Quantum tunnelling effects revealed in collisions of antiprotonic helium with hydrogenic molecules at low temperatures

Quenching of individual metastable states of antiprotonic helium (pHe /sup +/) in collisions with H/sub 2/ and D/sub 2/ in gas phase was studied using a laser-resonance method at CERN's antiproton decelerator. The temperature dependence of the quenching cross- sections of the pHe/sup +/ states (n, l)=(37,34), (38,35), and (38,37) in the reactions pHe/sup +/sub (n, l)/+H/sub 2/ and pHe /sup +/sub (n, l)/+D/sub 2/ was investigated in the temperature range of 25-300 K. Significant quantum tunnelling effects were revealed at low temperatures and (n, l)-dependent reaction barriers were deduced, in good agreement with recent theoretical calculations. (24 refs).