Laser spectroscopy of atomic inner shells

New experiments are reported in which the inner valence shell spectrum of Hg I has been excited by two-colour laser spectroscopy. The excitation scheme involved shakedown from an extended intermediate state, which favours transitions to states outside the centrifugal barrier, thus enhancing the nf series as compared with np, in complete contrast to single-photon excitation of the same states from the ground state. New multiplet splittings for the nf states have been observed, and the interpretation is consistent with an extended alkali model previously proposed. The 5d⁹nf resonances, although embedded in the photoionisation continuum, are found to be remarkably narrow, implying lifetimes in the tens of picoseconds range.