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

Coincidence ion pair production (cipp) spectra of F were recorded on the DELICIOUS III coincidence spectrometer in the one-photon excitation region of 125 975-126 210 cm. The F + F signal shows a rotational band head structure, corresponding to F 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 (F**): one Π and four Σ states. The lowest-energy Rydberg state spectrum observed (T = 125 999 cm) lacked some of the predicted rotational structure, which allowed an accurate determination of the ion pair production threshold of 15.6229± 0.0004 eV. Using the well-known atomic fluorine ionization energy and electron affinity, this number leads to a ground state F-F dissociation energy of 1.6012± 0.0004 eV. Photoelectron photoion coincidence (PEPICO) experiments were also carried out on F 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 F dissociation energy of 1.6013± 0.0006 eV, further confirming the cipp-derived value above. Because the two experiments were independently energy-calibrated, they can be averaged to 1.6013± 0.0003 eV and this value can be used to derive the fluorine atom's 0 K heat of formation as 77.25± 0.01 kJ mol. This latter is in excellent agreement with the latest Active Thermochemical Table (ATcT) value but improves its accuracy by almost a factor of three.