1ST LASER EXCITATION OF THE HE-4 1(1)S-2(1)P RESONANCE LINE AT 58-NM

We have observed the 1 1S-2 1P resonance line of He-4 at 58.4 nm (linewidth 0.3 cm-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) cm-1. From this value a Lamb shift for the ground state of +1.38(3) cm-1 is extracted, in good agreement with the theoretical value of +1.3745 cm-1.