Density dependence of molecular autoionization in a cold gas

Molecular autoionization (MAI) is a Penning-like process involving high Rydberg state electrons from two neighbouring excited atoms and can be the dominant mode of free electron production in a cold excited gas. The density of a gas may be determined employing the sensitivity of MAI to the average inter-ionic separation RN. We show by explicit calculation that a sharp correlation exists between the gas density and Rydberg state quantum number n, manifested through an abrupt increase in the number of ejected electrons as n is increased to a critical value n(th) = R-N(12)/2. The MAI electrons have discrete energies allowed by energy conservation, at 6/n(4) Ry, 2/n(3) Ry, 4/n(3) Ry, and so on.