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 R N . 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 1/2 /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.