Phosphorescence Emissions and Nonradiative Properties of Vapors of Aromatic Carbonyl Compounds at Low Pressure

Abstract

Abstract The phosphorescence quantum yields of benzaldehyde, acetophenone, p-fluorobenzaldehyde, p-chlorobenzal-dehyde, and benzophenone vapors, measured at low pressure down to the order of 10−3 Torr as a function of excitation wavelength, are essentially constant in the region of excitation energy corresponding to the S1(n, π*) state, whereas the yields decrease very sharply as the excitation energy is raised to the value corresponding to the S2(π, π*) state. This indicates that the nonradiative pathway in the carbonyl compound varies according to the type of the singlet state to which the molecule is initially excited. When excited to the S1 and S2 states, the molecule passes to the phosphorescent triplet state and a decomposing state, respectively. Two mechanisms are suggested and examined for understanding the nonradiative properties of the carbonyl compound vapors: (I) The internal conversion S2→S1 is slow compared with the conversion from S2 to the decomposing state; (II) the vibrational energy redistribution in S1 between the vibrational mode populated through S2→S1 and the mode to be populated optically is slow compared with the conversion from the former mode to the decomposing state.