Threshold-photoelectron-spectroscopy-coincidence study of the double photoionization of HBr

Abstract

A threshold-photoelectron-coincidence spectrum of HBr has been recorded in the 32.2–35.8 eV photon energy range, with a resolution of ∼0.01 eV, using a synchrotron radiation source. The ${\mathrm{X }}^3{\Sigma }^{-}$ and ${\mathrm{a }}^1{\Delta }_2$ states of the ${\mathrm{HBr}}^{\text{2+}}$ dication are clearly observed in the spectrum, while there is no clear evidence for the formation of the ${\mathrm{b }}^1{\Sigma }^{+}$ electronic state. For the first two states, the vibrational states $\text{v=0–3}$ have been resolved, while for the ground ${\mathrm{X }}^3{\Sigma }^{-}$ state also spin–orbit splitting has been detected. The results appear in good agreement with previous experimental observations. A comparison with theoretical predictions indicates the role of “noncovalent” contributions to the interaction between the two atomic partners for the formation of metastable states.