Laser spectroscopic studies of state-dependent collisional quenching of the lifetimes of metastable antiprotonic helium atoms

A laser spectroscopic method was developed to directly observe the lifetimes of individual states in metastable antiprotonic helium atoms [Formula Presented] Collisional effects with surrounding helium atoms were studied in cryogenic gas targets at pressures between 0.1 and 9.0 bars and at temperatures between 5.5 and 7.0 K. The metastable state [Formula Presented] showed a lifetime of 1.5 [Formula Presented], regardless of density, whereas the lifetime of the energetically lower-lying state [Formula Presented] was found to shorten from 1.3 to 0.1 [Formula Presented]s with increasing density. The lifetimes of the states [Formula Presented] and [Formula Presented] were determined to be 10 ns and 5 ns, respectively, showing the effects of the internal Auger process. When extrapolated to zero density, the measured lifetimes agreed with the results of calculations for the radiative and Auger widths. Using the fact that the state [Formula Presented] becomes short lived at high density, the resonance [Formula Presented] at 529.62 nm was detected.