The species O2−, F−, Ne, and Na+ are isoelectronic. Based on their photoelectron spectra, which ordering from lowest to highest binding energy for the valence electrons is most consistent with experimental data?
a) Na+<Ne<F−<O2−
b) Ne<F−<O2−<Na+
c) O2−<F−<Ne<Na+
d) F−<O2−<Ne<Na+