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 (p̄ ⁴He⁺). 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 (n, l) = (39,35) showed a lifetime of 1.5 μs, regardless of density, whereas the lifetime of the energetically lower-lying state (37,34) was found to shorten from 1.3 to 0.1 μs with increasing density. The lifetimes of the states (38,34) and (36,33) 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 (37,34) becomes short lived at high density, the resonance (38,35)→(37,34) at a wavelength of 529.62 nm was detected.