Populations and lifetimes in the v=n-l-1=2 and 3 metastable cascades of (p)over-barHe+ measured by pulsed and continuous antiproton beams -: art. no. 012504

The time evolution of the state populations in the v=n-l-1=2 and 3 metastable cascades of antiprotonic helium ((p) over bar He-4(+)) atoms were studied using laser spectroscopy. The antiprotonic states (n,l)=(39,35), (40,36), and (41,37) in the v=3 cascade were estimated to initially contain, respectively, (0.28+/-0.04)%, (0.06+/-0.02)%, and (0.02+/-0.01)% of the antiprotons stopped in the helium target, while the corresponding values for states (37,34), (38,35), and (39,36) in the v=2 cascade were (0.21+/-0.04)%, (0.49+/-0.07)%, and (0.19+/-0.07)%. The shortening of the state lifetimes due to collisions between (p) over bar He+ and helium atoms was studied. As the atomic density of the target was increased from rho=2x10(20) to 2x10(21) cm(-3), the lifetime of state (37,34) decreased by an order of magnitude, whereas the higher states (38,35) and (39,35) were relatively unaffected. Between densities rho=6x10(20) and 2x10(22) cm(-3), a short-lived component with a time constant tausimilar to0.2 mus appeared at early times in the delayed annihilation time spectrum of (p) over bar He+, while the total fraction of long-lived antiprotons decreased from 3.0 to 2.5%. These effects were caused by the lifetime-shortening of specific (p) over bar He+ states such as (37,34).