Spectroscopy refers to the study of the absorption, emission, or scattering of light by matter. When exposed to electromagnetic radiation, the molecular system under study is no longer found in a stationary state and it becomes necessary to consider one or more of its excited states.
Several methods have been developed in theoretical spectroscopy and in this part of the book, we will present some of these. Before doing so, however, this chapter will discuss some general aspects and set the stage.
First, a presentation of the molecular rovibronic states is given. Broadly speaking, the microwave, infrared, and visible parts of the spectrum relate to transitions between rotational, vibrational, and electronic levels of the system, respectively, and computational methods are specifically designed for the different cases. Vibrational transitions involves the motions of nuclei and is treated in more detail in the part of the book titled Molecular Structure and Dynamics. The methods presented in this part of the book are primarily designed to describe the electronic transitions found in UV/vis and X-ray spectroscopies.
Second, it is discussed how the responses to external fields give rise to time-dependent wave functions from which response functions can be defined. These response functions serve as meeting points between theory and experiment and computational methods are designed for their efficient evaluation, which is the main focus of the subsequent chapters of this part of the book.