ABSTRACT
This chapter reviews the properties of light and its interaction with molecules that are relevant for spectroscopy. Light is an electromagnetic wave that consists of oscillating electric and magnetic fields and propagates in a certain direction. Spectroscopy is based on the interaction of molecules with the electric and/or the magnetic component of the electromagnetic wave. The electronic, vibronic, and rotational energy levels of a molecule can be depicted schematically in an energy diagram, the Jablonski diagram. In contrast, the thermal energy is much smaller than the energy differences between vibronic or electronic states: transitions between vibronic or electronic states cannot be induced by thermal energy at ambient temperatures. Generally, transitions between two states of a molecule can be induced by perturbations, by external electric or magnetic fields. The transition dipole moment is a measure of the capacity of the incident light to distort the initial state towards the final state.
