Abstract

By electron impact inner-shell ionization of the metastable level of Al II, a quasi-metastable level (2p⁵ 3s3p ⁴S_3/2) of Al III can be populated. An experimental estimate of the excitation rate is 5 × 10^-10 cm³ sec^–1 for an electron energy approximately 300 electron volts. Using a multiconfigurational code the lifetime is calculated to be 39 nsec with radiative decay (at 17.5 nm) to the 2p⁶ 3p lower level being dominant over autoionization. Since the lifetime of the upper level greatly exceeds that of the lower, a population inversion can be maintained even in the presence of decay from other 2p⁵ 3s3p levels. Because of the large excitation rate from the ground state of Al III to the 2p⁶ 3p level the population inversion can only exist when the Al III density is reduced well below that of the Al II metastable density. Heating at the Al III ion cyclotron frequency may be used to selectively remove this ion from the discharge.

© 1986 Optical Society of America