Nonsteady-state molecular luminescence behavior under narrowband laser excitation

The time behavior of a two- and a three-level molecular system under narrowband pulsed-laser excitation is discussed, and relations are presented for the temporal variation of the concentrations of the levels involved. It is shown that if saturation is approached, the response time of the molecular system is strictly related to the pumping time, which is given by the reciprocal of the product of the laser spectral density and the Einstein coefficient of stimulated absorption. Typical calculations of the fractional number densities of the levels are presented as a function of the value of the laser spectral density required to achieve saturation for a few typical molecules such as benzophenone, fluorene, and rhodamine 6G.