Triplet-Triplet Absorption in Microcrystalline Benzil Detected by Diffuse Reflectance Laser Flash Photolysis

Abstract

Following pulsed excitation at 354 nm, a transient absorption, maximizing at 510 nm, is readily observed for microcrystalline benzil, by diffuse reflectance laser flash photolysis. The transient absorption reduces the level of diffusely reflected analyzing light by 80%, and its decay appears to be a mixture of first- and second-order processes. Decay constants obtained from the slope of the first-order plot vary over a factor of about 2.5, with an initial slope = 0.18 × 10⁵ (±0.01 × 10^s) s⁻¹, and a final slope = 0.66 × 10⁴ (±0.04 × 10⁴) s⁻¹. Laser excitation also induces phosphorescence in microcrystalline benzil, which has a similar kinetic decay. It is confirmed that in diffuse reflectance mode the percentage transient absorption is directly proportional to the excited state concentration.