On the Nature of Stimulated Emission in Rigid Organic Solutions

Abstract

A dilute solution of an aromatic amine in a saturated hydrocarbon is frozen to a rigid state in liquid nitrogen and irradiated with ultraviolet light in the absorption band of the amine. The amine undergoes a one-electron ionization to give its colored radical cation and an electron trapped in the matrix. Subsequent infrared illumination causes a burst of blue-green luminescence called stimulated emission, which is investigated here. It is found to be a recombination luminescence composed of the fluorescence and phosphorescence bands of the amine, though the phosphorescence appears in much greater proportion than when the amine is excited by ultraviolet light. The kinetics of the two bands are analyzed and it is shown that the phosphorescing state and the fluorescing state are both populated directly by the recombination process, and through the same rate-determining step. The recombination is also observed as a bleaching of the colored cation, which closely follows the kinetics of the stimulated fluorescence. It is shown that the system possesses electron traps of several different depths, ranging from shallow, thermally ionizable levels to depths of more than 3 eV.