Ultraviolet Excitation of M‑O₂ (M = Phenalenone, Fluorenone, Pyridine, & Acridine) Complexes Resulting in ¹O₂

In our experiment, a trace amount of an organic molecule (M = 1<i>H</i>-phenalen-1-one, 9-fluorenone, pyridine, or acridine) was seeded into a gas mix consisting of 3% O₂ with a rare gas buffer (He or Ar) and then supersonically expanded. We excited the resulting molecular beam with ultraviolet light at either 355 nm (1<i>H</i>-phenalen-1-one, 9-fluorenone, or acridine) or 266 nm (pyridine) and used resonance enhanced multiphoton ionization (REMPI) spectroscopy to probe for the formation of O₂ in the a-¹Δ_g state, ¹O₂. For all systems, the REMPI spectra demonstrate that ultraviolet excitation results in the formation of ¹O₂ and the oxygen product is confirmed to be in the ground vibrational state and with an effective rotational temperature below 80 K. We then recorded the velocity map ion image of the ¹O₂ product. From the ion images, we determined the center-of-mass translational energy distribution, <i>P</i>(<i>E</i>_T), assuming photodissociation of a bimolecular M-O₂ complex. We also report results from electronic structure calculations that allow for a determination of the M-O₂ ground state binding energy. We use the complex binding energy, the energy to form ¹O₂, and the adiabatic triplet energy for each organic molecule to determine the available energy following photodissociation. For dissociation of a bimolecular complex, this available energy may be partitioned into either center-of-mass recoil or internal degrees of freedom of the organic moiety. We use the available energy to generate a Prior distribution, which predicts statistical energy partitioning during dissociation. For low available energies, less than 0.2 eV, we find that the statistical prediction is in reasonable agreement with the experimental observations. However, at higher available energies, the experimental distribution is biased to lower center-of-mass kinetic energies compared with the statistical prediction, which suggests the complex undergoes vibrational predissociation.