This book is concerned with the spectroscopy of molecules, primarily in the gas phase. Broadly speaking, there are two types of gas source that are commonly used in laboratory spectroscopy. One is a thermal source, by which we mean that the ensemble of molecules is close to or at thermal equilibrium with the surroundings. An alternative, and non-equilibrium, source is the supersonic jet. Both are discussed below. Individual molecules can also be investigated in the condensed phase by trapping them in rigid, unreactive solids. This matrix isolation technique will also be briefly described.

Thermal sources

A simple gas cell may suffice for many spectroscopic measurements. This is a leak-tight container that retains the gas sample and allows light to enter and leave. It may be little more than a glass or fused silica container, with windows at either end and one or more valves for gas filling and evacuation. The cell can be filled on a vacuum line after first pumping it free of air (if necessary). If the sample under investigation is a stable and relatively unreactive gas at room temperature, this is a trivial matter.

If the sample is a liquid or solid with a low vapour pressure at room temperature, then the cell may need to be warmed with a heating jacket to achieve a sufficiently high vapour pressure. Residual air, together with volatile impurities that may be trapped in the condensed sample, can be removed using one or more freeze–pump–thaw cycles.