Small molecule behavior and the 3Bg state of biacetyl

Abstract
Below excitation energies of 22 500 cm−1 biacetyl behaves as a small molecule as evidenced by the pressure dependence of its phosphorescence and the structure of the excitation spectrum. The sudden transition to large (statistical) molecule behavior at this energy can be explained by the presence of a 3Bg state for which there is also theoretical evidence.
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