First observation of a photon-induced triply excited state in atomic lithium

Using a dual laser plasma technique we have measured for the first time the photoabsorption spectrum of atomic lithium at energies corresponding to the simultaneous excitation of both 1s electrons. We have observed the main triply excited 1${\mathit{s}}^2$2s${(}^2$S)→2${\mathit{s}}^2$p${(}^2$P) transition with Fano parameters ${\mathit{E}}_0$=142.32±0.05 eV Γ=0.20±0.04 eV, and q=-2.2±0.6. Our experiment represents the first step in photoionization studies of this fundamental atomic system in which the motion of the three electrons in the field of the nucleus is so highly correlated.