  Resonant mm-wave fields drive $n_0f(2) \rightarrow (n_0 \pm 1)g(2)$
transitions in a state-selected $n_0f(2)$ Rydberg gas of NO. This
transformation produces a clear signature in the selected field ionization
spectrum and dramatically increases the intensity of corresponding features in
the spectrum of the $n_0f(2)$ Rydberg series observed in UV-UV double resonant
transitions via the $A ~^2\Sigma^+, ~N'=0$ state. Here, $n_0$ refers to the
principal quantum number of an $f$ Rydberg state converging to the $N^+=2$
rotational state of NO$^+ X~ ^1\Sigma^+$. Enhancement owing to transitions from
$\ell=3$ ($f$) to $\ell=4$ ($g$) appears both in the electron signal detected
at early time by the field ionization of Rydberg molecules and, 40 $\mu$s
later, as the late-peak signal of plasma in a state of arrested relaxation.
Similar stabilization and enhanced intensity also occurs for shorter-lived
interloping complex resonances, $44p(0) - 43d(1)$ and $43p(0) - 42d(1)$. We
conclude from these observations that avalanche alone does not guarantee a
plasma state of arrested relaxation. But rather, the formation of an arrested
phase requires both avalanche-produced NO$^+$ ions and a persistent population
of long-lived Rydberg molecules.
