Interband transitions in molecular crystals

Abstract

Absorptions from the triplet exciton bands in benzophenone, benzil and benzoin have been observed in crystals excited by 337 nm light from a N₂ laser. In benzophenone and benzil, the crystal absorptions are much wider than those of the isolated molecules, showing that considerable mixing between the upper triplet exciton states and singlet and CT states is taking place. The phosphorescence spectrum of benzophenone crystals at high density excitation shows a (0, 0) band which is much weaker relative to the (0, 1) band than at low density excitation : this has been attributed to a high concentration of phonons in the crystal. The high excitation density also enabled the triplet-triplet exciton annihilation rates for benzophenone to be measured : _γTT (295 K) = 1 × 10^-18 m³ s^-1 and _γTT (77 K) = 6 × 10^-9 m³s^-1.