High-precision structural studies of the antiprotonic helium atom p-|Am4He+by observing laser resonances with Δv=Δ(n-l-1)=2

Very weak laser resonances of unfavored transitions (n,l)→(n+1,l-1) have been observed in metastable p-|${\mathrm{Am}}^4$ ${\mathrm{He}}^{+}$ atoms by using a pulsed antiproton beam. The observed wavelengths (713.578±0.006 nm for l=34 and 726.095±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 [Phys. Rev. A 54, R1749 (1996); and (unpublished)] which includes the relativistic correction to the motion of the electron.