In this workshop lecture we will abandon the first-year chemistry view of a spectroscopic transition in terms of a jump between two energy levels and will instead look at the dynamics of a spectroscopic transition. To achieve that, the concept of an eigenstate will be reviewed and the idea of a superposition state will be introduced. A set of equations that governs the interaction of light and matter in a spectroscopic transition, the Bloch equations, will be derived in a simplified fashion. The Bloch equations will then be used to gain qualitative insights into the behavior of atoms and molecules in the presence and absence of electromagnetic radiation.
Microwave rotational spectroscopy will be used as an example in this introduction, but the concepts can be generalized and apply also to other spectroscopic techniques.