UV photofragmentation dynamics of acetaldehyde cations prepared by single-photon VUV ionization

Abstract

Acetaldehyde cations (CH₃CHO⁺) were prepared using single-photon vacuum ultraviolet ionization of CH₃CHO in a molecular beam and the fragmentation dynamics explored over the photolysis wavelength range 390–210 nm using velocity-map ion imaging and photofragment yield (PHOFY) spectroscopy. Four fragmentation channels are characterized: CH₃CHO⁺ → C₂H₃O⁺ + H (I), CH₃CHO⁺ → HCO⁺ + CH₃ (II), CH₃CHO⁺ → CH₃⁺ + HCO (III), CH₃CHO⁺ → CH₄⁺ + CO (IV). Channels (I), (II), and (IV) are observed across the full photolysis wavelength range while channel (III) is observed only at λ < 317 nm. Maximum fragment ion yields are obtained at ∼250 nm. Ion images were recorded over the range 316–228 nm, which corresponds to initial excitation to the ²A′ and ²A′ states of CH₃CHO⁺. The speed and angular distributions are distinctly different for each detected ion and show evidence of both statistical and dynamical fragmentation pathways. At longer wavelengths, fragmentation via channel (I) leads to modest translational energies (E_T), consistent with dissociation over a small barrier and production of highly internally excited CH₃CO⁺. Additional components with E_INT greater than the CH₃CO⁺ secondary dissociation threshold appear at shorter wavelengths and are assigned to fragmentation products of vinyl alcohol cation or oxirane cation formed by isomerization of energized CH₃CHO⁺. The E_T distribution observed for channel (III) products peaks at zero but is notably colder than that predicted by phase space theory, particularly at longer photolysis wavelengths. The colder-than-statistical E_T distributions are attributed to contributions from secondary fragmentation of energized CH₃CO⁺ formed via channel (I), which are attenuated by CH₃CHO⁺ isomerization at shorter wavelengths. Fragmentation via channels (II) and (IV) results in qualitatively similar outcomes, with evidence of isotropic statistical components at low-E_T and anisotropic components due to excited state dynamics at higher E_T.