THERMALLY STIMULATED LUMINESCENCE OF UV-AND X-IRRADIATED CAFFEINE

Abstract— Thermally stimulated luminescence studies of UV- and X-irradiated caffeine have been conducted in the interval77–300 K. The X-ray-induced glow curve exhibits peaks at 102, 128, 158, and 198 K, whereas the UV-induced curve is characterized by peaks with maxima at 120 and 208 K. Analysis of the X-ray-induced glow peaks revealed that the 102 and 128 K peaks obeyed first-order kinetics, while the 158 and 198 K ones followed orders of 1.3 and 1.8, respectively. Trapping parameters associated with the peaks were also elucidated. Concomitant emission spectra of each glow peak (both UV and X-ray induced) were identical and consisted of a band with well-defined maxima at 405 and 480 nm which are attributed to singlet (S₁→ S₀+ h v ) and triplet (T₁→ S₀+ h v ) transitions, respectively. At elevated temperatures the triplet transition is not observed due to quenching of the triplet state or to triplet-triplet interactions. An energy-level diagram depicting the thermally stimulated processes in irradiated caffeine is presented.