Dynamics and quantum yields of H₂ + CH₂CO as a primary photolysis channel in CH₃CHO

Abstract

The first experimental observation of the primary photochemical channel of acetaldehyde leading to the formation of ketene (CH₂CO) and hydrogen (H₂) molecular products is reported. Acetaldehyde (CH₃CHO) was photolysed in a molecular beam at 305.6 nm and the resulting H₂ product characterized using velocity-map ion (VMI) imaging. Resonance-enhanced multiphoton ionization (REMPI), via two-photon excitation to the double-well EF ¹Σ state, was used to state-selectively ionize the H₂ and determine angular momentum distributions for H₂ (ν = 0) and H₂ (ν = 1). Velocity-map ion images were obtained for H₂ (ν = 0 and 1, J = 5), allowing the total translational energy release of the photodissociation process to be determined. Following photolysis of CH₃CHO in a gas cell, the CH₂CO co-fragment was identified, using Fourier transform infrared spectroscopy, by its characteristic infrared absorption at 2150 cm^-1. The measured quantum yield of the CH₂CO + H₂ product channel at 305.0 nm is φ = 0.0075 ± 0.0025 for both 15 Torr of neat CH₃CHO and a mixture with 745 Torr of N₂. Although small, this result has implications for the atmospheric photochemistry of carbonyls and this reaction represents a new tropospheric source of H₂. 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.