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 3S2 and a 1D2 states of the HBr21 dication are clearly observed in the spectrum, while there is no clear evidence for the formation of the b 1S1 electronic state. For the first two states, the vibrational states v50 – 3 have been resolved, while for the ground X 3S2 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.