State-Selected Photodissociation of Acetaldehyde Molecular Ion:  Hydrogen Scrambling and the Product Branching Ratio

Acetaldehyde molecular ions were produced in an ICR cell using (2 + 1) resonance-enhanced multiphoton ionization of a pulsed molecular beam of acetaldehyde via a two-photon resonant 3s ← n Rydberg transition in the wavelength range 364−354 nm. Mass-selected REMPI spectra of CH 3 CHO are presented. The power dependencies of ion intensities were examined to help establish the mechanisms of REMPI dissociation processes. The product branching ratios from REMPI dissociations of CH 3 CHO and CH 3 CDO were determined. A multireference configuration interaction method was employed to obtain structural and energetic information about the X̃ 2 A‘, Ã 2 A‘‘, and B̃ 2 A‘ states of acetaldehyde molecular ion. Comparisons of theory with the experiments lead us to conclude that acetaldehyde molecular ions are excited to the B̃ 2 A‘ state and undergo both α and β cleavage processes. The α C−C cleavage yields the formyl ion without hydrogen scrambling. The product branching ratio of [HCO + ]/[CH 3 CO + ] remains constant at 2.1 ± 0.2 over the entire wavelength range. CH 3 CDO yields the branching ratio of α C−D to β C−H cleavage, [CH 3 CO + ]/[CH 2 DCO + ] = 4.7. An aldehydic deuterium substitution reduces the branching ratio of α C−H(D) to α C−C cleavage.