Photoquenching: The dependence of the primary quantum yield of a monophotonic laser-induced photochemical process on the intensity and duration of the exciting pulse

Excited state absorption in large molecules leads to a decrease of the primary quantum yield of a photochemical or a photophysical process. Since then the quantum yield decreases with increasing light intensity this effect is called photoquenching.Kinetic analysis of the excitation in a general level scheme of a large molecule yields expressions for the quentum yield of a laser-induced photochemical process. Calculation of the quantum yield for various combinations of molecular parameters and laser pulse characteristics shows quenching of the photochemical process due to excited state absorption, at laser intensities for which bleaching effects and other nonlinear processes are negligeable. The applicability of the steady state approximation in analyzing laser-induced processes is discussed. Experiments are reported, which confirm the calculated intensity-dependent quantum yield function. Previous measurements of intensity-induced quenching can now be discussed quantitatively. Care should be taken in interpreting laser-induced photochemical yields, especially at mode locked laser intensities; correct values can only be obtained by extrapolation to zero laser intensity