Abstract The features of fluorescence and transmission signals in optical pumping of dilute gases with non‐coherent light sources are discussed in terms of a generalized signal function. Using the spherical tensor operator formalism kinetic equations for the atomic state multipoles as well as rate equations for spontaneous emission and absorption are derived. In these rate equations the kinetics of the atomic system expressed by the, generally, time dependent state multipoles of the atomic distribution in the ground or excited state is weighted by the optical signal parameters to give a signal function, which is explicitly adapted to the experimental conditions of observation.
We present experimental data showing destruction of degenerate four-wave-mixing signals by collisions at extremely low pressures of added buffer gases (\ensuremath{\sim}100 mtorr). These experiments are performed on the $3s^{2}S_{\frac{1}{2}}(F=2)\ensuremath{-}3p^{2}P_{\frac{3}{2}}(F=3)$ transition of the
D2 line in atomic sodium for which, in the absence of buffer gases, the lower state is not optically pumped to the
3s2S21(F=1) ground state. However, using a simple rate equation model we are able to qualitatively explain the observed behavior by showing that state-changing collisions have the effect of generating a collision-induced optical pumping. This transfers the
F=2 ground-state population to the
F=1 ground state, thus reducing the density of atoms available for generating a signal.