[en] A recent energy pathways model for the de-excitation of excited helium gas [Payne et al., J. Chem. Phys. 63, 1422 (1975)] suggested that a key step He(2¹P) + He(1¹S) → He(2¹S) + He(1¹S) had an unusually large cross section. This led to the need to measure the population of He(2¹S) as a function of time following charged-particle excitation of He(2¹P). The principle of resonance ionization spectroscopy is explained, and the ratio of He(2¹S) to direct ionization is shown as a function of laser energy per pulse. Conditions for resonance ionization of atoms in their ground states are described; studies were made of two-photon ionization of Cs--Ar mixtures with the broadened Cs(7p) as an intermediate state. The topic of photodissociation of molecules: time-resolved sources of free atoms is discussed; all of the CsI molecules in the central portion of a laser beam could be dissociated to neutral Cs and I atoms in their ground states. The method is well suited to rather direct measurements of the diffusion of a small number of alkali atoms in various gases and to a remarkably simple determination of the rate of reaction of free atoms with various other atoms or molecules. One-atom detection of Cs in a background of 10¹⁹ atoms of Ar was demonstrated with a proportional counter. 12 figs