Photodissociation of spatially aligned acetaldehyde cations

Photofragment translational energy and angular distributions are reported for the photodissociation of acetaldehyde cations in the wavelength range 354-363 nm obtained using the DC slice ion imaging technique. Vibrationally selected parent ions were produced by 2+1 resonance-enhanced multiphoton ionization (REMPI) via the 3s ← n Rydberg transition, with photodissociation resulting from absorption of a fourth additional photon. Three product channels were observed: HCO ⁺, CH ₃CO ⁺, and CH ₄ ⁺. The angular distributions reveal that all product channels have a predominantly parallel recoil anisotropy although the lower β ₂ parameter of CH ₃CO ⁺ indicates the concomitant presence of a perpendicular component. Furthermore, the distinct angular distribution of the CH ₃CO ⁺ fragments shows a large value of the higher order Legendre polynomial term, providing evidence that acetaldehyde cations are spatially aligned during the ionization process.