The elusive 2s3s1S level in B II

It has been known for nearly 30 years that the theoretical and experimental values for the energy of the 2s3s 1 S level in singly ionized boron, B II, differ strongly. Since there is much better agreement for other B II levels, it has been concluded that the experimental value for 2s3s 1 S must be revised. Despite a number of recordings over the years of sliding-spark, hollow cathode and beam–foil spectra, this level has not been located. We have now performed another beam–foil experiment, using higher resolution and sensitivity than in most previous studies. By combining these new data with previous results, we have identified transitions from the 2s4p, 2s5p and 2p3s 1 P levels to 2s3s 1 S, the excitation energy (137 622 ± 3 cm −1 ) of which is now well established and in excellent agreement with theoretical predictions.