The measurement of lifetimes in atoms, molecules and ions

The field of the direct measurement of atomic and molecular lifetimes has been characterized in the past few years by the development of several new and powerful techniques and by an improving ability to control and limit the measurement inaccuracies of older techniques. The establishment of a wide range of accurate values for atomic and molecular lifetimes is now possible. This review deals with the measurement of lifetimes of atoms and molecules in the gas phase, in the range 10-10-10-5 s, and concentrates on the more accurate and promising techniques, which are grouped together according to whether their most prominent feature is the means of excitation or the interference between excited levels or the coincident detection of two decay events. Possible sources of systematic error are discussed at length and the theoretical background appropriate to the measurements is given when necessary. The techniques and their areas of applicability are compared and the likely future development of them is discussed.