Radiative decay modes of the antiprotonic helium atom

Abstract

The authors discuss the decay modes of the 'exotic' He atom obtained by replacing one electron in ordinary He by an antiproton. They use the Born-Oppenheimer approximation as a first step in the description of this three-body Coulomb system, and concentrate on states with angular momentum approximately 20 to 40. The authors find that within the Born-Oppenheimer approximation strong correlations between the motion of the antiproton and electron are critical in determining the radiative lifetimes of "vibrationally" excited states. These correlations vastly increase the lifetimes of states formed in the potential determined by the electronic ground state. Going beyond the Born-Oppenheimer approximation, and including states based on excited electronic configurations leads to a new decay mechanism, a radiative transition with simultaneous electron excitation. Surprisingly this is the dominant radiative decay mode for highly excited states of antiprotonic helium. The inclusion of pi states can further alter the picture; strong angular coupling between the 2p sigma and 2p pi states leads to a complete breakdown of the Born-Oppenheimer approximation for states of total angular momentum <or approximately=20 and provides a spectacularly virulent example of Lambda doubling, so strong that the ground vibrational states of angular momentum <or approximately=20 are actually in an excited electronic state well. The auhtors discuss the implications for the long lived states observed experimentally in He.