ΔHf: 0.0 kcal/mol, REF: Element in its standard state. I.P.: 15.4 eV, REF: R. D. Levin, S. G. Lias, "Ionization Potentials and Appearance Potential Measurements," 1971-1981, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand. 71, (1982), Cat. No. C13.48:71.