The structure of spin-orbital transitions in N, O, S azoaromatic molecules with allowance for high-lying singlet and triplet states populated with a pump in LCAO MO CSFCIINDO/S models

This paper investigates the structure of spin-orbital transitions and processes of inner conversion in laser-active organic compounds with allowance for the interaction of both low- and high-lying singlet and triplet states, efficiently populated by a high-power UV lamp or laser pump. Models of the LCAO MO CSF CI INDO/S quantum-chemical method are applied to calculate a broad spectrum of excited states of different spin and orbital nature for a 2-(n-aminophenyl)-5-phenyl-1,3,4-oxadiazole (n-NH2-PDP) molecule, which is capable of fluorescing with a high quantum yield, gamma approximate to 0.8-0.9, and lasing within the UV spectral range in various solvents. We provide a mathematical background and present the basic formulas for the calculation of matrix elements corresponding to spin-orbital coupling between pi pi*- and pi pi*-type states for polyatomic systems. The Robinson-Frosch formula modified for problems of laser physics is employed to calculate the constants of optical and nonoptical deactivation of electron-vibrational excitation energy and cross sections of stimulated emission and secondary reabsorption in spectra of pump-induced reabsorption through S-1* --> S-n* and T-1 --> T-n transitions. The calculated rate constants of radiative decay and intercombination conversion and fluorescence quantum yields of molecules in vapors and solutions are used to determine the characteristic limiting duration of the pumping pulse that allows the lasing of an n-NH2-PDP molecule. The results of these calculations are compared with characteristics measured for different experimental conditions.