Investigation of Acetyl Chloride Photodissociation by Photofragment Imaging

The photofragment ion imaging technique is used to measure the velocity distributions of atomic chlorine, methyl, and carbon monoxide fragments generated in the photodissociation of acetyl chloride at 236 nm. The fragments are selectively ionized by (2 + 1) multiphoton ionization and projected onto a two-dimensional position-sensitive detector to obtain speed and angular distributions. The Cl images display anisotropic angular distributions, characteristic of a prompt, impulsive dissociation of the C-Cl bond. A fraction of the CH[sub 3]CO, produced as a primary photoproduct, subsequently decomposes to form CH[sub 3] and CO. The CH[sub 3] and CO images are isotropic, suggesting that rotation of the acetyl radical intermediate occurs prior to the secondary dissociation. The internal state distribution of CO is probed using (2 + 1) multiphoton ionization via the B[sup 1][Sigma][sup +] state near 230 nm. The rotational state distribution of CO extends to J[double prime] = 30, while no vibrational excitation is observed. The transition state structure of the CH[sub 3]CO intermediate, leading to dissociation into CH[sub 3] and CO, is computed via ab initio quantum mechanical methods. 42 refs., 7 figs., 4 tabs.