Giant-Dipole-Resonance Absorption in Atomic Thorium by a Novel Two-Laser Technique

A novel two-laser technique for photoabsorption studies of refractory and corrosive elements is described and applied to observe for the first time the absorption spectrum of atomic thorium. Results on the giant $5d$ dipole resonance in the extreme uv are found to be in good qualitative agreement with recent theoretical predictions. The spectrum is very similar to that reported for the metal, indicating that the $5f$ wave function is largely unaffected by its solid environment.