Axialisation, cooling and quenching of Ba+ ions in a Penning trap

Collisions of ions, stored in a Penning trap, with neutral background molecules usually lead to rapid ion loss from the trap unless the ions are excited by the sum of the frequencies of the reduced cyclotron and magnetron motion. Then the ions are cooled by collisions and are driven to the trap centre leading to substantial increase of the storage time. Furthermore in a three level system including a long living metastable state collisions deexcite this metastable state and increase the population density in the ionic ground state. In a laser spectroscopic experiment we demonstrate the advantages of collisions on Ba+ ions stored in a Penning trap. The combined action of metastable state quenching, axialisation and cooling leads to a significantly enhanced laser induced fluorescence signal. The cross section for collisional relaxation of the ground state Zeeman levels has been determined and we find that it is of the same order of magnitude as quenching cross section for a metastable state. Cooling and increased signal strength allows us to observe extremely narrow resonances and motional sidebands in microwave induced Zeeman transitions in the Ba+ electronic ground level.