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.

Saturation in laser excited atomic fluorescence spectrometry: experimental verification.

The use of lasers to saturate an energy level of an atomic or molecular species is becoming almost commonplace. In this paper measurements of fluorescence signal vs laser power are reported. When a log-log plot of these data departs from the slope of unity saturation is invoked, but when the curve does not roll over B(F max) (fluorescence radiance) is lost. The shapes of the experimental log B(F) vs log E(nu) curves are investigated and also their absolute values with respect to theory.

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.

Steady state molecular luminescence radiance expressions assuming narrowband excitation.

Steady state luminescence radiance expressions for two-level and three-level molecular systems are derived in terms of transition probabilities, source irradiance, emission pathlength, absorption band halfwidth, total molecule concentration, frequencies of the transitions, statistical weights after transitions, and nonradiational activation and deactivation rate constants. Limiting radiance expressions are given for both low (conventional sources) and high (lasers) intensity sources. Saturation irradiances for the two-level and three-level molecular systems are also given. Conclusions concerning the expressions are given. One conclusion, which has special importance, involves the production of similar phosphorescence intensities whether the first triplet electronic state is excited directly or is excited via the first excited singlet with intersystem crossing to the first triplet state.