We have developed a method to measure triplet optical densities ODr(λ) when considerable photodecomposition is present. The method is demonstrated on Coumarin 120. By focusing an UV cw laser beam on a sample with the aid of a lens, high triplet optical densities ODT(λ) can be recorded. However, the excitation of organic compounds with light causes varying degrees of photodecomposition. In order to take photodecomposition into account, one records triplet optical-density values as well as the accumulation of absorbing photoproducts as a function of time. Turning off the cw laser excitation, one records the accumulation of photoproducts only. Separating the two processes, one can determine how the triplet optical density ODT declines as a function of time t. The obtained curve can be expressed with an equation that appears to decline exponentially with time. This allows one to extrapolate back to t = 0 and recover ODT when there was no photodecomposition. The extrapolated ODT values are used to obtain triplet-extinction coefficients ∊T(λ) by McClure’s method of Coumarin 120.
Theodore G. Paviopoulos, "Measurement of triplet optical densities of organic compounds in the presence of photodecomposition," Appl. Opt. 31, 1261-1266 (1992)