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 B[combining tilde]2A' and C[combining tilde]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 (ET), consistent with dissociation over a small barrier and production of highly internally excited CH3CO+. Additional components with EINT 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 ET 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 ET 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-ET and anisotropic components due to excited state dynamics at higher ET.