Resolution of $\left|m_l\right|$ and $\left|m_j\right|$ Levels in the Electric Field Ionization of Highly Excited $d$ States of Na

In the electric field ionization of highly excited $s$, $p$, and $d$ states of Na, we have observed one, two, and three ionization thresholds, respectively, reflecting the number of high-field $\left|m_l\right|$ states coming from the $s$, $p$, and $d$ states. Our observations indicate that the passage from low to intermediate field is adiabatic, and as a consequence we are able to achieve selective ionization of essentially degenerate low-field $\left|m_l\right|$ fine-structure levels.