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Physical Review Letters

Negative-ion formation in Rydberg atom interactions (n=7–40)

M. Ciocca, M. Allegrini, E. Arimondo, Charles E. Burkhardt, W. P. Garver, Jacob J. Leventhal

Published Feb 17, 1986• 19 citations
Publisher

Abstract

We describe the first experiments in which negative ions formed in interactions involving highly excited (laser-selected) atoms are identified by mass. We therefore report the first observations of negative ions produced by Rydberg--ground-state interactions (at low n) and by Rydberg-Rydberg interactions (at high n).
DOI
10.1103/physrevlett.56.704

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Excited states of atoms and molecules exhibit a rich array of diverse phenomena. This dissertation examines two exotic states of atoms at such excited levels: Rydberg molecules and atomic negative ions. Rydberg molecules are formed by a Rydberg atom and one or more ground state atoms, and can be highly polar due to their unusual electronic wave functions and enormous bond lengths. This dissertation expands the theory of these molecules by studying the formation and structure of polyatomic molecules, multichannel Rydberg molecules formed from divalent atoms, and spin effects and relativistic interactions. It also details intermolecular forces between Rydberg molecules, their manipulation via external fields, and their dependence on the intricacies of electron-atom scattering. This electron-atom interaction is also the main component of the latter portion of this thesis, which studies doubly excited states of alkali negative ions in very polarizable and nearly degenerate atomic states. Photodetachment of these states reveals electron correlation and long-range forces stemming from their high excitation.
The formation of Li− by dissociative attachment in e+Li2 collisions is characterized by a large cross section for electron attachment to highly vibrationally excited Li2 molecules. However, the electronic structure of the Li−2 system dictates that low energy electrons (≲0.2 eV) are needed to minimize loss processes. An alternate production route for Li− is possible via direct ion‐pair formation through collisions of highly excited (Rydberg) Li atoms. The mechanisms for ion‐pair formation will be discussed in terms of the high‐lying molecular Rydberg states of Li2.
Atoms in highly excited Rydberg states possess physical characteristics quite unlike those associated with atoms in the ground or low-lying excited states. In particular, they are physically very large and are only very weakly bound. In consequence, collisions can lead to a wide variety of reaction processes many of which are unique to Rydberg species and have very large collision cross sections. In collisions with neutral targets, Rydberg atoms behave not as an atom but rather as a pair of well-separated independent scatterers, namely the core ion and the excited Rydberg electron. In the present article we discuss many of the different reactions that can occur when Rydberg atoms collide with neutral targets, focusing principally on reactions that are dominated by (binary) Rydberg electron-target interactions and include collisions with molecules that attach free low-energy electrons and with polar targets. In certain situations, however, interactions involving the Rydberg core ion are important. This is illustrated using as an example the destruction of ultralong-range Rydberg molecules excited in a cold dense gas.