Transient Decay Following Pulse Excitation of Diffuse Scattering Samples

Analytical solutions are examined in order to predict how the measured decrease in diffuse reflectance observed in the flash photolysis of powdered samples is related to the concentration of a transient absorbing species. It is shown on the basis of these calculations that when the transient reflectance changes by less than 10 per cent, the change in reflectance is a linear function of transient concentration. Experimental evidence supporting this conclusion is presented from studies of the triplet state of coronene chemisorbed on γ alumina where the concentration of the excited state can also be monitored by measuring luminescence intensity, which is shown to have a virtually identical dependence on the excited state concentration as the transient change in reflectance due to triplet-triplet absorption. Since the luminescence intensity is a linear function of the transient concentration, it follows that this is also true for the transient reduction in diffuse reflectance. Non-exponential decay of the transient reflectance of electronically excited molecules as a function of time may be observed because of heterogeneous adsorption, i.e., where the adsorbed molecules occupy a variety of different surface environments which give rise to different decay probabilities, or as a result of energy transfer due to impurity quenching.