Spectroscopic observation of a metastable electronically excited He4+

In the size distribution of ionized helium clusters produced by electron impact ionization of very large neutral clusters (N > 105 atoms), He4+ is the most abundant small cluster ion. This observation has recently received considerable attention, since in theoretical calculations no specially stable structure for the tetramer ion could be found. Measurements of the photoabsorption cross section and of the fragment kinetic energy release now indicate that this ‘magic’ He4+ is in a metastable electronically excited state.