positroniumn.

1945 A. E. Ruark in Physical Rev. 68 278/1 In 1937 I conceived the idea that an unstable atom composed of a positron and a negative electron may exist in quantities sufficient for spectroscopic detection. The name positronium is suggested. The spectrum of positronium would have lines at wave-lengths twice as great as the hydrogen lines.

1957 New Scientist 26 Dec. 28/3 When positronium is in free (i.e., near-vacuum) conditions, spectroscopic measurement shows that it has a lifetime of about one and a half ten-millionths of a second.

1966 McGraw-Hill Encycl. Sci. & Technol. (rev. ed.) X. 524/1 Positronium is of particular interest because it is the two-body system to which quantum electrodynamics is applicable, and its study has served as an important confirmation of the theory of quantum electrodynamics.

1976 Science 19 Nov. 826/3 If ψ were a bound system of two spin ½ particles, there could be excited levels with a spectrum similar to that of positronium (the system of electron and positron, bound to each other by the electromagnetic force).

1994 P. Davies Last Three Minutes vii. 97 In the laboratory, positronium decay takes place in a tiny fraction of a second, but in outer space, with little to disturb them, electrons and positrons could become bound in a huge orbit.