Observation of an optical transition in the negative beryllium ion

The theoretically predicted 1s 2 2p 3 4 S-1s 2 2s2p 2 4 P optical transition in Be - has been observed experimentally. The 4 S state decays at 2653.01+or-0.09 AA to the metastable 4 P state with tau =1.25+or-0.10 ns. Thus the electron affinity for Be(1s 2 2p 2 ) 3 P is 4.2 meV larger and not 10 meV lower than the electron affinity for Be(1s 2 2s2p) 3 P as predicted by the most accurate electron correlation calculation available.