Electronic and vibrational relaxation in biacetyl vapor

The fluorescence decay of biacetyl vapor has been directly measured as a function of excitation wavelength with medium resolution (10–30 Å) using a tunable dye laser. At low pressures the decay appears to be reasonably exponential with no evidence for quantum beats or other reversible behavior which might have been anticipated due to the low density of triplet vibronic levels to which energy conserving transitions apparently take place. The low pressure lifetime varies smoothly from $\text{∞ 14\hspace{0.17em}}\mathrm{nsec}$ for excitation near 4450 Å to $\text{∞ 6\hspace{0.17em}}\mathrm{nsec}$ for excitation at 3720 Å. Molecular collisions with ground state biacetyl and with added Ar vibrationally relax the excited molecules but do not quench the electronic excitation. Biacetyl removes $\text{∞ 700\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ of vibrational energy per collision while Ar removes $\text{∞ 90\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ .