High-Precision Structural Studies of the Antiproton Helium Atom $\overline{p}^{4}He^+$ by Observing Laser Resonances with $\Delta v = \Delta (n-l-1) = 2$

Very weak laser resonances of unfavoured transitions $(n,l) \rightarrow (n+1,l-1)$ have been observed in metastable \={p}$^4$He$^+$ atoms by using a pulsed antiproton beam. The observed wavelengths (713.578 $\pm$ 0.006 nm for $l$ = 34 and 726.095 $\pm$ 0.004 nm for $l$ = 35), as well as those of the known favoured transitions, are to a few ppm accounted for by a recent variational calculation of Korobov, which includes the relativistic correction.