Identification of Rydberg States in the Atomic Lanthanides and actinides

Abstract

An accurate determination of an atomic ionization potential usually requires an observation of a Rydberg progression or the determination of a photoionization threshold. In the lanthanides and the actinides direct measurements of limit values using conventional techniques are prohibitively difficult. For example, a uv absorption spectrum is usually hard to interpret due to the existence of many high (low) lying states of low (high) lying valence configurations. Transitions to these high lying valence states are stronger and more numerous than transitions to Rydberg states. As a rule, Rydberg series are hopelessly obscured in the absorption spectrum. Similarly, uv photoionization spectra are complicated by transitions from many low lying metastable levels that have significant populations at the temperatures at which the atomic species have reasonable vapor pressures. Usually, photoionization occurs at wavelengths significantly longer than that necessary to ionize ground state atoms. It is nearly an impossible task to separate the photoionization spectrum originating on the atomic ground state from that on the metastable states. While long Rydberg series in 3 lanthanide elements have been identified using conventional methods [1,2], more sensitive and flexible methods are required to systematically study these heavy multielectron atoms.

Work performed under the auspices of the U.S. Energy Research and Development Administration under Contract No. W-7405-Eng-48.