Resolving the F₂ bond energy discrepancy using coincidence ion pair production (cipp) spectroscopy

Coincidence ion pair production (cipp) spectra of F₂ were recorded on the DELICIOUS III coincidence spectrometer in the one-photon excitation region of 125 975-126 210 cm^-1. The F⁺ + F^- signal shows a rotational band head structure, corresponding to F₂ Rydberg states crossing over to the ion pair production surface. Spectral simulation and quantum defect analysis allowed the characterization of five new molecular Rydberg states (F₂**): one Π and four Σ states. The lowest-energy Rydberg state spectrum observed (T₀ = 125 999 cm^-1) lacked some of the predicted rotational structure, which allowed an accurate determination of the ion pair production threshold of 15.6229₄± 0.0004₃ eV. Using the well-known atomic fluorine ionization energy and electron affinity, this number leads to a ground state F-F dissociation energy of 1.6012₉± 0.0004₄ eV. Photoelectron photoion coincidence (PEPICO) experiments were also carried out on F₂ and the dissociative photoionization threshold to F⁺ + F was determined as 19.0242 ± 0.0006 eV. Using the atomic fluorine ionization energy, this can be converted to an F₂ dissociation energy of 1.6013₂± 0.0006₂ eV, further confirming the cipp-derived value above. Because the two experiments were independently energy-calibrated, they can be averaged to 1.6013₀± 0.0003₆ eV and this value can be used to derive the fluorine atom's 0 K heat of formation as 77.25₁± 0.01₇ kJ mol^-1. This latter is in excellent agreement with the latest Active Thermochemical Table (ATcT) value but improves its accuracy by almost a factor of three.