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; Hückel molecular orbitals; Frost–Musulin diagram; Walsh diagram; Nonradiative transitions; Correlation diagrams