states is very nearly adiabatic. These findings suggest a general approach for predicting the threshold fields for ionization of atomic Rydberg states.