The photoelectron spectra and ionized states of the halogen acids

Abstract

Photoelectron spectra have been obtained for HF, HCI, HBr and HI. These yield the following values (eV) for ionization to the $^2$II and $^2\Sigma^+$ states; HF, 16.05, 18.6, (20.0); HCl, 12.74, 12.82, 16.23 (16.6); HBr; 11.67, 12.00, 15.29 (15.6); HI 10.38, 11.05, 13.85 (14.25), the experimental error being $\pm$ 0.01 eV in most cases. The values are adiabatic except those in brackets which represent the vertical ionization associated with the values immediately preceding them which are minimum ionization energies to the $^2\Sigma^+$ states. The ionized states of HF are appreciably affected by interaction between states of the same species in a manner directly related to the mechanism of covalent-ionic resonance in the neutral molecule. Vibrational structure is found in the $^2\Sigma^+$ states of HCI and HBr but not in those of HF and HI. Potential energy diagrams are derived to explain this. Diagrams are also given correlating the geometrical and energetic parameters of this set of related molecules.