Chapter 3 - Structure and Dynamics of High Rydberg States Studied by High-Resolution Spectroscopy and Multichannel Quantum Defect Theory

Abstract

Rydberg states are a common feature of atoms and molecules. These electronically excited states form series that converge to each quantum state of a cation. The properties of high atomic and molecular Rydberg states and their dependence on the principal quantum number n are presented together with the spectroscopic methods used to study these states at high resolution. Such studies require multiphoton excitation schemes or single-photon excitation with vacuum ultraviolet (VUV) radiation. Applications of high-resolution spectroscopy to the study of the hyperfine structures of Rydberg states of exemplary atomic and molecular systems are described, and the evolution of the hyperfine pattern and the angular momentum coupling hierarchy with increasing n values is discussed. It also discusses the development of narrow-bandwidth VUV laser systems. Narrow-bandwidth laser systems are used in pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy and Rydberg-state-resolved threshold-ionization spectroscopy experiments, which yield precise values of ionization energies and rovibrational energy structures of cations. The main limitation in resolution originates from the Doppler effect and can be overcome by VUV-millimeter wave double-resonance experiments. In such experiments, even the hyperfine structure of Rydberg states can be resolved as is demonstrated for the rare gases krypton and xenon and for ortho-hydrogen. Multichannel quantum defect theory is the most appropriate and reliable tool for the analysis of the spectra of high atomic and molecular Rydberg states.