A theoretical and experimental study of the double photoionisation of molecular bromine and a new double ionisation mechanism

Complete double photoionisation spectra of Br2 have been measured at 30.4, 37.9 and 40.7 nm. The spectra are compared with high level relativistic calculations of the state energies and potential energy surfaces of Br22+ carried out within a four-component relativistic framework. Excellent agreement is obtained between experimental and theoretical relative state energies which allows for an assignment of the main part of the spectrum. Spectroscopic constants and tunnelling lifetimes have been determined for quasi-bound states. From the experimentally determined electron distributions we deduce that a new form of dissociative autoionisation plays a major role in double photoionisation of bromine and other molecules where certain atomic ions are major fragmentation products.