Photodissociation of acetone from 266 to 312 nm: Dynamics of CH 3 + CH 3 CO channels on the S 0 and T 1 states.

The photodissociation dynamics of acetone (CH ₃ ) ₂ CO, cooled in a molecular beam, have been explored over the wavelength range 266-312 nm. Nascent CH ₃ fragments were detected by resonance-enhanced multiphoton ionization, followed by mass-selected ion imaging. For photolysis at λ = 306 nm, the image shows a sharp ring, which, when converted to a translational energy distribution, reveals a narrow Gaussian peak with a maximum at 90% of the available energy. As the photolysis energy is increased, the distribution slowly broadens and shifts to higher recoil translational energy. The fraction of available energy in translation energy decreases in favour of internal energy of the CH ₃ CO fragment. These observations are consistent with a dynamical model in which the energy of the exit channel barrier on the T ₁ surface evolves mostly into relative translational energy. Energy in excess of the barrier is partitioned statistically into all degrees of freedom. No evidence was found for any other dynamical pathway producing CH ₃ fragments, including reaction on S ₀ or S ₁ , for dissociation between 306 and 266 nm. For λ > 306 nm, a diffuse, slow recoil component to the image appears. The translational energy distribution for this component is fit well by a statistical prior distribution of energy. We attribute this component to dissociation on the S ₀ , ground state surface; to our knowledge, this is the first direct observation of this channel. The appearance of S ₀ dynamics and the disappearance of the T ₁ component are consistent with previously inferred barrier height on T ₁ for the production of CH ₃ CO + CH ₃ . The possible atmospheric implications of our findings are discussed.