Photoionization Mass Spectrometric Study of F2, HF, and DF

Abstract

From an analysis of the threshold region for formation of F₂⁺ by photoionization of F₂, the value 15.686 ± 0.006 eV has been deduced as the first ionization potential. The process ${\mathrm{F}}_2\mathrm{+h\nu \to }{\mathrm{F}}^{+}+{\mathrm{F}}^{-}$ begins at slightly lower energy and is traced to Landau–Zener‐type crossing from certain Rydberg states to a potential curve leading to the separated ions in their ground states. The Rydberg states are those converging to the ground state of F₂⁺, ${}^2\Pi$ . The ion‐pair process is still significant at higher energy, in the vicinity of the threshold for the process ${\mathrm{F}}_2\mathrm{+h\nu \to }{\mathrm{F}}^{+}+\mathrm{F}\mathrm{+e}$ . Subtracting the intensity of F⁻ from that of F⁺ in this energy region yields a corrected threshold of 19.01 eV, which gives $D_0({\mathrm{F}}_2\text{)\hspace{0.17em}=\hspace{0.17em}1.59\hspace{0.17em}±\hspace{0.17em}0.01}\mathrm{eV}$ . The photoionization of HF (and DF) also gives rise to these three prominent processes, i.e., parent ionization, pair formation, and dissociative ionization. The ionization potential is found to be 16.00₇ ± 0.01 eV for HF (16.03₀ ± 0.01 eV for DF). The corrected threshold for H⁺ is 19.44₅ eV (19.51₃ eV for D⁺) which gives $D_0(\mathrm{HF}{\text{)\hspace{0.17em}=\hspace{0.17em}5.84}}_5\text{\hspace{0.17em}±\hspace{0.17em}0.01}\mathrm{eV}$ . Quantitative descriptions are given for the autoionization peaks observed in both the F₂ and HF spectra.