Molecular autoionization (MAI) is a Penning-like process involving high Ryd
berg state electrons from two neighbouring excited atoms and can be the dom
inant mode of free electron production in a cold excited gas. The density o
f a gas may be determined employing the sensitivity of MAI to the average i
nter-ionic separation RN. We show by explicit calculation that a sharp corr
elation 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 ha
ve discrete energies allowed by energy conservation, at 6/n(4) Ry, 2/n(3) R
y, 4/n(3) Ry, and so on.