Chlorine K shell photoabsorption spectra of gas phase HCl and Cl2 molecules

High resolution photoabsorption spectra of HCl and Cl₂ have been measured near the chlorineK edge in the 2810–2850 eV photon energy range. Below the ClK edge, the strongest resonance is interpreted as a simple core excitation into the unoccupied σ* valence orbital for both molecules, leading to a markedly repulsive state. Higher resonances due to low lying Rydberg states, are observed in both systems, but with a larger oscillator strength for HCl as compared to Cl₂. In Cl₂, the σ* orbital is deep enough to avoid any mixing with Rydberg orbitals. In HCl, we observe the dipole forbidden Cl 1s → 4s transition which denotes a strong 4s–4p hybridization. Above the ClK edge, the multiplet features seen for HCl are analysed in terms of double-core-valence excited vacancy states. In Cl₂, their counterpart are found very close to the ionization threshold because of the deep σ* orbital and possibly because the excited core and valence electrons originates either from the same atomic site or from different ones.