Photoionization Study of the Dissociation Energy of Fluorine and the Heat of Formation of Hydrogen Fluoride

Abstract

Mass spectra and photoionization yield curves are obtained for the molecular and atomic ions of fluorine and hydrogen fluoride. The atomic ions of both molecules are formed by ion-pair and by dissociative ionization processes. The F2+ curve has a weak onset at 15.58 eV, ascribed to a hot band, followed by an abrupt, intense onset at 15.69 eV, ascribed to the 0–0 transition. Several broad bands of autoionization are observed. The onset of the dissociative ionization process occurs at 18.76 eV, from which we calculate D°0(F2) = 1.34 ± 0.03 eV (30.9 kcal mole−1). The only observed ion-pair process is spin forbidden, producing F+(3P) + F−(1S), for which the thermodynamic threshold is not reached. The HF+ curve has a slightly tailing onset at 15.92 eV. No F+ ion is observed. The dissociative ionization process yielding H+ occurs at 19.34 eV, permitting the calculation of D°0(HF) = 5.74 eV, and ΔHf°0(HF) = −2.83 ± 0.03 eV (−65.3 kcal mole−1). Identical ion-pair curves have a threshold at 15.87 eV resulting in EA (F) = 3.47 eV. The indicated uncertainties are estimated limits of error.