Local dissipation drives global relaxation in a molecular ultracold plasma with on-site disorder and long-range dipolar interactions

The nitric oxide ultracold plasma evolves from a state-selected Rydberg gas to form a disordered ensemble of dipoles that populates a distribution of Rydberg and excitonic states concentrated in an energy interval within a few hundred GHz of the ionization threshold, doped by a trace population of more deeply bound Rydberg molecules. This residue of lower-n molecules includes a fraction that retains the initially selected principal quantum number, n₀. l-mixing collisions drive these molecules to occupy non-penetrating states of high-l. Excitation by mm-wave radiation tuned to resonance with n₀l(2) to (n₀ ± 1)d(2) transitions depletes the plasma signal to an amplitude near zero, even though delayed selective field ionization spectra show that the distribution of states evolves by then to contain fewer than one percent in the n₀ level resonant with the mm-wave field. Reading the nature of this coupling in the linewidths and depths of these depletion resonances, we see direct evidence that predissociation, which transfers population from the spectroscopically active (n₀ ± 1)d(2) states to a system of free N(⁴S) + O(³P) atoms, bridges the closed plasma ensemble to a thermal continuum, in effect creating an open quantum system that combines the plasma with the reservoir of free atoms. Motivated by these results, we have constructed a minimal model of disordered one-dimensional spins evolving under a Lindblad master equation with local dissipation that qualitatively reproduces the experimental observations.