Introduction

Many important laser systems operate using molecular species. The laser transition occurs between energy levels of the molecule, which may be in the gaseous, liquid, or solid state. To understand more about molecular lasers it is important to consider the additional complexity of the energy-level structure of a molecule compared with that of an atom. In this chapter we will explain the three different kinds of energy level – electronic, vibrational, and rotational – which occur for molecular species, and then go on to explain how such a complex energy-level structure allows the possibility of laser oscillation over a very broad wavelength range.

The energy levels of molecules

Electronic energy states

In an atom, the orbiting electrons move in the spherically symmetric potential of the nucleus, and the various energy levels of the system correspond, in a simple sense, to different orbital arrangements of these electrons. For example, an excited electron frequently moves into an orbit that takes it further from the nucleus. In a molecule the electrons travel in orbits that surround all the nuclei of the molecule, although quite often there will be considerable localization of some electrons near a particular nucleus. The electronic energy states of the molecule result from different arrangements of the orbiting electrons about the nuclei. Electrons that move from one electronic energy level of a molecule to another experience changes in energy that are broadly comparable to such jumps in atoms.