The elusive 2s3s S-1 level in BII

It has been known for nearly 30 years that the theoretical and experimental values for the energy of the 2s3s S-1 level in singly ionized boron, B it, differ strongly. Since there is much better agreement for other B II levels, it has been concluded that the experimental value for 2s3s S-1 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 P-1 levels to 2s3s S-1, the excitation energy (137 622 +/- 3 cm(-1)) of which is now well established and in excellent agreement with theoretical predictions. (Less)