First laser excitation of theHe411S-21Presonance line at 58 nm

We have observed the 1 ${}_{}{}^1{}_{}{}^{}$S-2 ${}_{}{}^1{}_{}{}^{}$P resonance line of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at 58.4 nm (linewidth 0.3 ${\mathrm{cm}}^{\mathrm{-}1}$) in a laser-atomic-beam experiment. The coherent radiation is produced by fifth-harmonic generation of UV light from a frequency-doubled, pulsed dye laser at 584 nm in a pulsed expansion of acetylene gas. The frequency of the fundamental radiation is calibrated against the iodine absorption spectrum, resulting in a transition frequency of 171 134.89(3) ${\mathrm{cm}}^{\mathrm{-}1}$. From this value a Lamb shift for the ground state of +1.38(3) ${\mathrm{cm}}^{\mathrm{-}1}$ is extracted, in good agreement with the theoretical value of +1.3745 ${\mathrm{cm}}^{\mathrm{-}1}$