EVALUATION OF THE EXOTHERMICITY OF THE CHEMI-IONIZATION REACTION Sm + O _ SmO+ + e_

Abstract

The chemi-ionization reaction Sm + O $rightarrow$ SmO$^{+}$ + e$^{-}$ has been used for chemical release experiments in the thermosphere. This reaction was chosen, in part, because the best available data indicated that it is exothermic by 0.35 $pm$ 0.12 eV. Low ion yields in the initial atmospheric release experiments raised questions concerning the accuracy of the ionization energy (IE) for SmO and the bond dissociation energy (BDE) of SmO$^{+}$. New measurements of the BDE, obtained using a selected ion flow tube and guided ion beam techniques, yielded a more precise value of 5.73 $pm$ 0.07 eV. The ionization energy of SmO was reexamined using pulsed-field ionization zero kinetic energy (ZEKE) photoelectron spectroscopy. The value obtained, 5.7427 $pm$ 0.0006 eV, was significantly higher than the literature value. Combined with literature bond energies of SmO, this IE indicates an exothermicity for Sm + O $rightarrow$ SmO$^{+}$ + e$^{-}$ of 0.14 $pm$ 0.17 eV, independent from and in agreement with the value deduced from the guided ion beam measurements. The implications of these results for interpretation of chemical release experiments are considered.