The influence of the fluence, intensity, and time on rate coeffs. and product yields in collisionless unimol. reactions induced by monochromatic IR is discussed in terms of the logarithmic reactant fluence plots. Model calcns. for several archetypes of such plots are presented, based on solns. of the Pauli master equation and solns. of the quantum mech. equations of motion for spectra involving many states at each level of excitation. Exptl. examples re-evaluated from the literature and new measurements on the laser induced decompn. of CF2HCl are reported which illustrate the various theor. possibilities. The role of the reducibility of the rate coeff. matrix is discussed in connection with turnovers and with the strong influence of initial temp. that is found in the laser induced decompn. of CF2HCl.