Resolving The F-2 Bond Energy Discrepancy Using Coincidence Ion Pair Production (Cipp) Spectroscopy Dagger

Coincidence ion pair production (cipp) spectra of F-2 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 F-2 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-2**): one pi and four sigma states. The lowest-energy Rydberg state spectrum observed (T-0 = 125 999 cm(-1)) lacked some of the predicted rotational structure, which allowed an accurate determination of the ion pair production threshold of 15.6229(4) +/- 0.0004(3) 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(9) +/- 0.0004(4) eV. Photoelectron photoion coincidence (PEPICO) experiments were also carried out on F-2 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-2 dissociation energy of 1.6013(2) +/- 0.0006(2) eV, further confirming the cipp-derived value above. Because the two experiments were independently energy-calibrated, they can be averaged to 1.6013(0) +/- 0.0003(6) eV and this value can be used to derive the fluorine atom's 0 K heat of formation as 77.25(1) +/- 0.01(7) 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.