Measurements of the phosphorescences of organic compounds at low pressures are expected to offer important information concerning the photophysical nature of molecules. However, only few such measurements have been reported, possibly because optical excitations are not efficient for the formation of triplet states at low pressures. The electron-impact method, on the other hand, has been expected to have larger excitation efficiencies, even at low pressures. However, the method had not been successfully applied to phosphorescence measurements in the vapor phase until we observed phosphorescences of benzaldehyde, acetophenone, and pyrazine by electron impact. Phosphorescences of these molecules were only observable at pressures higher than 0.3 Pa. (These pressures are still "very low" from the view point of excited-state dynamics of organic molecules in the gas phase. Luminescences of organic molecules have seldom been measured at pressures lower than a few Pa.) Since electron-impact experiments are usually conducted at pressures around 10 mPa or lower, care must be taken when this method is applied to studies of phosphorescences at higher pressures.

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