The decay curve of a spectral line with unresolved structure produced by a foil-excited atomic beam is considered in terms of a spherical tensor analysis of the density operator. In the case of L-S coupling, no external field and no cascades, the general expression is displayed and the necessary conditions for the appearance of quantum beats are discussed. By considering a series of special cases, some observable implications of the commonly assumed properties of the beam–foil excitation process are derived.