Evaluation of the exothermicity of the chemi-ionization reaction Sm + O → SmO⁺ + e⁻

The exothermicity of the chemi-ionization reaction Sm + O → SmO⁺ + e⁻ has been re-evaluated through the combination of several experimental methods. The thermal reactivity (300–650 K) of Sm⁺ and SmO⁺ with a range of species measured using a selected ion flow tube-mass spectrometer apparatus is reported and provides limits for the bond strength of SmO⁺, 5.661 eV ≤ D₀(Sm⁺-O) ≤ 6.500 eV. A more precise value is measured to be 5.72₅ ± 0.07 eV, bracketed by the observed reactivity of Sm⁺ and SmO⁺ with several species using a guided ion beam tandem mass spectrometer (GIBMS). Combined with the established Sm ionization energy (IE), this value indicates an exothermicity of the title reaction of 0.08 ± 0.07 eV, ∼0.2 eV smaller than previous determinations. In addition, the ionization energy of SmO has been measured by resonantly enhanced two-photon ionization and pulsed-field ionization zero kinetic energy photoelectron spectroscopy to be 5.7427 ± 0.0006 eV, significantly higher than the literature value. Combined with literature bond energies of SmO, this value indicates an exothermicity of the title reaction of 0.14 ± 0.17 eV, independent from and in agreement with the GIBMS result presented here. The evaluated thermochemistry also suggests that D₀(SmO) = 5.83 ± 0.07 eV, consistent with but more precise than the literature values. Implications of these results for interpretation of chemical release experiments in the thermosphere are discussed