Near Threshold Unimolecular Dissociation of Peroxyformic Acid Initiated by Vibrational Overtone Excitation

Unimolecular dissociation is an important loss mechanism for atmospheric hydroperoxides (R-O-OH) specially at high solar zenith angles. Here we present the first results of the unimolecular dissociation of peroxyformic acid (PFA), a hydroperoxide molecule exhibiting internal hydrogen bonding and concomitant hydrogen atom chattering. We initiate unimolecular dissociation by exciting the molecule in the vicinity of its fifth OH stretching overtone state (6ν_OH) at 626 nm as well as a combination band at 615 nm. We probe the resulting OH fragments from this near threshold unimolecular dissociation using laser-induced fluorescence and determine the partitioning of the available energy into the OH fragment's internal and translational degrees of freedom. By applying energy and momentum conservation, we also determine the average translational and internal energy of the undetected partner HCO₂ radical. Our measurements provide insight into the unimolecular dissociation of PFA and indicate that, in the dissociation process, most of the available energy (∼70%) is partitioned into the translational degrees of freedom of the two fragments and that the HCO₂ fragments are formed predominately in their ²A₁ ground electronic state. Further, fitting the OH fragment rotational state distribution with phase space theory allows us to obtain an improved estimate for the heat-of-formation of PFA that we determine to be ΔH ⁰_f (0) = -66 ± 1 kcal/mol.