Observation of an optical transition in the negative beryllium ion

Abstract
The theoretically predicted 1s22p3 4S-1s22s2p2 4P optical transition in Be- has been observed experimentally. The 4S state decays at 2653.01+or-0.09 AA to the metastable 4P state with tau =1.25+or-0.10 ns. Thus the electron affinity for Be(1s22p2)3P is 4.2 meV larger and not 10 meV lower than the electron affinity for Be(1s22s2p)3P as predicted by the most accurate electron correlation calculation available.