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Two experiments have been performed at the CERN Low Energy Antiproton Ring ~LEAR! facility to study the lifetime shortening of metastable antiprotonic helium atoms by the presence of ppm admixtures of oxygen. The change in shape and lifetime of the delayed annihilation time spectra can be reproduced fairly well by fitting a simple four-level model to the data and assuming equal quenching cross sections for all metastable levels. From this we deduce a quenching cross section of (1.35 60.03)310 cm at a temperature of 100 K. An analysis of data from a previous experiment gives a value for the quenching cross section at room temperature of (1.34 60.07)310 cm. Resonant laser deexcitation of the levels ( n,l )5(37,34) and (39,35)~the lowest metastable levels of the cascades y5n2 l 2152 and 3, respectively ! was used to examine the time development of the population of these levels. Quenching cross sections of (1.32 60.10) 310 cm and (1.9060.10)310 cm were deduced for the y52 andy53 cascades from these studies at 100 K.@S1050-2947 ~98!06110-1#

Influence of oxygen admixtures on the lifetime of metastable antiprotonic helium atoms