Dynamics and quantum yields of H + CHCO as a primary photolysis channel in CHCHO.

The first experimental observation of the primary photochemical channel of acetaldehyde leading to the formation of ketene (CHCO) and hydrogen (H) molecular products is reported. Acetaldehyde (CHCHO) was photolysed in a molecular beam at 305.6 nm and the resulting H product characterized using velocity-map ion (VMI) imaging. Resonance-enhanced multiphoton ionization (REMPI), via two-photon excitation to the double-well EF Σ state, was used to state-selectively ionize the H and determine angular momentum distributions for H (ν = 0) and H (ν = 1). Velocity-map ion images were obtained for H (ν = 0 and 1, J = 5), allowing the total translational energy release of the photodissociation process to be determined. Following photolysis of CHCHO in a gas cell, the CHCO co-fragment was identified, using Fourier transform infrared spectroscopy, by its characteristic infrared absorption at 2150 cm. The measured quantum yield of the CHCO + H product channel at 305.0 nm is φ = 0.0075 ± 0.0025 for both 15 Torr of neat CHCHO and a mixture with 745 Torr of N. Although small, this result has implications for the atmospheric photochemistry of carbonyls and this reaction represents a new tropospheric source of H. Quasi-classical trajectory (QCT) simulations on a zero-point energy corrected reaction-path potential are also performed. The experimental REMPI and VMI image distributions are not consistent with the QCT simulations, indicating a non reaction-path mechanism should be considered.