Fluorescence Excitation and Photodecomposition of the First Excited Singlet Cyclobutanone (1A2): A Study of Predissociation of and Collisional Energy Transfer from the Vibronically Selected Species

Fluorescence emission quantum yields (Φ_F) from the first excited singlet state of cyclobutanone (CB) in the gas phase have been measured in the exciting wavelength (λ_ex) range of 3050–3350 Å at $\text{∼0.5 Å}$ resolution. The value of Φ_F at 5 torr is $\text{∼0.002}$ above ${\lambda }_{\mathrm{ex}}\text{=3190 Å}$ , but it suffers a sharp decline below this wavelength because of the onset of molecular predissociation, e.g., ${\Phi }_F{\text{<4×10}}^{\text{−6}}$ for ${\lambda }_{\mathrm{ex}}\text{<3100 Å}$ . Fluorescence stabilization phenomena are studied by addition of C₃H₈ up to 2200 torr. A simple kinetic treatment of the data provides (i) the estimated singlet lifetime value (τ_s) of 10⁻⁹−10⁻¹² sec below λ_ex=3140 Å; and (ii) the collision efficiency value of $\text{∼0.5}$ for the upward transition as well as for the downward transition (of vibrational energy transfer). The primary photochemical transformation mechanism for the ${\mathrm{CB(S}}_1^{*})$ is discussed in light of the new estimated lifetime information.