Fluorescence, Phosphorescence, and Triplet Formation in Biacetyl at Low Pressures

The $S_1\mathrm{\hspace{0.17em}\to \hspace{0.17em}T}$ intersystem crossing in biacetyl vapor has been examined by direct observation of emission from both the initial and final states. When biacetyl is excited by absorption of 4358‐Å radiation, both the singlet‐ and the triplet‐emission yields are independent of pressure in the range 40–0.1 torr. Since isolated molecule conditions are established for singlet relaxation in the lower limit of this range, triplet formation must occur in biacetyl in the absence of collisional perturbations involving the excited singlet state. Triplet emission continues to be observable at pressures descending to 0.03 torr, but the emission yield decreases due to triplet deactivation at the walls. The results are discussed within the framework of recent theories of radiationless transitions.