The energy dependence of CO(v,J) produced from HCO via the transition state, roaming, and triple fragmentation channels.

The dynamics of CO production from photolysis of HCO have been explored over a 8000 cm energy range (345 nm-266 nm). Two-dimensional ion imaging, which simultaneously measures the speed and angular momentum distribution of a photofragment, was used to characterise the distribution of rotational and translational energy and to quantify the branching fraction of roaming, transition state (TS), and triple fragmentation (3F) pathways. The rotational distribution for the TS channel broadens significantly with increasing energy, while the distribution is relatively constant for the roaming channel.

Experimental and theoretical investigation of triple fragmentation in the photodissociation dynamics of H2CO.

The photodissociation dynamics of H2CO molecules at energies bracketing the triple fragmentation threshold were investigated using velocity map ion imaging of the H-atom fragments. An algorithm was developed to model the experimental results as a two-step process: initially barrierless C-H bond fission on the S0 potential energy surface to form H + HCO, followed by secondary fragmentation of those HCO radicals with sufficient internal energy to overcome the small exit channel barrier on the HCO surface to form H + CO. Our model treats the first step using phase space theory (PST) and the second using a combined PST-impulsive model, with a tunneling correction.

A phase space theory for roaming reactions.

We describe a new, simple theory for predicting the branching fraction of products in roaming reactions, compared to the analogous barrierless bond dissociation products. The theory uses a phase space theory (PST) formalism to divide reactive states in the bond dissociation channel into states with enough translational energy to dissociate and states that may roam. Two parameters are required, ΔEroam, the energy difference between the bond dissociation threshold and the roaming threshold, and the roaming probability, Proam, the probability that states that may roam do roam rather than recombine to form reactants.

Photo-tautomerization of acetaldehyde to vinyl alcohol: a potential route to tropospheric acids.

Current atmospheric models underestimate the production of organic acids in the troposphere. We report a detailed kinetic model of the photochemistry of acetaldehyde (ethanal) under tropospheric conditions. The rate constants are benchmarked to collision-free experiments, where extensive photo-isomerization is observed upon irradiation with actinic ultraviolet radiation (310 to 330 nanometers).

Near-threshold H/D exchange in CD₃CHO photodissociation.

Measuring the isotopic abundance of hydrogen versus deuterium atoms is a key method for interrogating reaction pathways in chemistry. H/D 'scrambling' is the intramolecular rearrangement of labile isotopes of hydrogen atoms and when it occurs through unanticipated pathways can complicate the interpretation of such experiments. Here, we investigate H/D scrambling in acetaldehyde at the energetic threshold for breaking the formyl C-H bond and reveal an unexpected unimolecular mechanism.