Laser spectroscopy of metastable states in the [Formula Presented] cascade of antiprotonic [Formula Presented]

The pressure dependence of the metastable-state lifetimes in antiprotonic [Formula Presented] atoms [Formula Presented] was studied using the recently observed laser resonance transition [Formula Presented] in the [Formula Presented] cascade. To this end antiprotons from the Low Energy Antiproton Ring (LEAR) at CERN were extracted in approximately 200-ns-long bunches with about [Formula Presented] per bunch and stopped in a [Formula Presented] gas target. Time spectra of delayed annihilation products were taken with the help of a Čerenkov counter with a gated photomultiplier. Under our experimental conditions, [Formula Presented] and [Formula Presented] the lowest metastable level [Formula Presented] was found to be much shorter lived than the corresponding state (37,34) in the [Formula Presented] cascade of [Formula Presented] It was strongly quenched at pressures above 500 mbars, whereas the higher states in the cascade remained nearly unaffected. This enabled us to investigate the [Formula Presented] transition by resonant laser deexcitation. Its wavelength was found to be [Formula Presented] only 6 ppm lower than the theoretical value given by Korobov and Bakalov [Phys. Rev. Lett. 79, 3379 1997)] after including relativistic corrections.