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Acetaldehyde cations (CH 3 CHO + ) were prepared using single-photon vacuum ultraviolet ionization of CH 3 CHO 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: CH 3 CHO + → C 2 H 3 O + + H (I), CH 3 CHO + → HCO + + CH 3 (II), CH 3 CHO + → CH 3 + + HCO (III), CH 3 CHO + → CH 4 + + 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 B̃ 2 A′ and C̃ 2 A′ states of CH 3 CHO + . 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 CH 3 CO + . Additional components with E INT greater than the CH 3 CO + 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 CH 3 CHO + . 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 CH 3 CO + formed via channel (I), which are attenuated by CH 3 CHO + 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 . 

The photodissociation dynamics of acetone has been investigated using velocity-map ion imaging and photofragment excitation (PHOFEX) spectroscopy across a range of wavelengths spanning the first absorption band (236–308 nm). The radical products of the Norrish Type I dissociation, methyl and acetyl, as well as the molecular product ketene have been detected by single-photon VUV ionization at 118 nm. Ketene appears to be formed with non-negligible yield at all wavelengths, with a maximum value of Φ ≈ 0.3 at 280 nm. The modest translational energy release is inconsistent with dissociation over high barriers on the S 0 surface, and ketene formation is tentatively assigned to a roaming pathway involving frustrated dissociation to the radical products. Fast-moving radical products are detected at λ ≤ 305 nm with total translational energy distributions that extend to the energetic limit, consistent with dissociation occurring near-exclusively on the T 1 surface following intersystem crossing. At energies below the T 1 barrier a statistical component indicative of S 0 dissociation is observed, although dissociation via the S 1 /S 0 conical intersection is absent at shorter wavelengths, in contrast to acetaldehyde. The methyl radical yield is enhanced over that of acetyl in PHOFEX spectra at λ ≤ 260 nm due to the onset of secondary dissociation of internally excited acetyl radicals. Time-resolved ion imaging experiments using picosecond duration pulses at 266 nm find an appearance time constant of τ = 1490 ± 140 ps for CH 3 radicals formed on T 1 . The associated rate is representative of S 1 → T 1 intersystem crossing. At 284 nm, CH 3 is formed on T 1 with two distinct timescales: a fast <10 ns component is accompanied by a slower component with τ = 42 ± 7 ns. A two-step mechanism involving fast internal conversion, followed by slower intersystem crossing (S 1 → S 0 → T 1 ) is proposed to explain the slow component.