Steady-state atomic fluorescence radiance expressions for continuum excitation

Several general expressions for the atomic fluorescence radiance excited by a laser source are presented for both two-level and three-level systems. The laser has been approximated to a spectral continuum since its bandwidth is assumed to be much larger than the absorption profile. Two types of three-level systems are considered: (1) systems where two excited states are close together, both coupled radiatively with the ground state (Na, K, Cs, etc.); and (2) systems where one level is close to the ground state, both being radiatively connected with a third excited state (T1, In, Ga, etc.). Expressions are given for both linear and saturating conditions, and the use of such expressions for absolute species measurement is discussed. In our treatment, we have made the assumption that the rate equations approach is valid.