Fundamentals of Electronic Spectroscopy

Abstract

The basic principles of electronic spectroscopy of atoms and molecules in the gas phase are presented. In the first part, the elementary concepts necessary to describe the electronic structure of atoms, diatomic molecules, and polyatomic molecules are introduced in a systematic manner, with an effort to classify the different interactions (electrostatic, spin–orbit, hyperfine) and types of motions (electronic, vibrational, rotational), which determine the energy level structures. In the second part, electronic transitions are discussed, with their spin-rovibrational structures. Examples ranging from the simple band structure of electronic transitions of homonuclear diatomic molecules to the highly complex band structure of polyatomic molecules subject to strong vibronic interactions are used to illustrate the richness of electronic spectra. Keywords: electronic spectra; Born–Oppenheimer approximation; selection rules and group theory; Jahn–Teller effect; Renner–Teller effect; Hund's angular momentum coupling cases; spin–orbit coupling, LS and jj coupling; autoionization; predissociation; intersystem crossing; internal conversion; Franck–Condon factors; exciton model; Herzberg–Teller intensity borrowing mechanism; Rydberg states; valence states; ion-pair states; diatomic molecules; atomic spectroscopy; Zeeman and Stark effects; hyperfine coupling; atomic and molecular orbitals; Huckel molecular orbitals; Frost–Musulin diagram; Walsh diagram; nonradiative transitions; correlation diagrams