Low Voltage Excitation of Sodium

Abstract

In an attempt to clarify the available data on excitation of atoms by low velocity electron impact, the author has studied the optical excitation function of the resonance lines in sodium vapor in the range from 1.8 to 4.2 volts. It is found that the excitation probability has three distinct maxima, corresponding to excitation to $3{}_{}{}^2{}_{}{}^{}P$, $4{}_{}{}^2{}_{}{}^{}S$ and $5{}_{}{}^2{}_{}{}^{}S$ states. Within two-tenths of a volt of the excitation potential the probability has dropped to zero and remains at that value until a new energy level is reached. No excitation to the $3{}_{}{}^2{}_{}{}^{}D$ appears, possibly because of the existence of a selection principle in electronic excitation.