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

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 X 3?- and a 1?2 states of the HBr2+ dication are clearly observed in the spectrum, while there is no clear evidence for the formation of the b 1?+ electronic state. For the first two states, the vibrational states v = 0-3 have been resolved, while for the ground X 3?- 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