Experimental Detection of Tetraoxygen This work was supported by the University of Rome, the Italian National Research Council (CNR), and the Ministero dellapos;Università e della Ricerca Scientifica e Tecnologica (MURST).

Although suggested by Lewis in 1924 and theoretically predicted, O4 has so far proved extremely elusive, defying all attempts at the positive experimental detection of a bound, intact O4 species. The search has now been brought to an end by the conclusive proof, achieved by neutralization–re-ionization mass spectrometry (see picture), of the existence of intact O4 as a gas-phase species with a lifetime in excess of 1 μs, whose dissociation requires overcoming a barrier of the order of 10 kcal mol−1.