  Prethermalization occurs as an important phase in the dynamics of many-body
systems when strong coupling drives a quasi-equilibrium in a subspace separated
from the thermodynamic equilibrium by the restriction of a gap in energy or
other conserved quantity. Here, we report the signature of an enduring
prethermal regime of arrested relaxation in the molecular ultracold plasma that
forms following the avalanche of a state-selected Rydberg gas of nitric oxide.
Electron collisions mix orbital angular momentum, scattering Rydberg molecules
to states of very high-$\ell$. Spontaneous predissociation purifies this
non-penetrating character, creating an extraordinary gap between the plasma
states of $n \approx \ell$, with measured $n>200$ and penetrating states of
$\ell = 0, ~1$ and 2. Evolution to a statistically equilibrated state of N and
O atoms cannot occur without Rydberg electron penetration, and this gap blocks
relaxation for a millisecond or more. Evolving through the critical phase,
electrons that balance the NO$^+$ charge behave as though localized in the
prethermal phase and play an ineffective role in bridging this gap. However,
the application of a weak radiofrequency (RF) field promotes a dramatic degree
of relaxation owing to electron collisions. On an entirely different scale,
exciting a quantum-state transition in an exceedingly small fraction of the
molecules in the prethermalized ensemble acts with even greater effect to drive
the entire system toward equilibrium. We ascribe this to dissipative character
added to a small fraction of the states in the prethermally localized ensemble.
Using the Lindblad master equation, we illustrate qualitatively similar
dynamics for a toy model of an open quantum system that consists of a localized
set of spins on which dissipation acts locally at a single site.
