ABSTRACT
This chapter reviews the underlying physics required for studying molecular dynamics using time-resolved spectroscopy. This material is treated in a time-independent manner, as it is the quantum properties of the molecules themselves that are relevant at this point, not the interaction with a time-dependent light field. While the hydrogen atom is perhaps within the domain of standard, time-independent undergraduate quantum mechanics, it is often one of the last subjects covered. Spin–orbit coupling generally leads to slow, intersystem crossing when exciting a molecule from an initial state with a well-defined total spin to a final state with the same total spin but which is not an eigenstate of the total Hamiltonian. The chapter aims to develop a general picture of electronic structure applicable to molecules. It discusses the Born—Oppenheimer approximation (BO) in the time-independent picture and addresses cases where the approximation breaks down. The BO approximation allows one to treat the electrons and nuclei separately, greatly simplifying the problem.
