Abstract

Earth-alkali atoms, particularly barium, are often used to investigate electron correlation effects in two electron systems. The reason for this is that the doubly excited states of barium are accessible for laser excitation, whereas for the more fundamental system of helium this is still be impossible. The complicating effects of the rather large 2⁺-core of barium, involving interactions with 54 core electrons, may be neglected when large angular momentum states of both valence electrons are considered. Double-Rydberg states have attracted interest¹ and have been analyzed, not without complications from the perspective of electron correlation. Doubly excited circular states with non-core penetrating orbits may also show (anti-) correlation effects on the electron positions in the atom. Recently we populated a 4f7h state in barium, which is a nearly double circular state (l₁ = n₁ − 1, l₂ = n₂ − 2). In a two-step pulsed laser experiment the 4fnh states are excited in an isolated-core excitation scheme (ICE) from 5dnh intermediate states using the frequency-doubled output (230 nm) of a pulsed dye laser. The low-lying autoionizing 5dnh states are weakly interacting with 5dnf states and can therefore be excited from 5d²-metastable states using the visible output of a second dye laser. Due to strong dipole coupling between the 6d and 4f ion levels the 4fnh states mainly autoionize in the 6deZ-continuum. Special attention is given to 4f_5/27h states that lie below the 6d_5/2 ionization limit but still above the 6d_3/2 ionization limit. We observed strong interaction of a broad 4f_5/27h resonance (Γ = 60 cm^-1) with a narrow autoionizing 6dnl Rydberg series converging to the 6d_5/2 ionization limit. Calculations based on an ab initio R-matrix formalism indicate that we are mainly dealing with l = 6. The agreement between the experimental and theoretical excitation spectrum shows that even at these high-lying energies with many contributing continuum channels electronic structure calculations are still feasible. Also the proto-typical interference pattern involving a q-reversal in the 6dni series on the 4f7h resonance is reproduced. The 6dni states appear to be extremely narrow and are currently under investigation with a narrower bandwidth laser. These states may serve as intermediate states for further excitation to high-l double Rydberg states.

© 1994 IEEE