Phosphorescence and Triplet←Singlet Absorption Spectra of Benzophenone Crystal at 4.2°K

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Abstract
The phosphorescence spectrum of benzophenone crystal at 4.2° has been measured to determine whether or not pure molecular crystals phosphoresce. The comparison of its vibrational structure with the infrared and Raman spectra leads to the conclusion that the phosphorescence really comes from benzophenone, not from some chemical impurities. Furthermore, the triplet←singlet absorption has been observed with the benzophenone crystal at 4.2°K for the purpose of comparison with the emission spectrum. The 0–0 band of the phosphorescence has been found to be lower in energy by 81 cm−1 than that of the triplet←singlet absorption. The origin of the gap between the absorption and the emission is discussed in an effort to understand the mechanism of the phosphorescence in the crystalline state. Finally, the oscillator strength of triplet←singlet absorption is theoretically evaluated to be 1.74 × 10−7, in agreement with the experimentally estimated value of 3.2 × 10−7.