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 lif etimes of individual states in metastable antiprotonic helium atoms (( p) over bar(4)He(+)). Collisional effects with surrounding helium atom s 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 stat e (n,l) = (39,35) showed a lifetime of 1.5 mu s, regardless of density , whereas the lifetime of the energetically lower-lying state (37,34) was found to shorten from 1.3 to 0.1 mu s 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 p rocess. When extrapolated to zero density, the measured lifetimes agre ed with the results of calculations for the radiative and Auger widths . Using the fact that the state (37,34) becomes short lived at high de nsity, the resonance (38,35) --> (37,34) at 529.62 nm was detected.