Bochum 1998 – scientific programme

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HK: Hadronen und Kerne

HK 56: Postersitzung

HK 56.80: Poster

Wednesday, March 18, 1998, 17:00–19:00, Foyer

Quenching of metastable antiprotonic helium atoms by H₂ — •B. Ketzer — CERN, CH-1211 Geneva 23, Switzerland

Antiprotonic helium atoms are formed by Coulomb capture of antiprotons stopped in a helium target. In a series of laser spectroscopy experiments at LEAR we have shown that the unusual longevity of some 3% of these exotic atoms occurs due to the formation of metastable states of the p-e⁻-He²⁺ system. The experimental technique used until now, however, was restricted to laser-induced transitions between metastable states (n,l), n being the principal quantum number and l the angular momentum quantum number, and adjacent nonmetastable ones (n−1,l−1). Recently, we have overcome this restriction by taking advantage of state dependent quenching effects observed when H₂ is added to the helium at ppm levels. By selectively shortening the lifetimes of states with higher n, six “inverse” resonant transitions (n,l)→ (n+1,l+1) between normally metastable states of antiprotonic helium atoms were observed [1]. We have employed this “H₂-assisted inverse resonance” (HAIR) method to determine the decay rates of the states (n,l)=(39,l), l=36,37,38 and (38,l), l=35,36,37 of pHe⁺ as a function of the H₂ admixture. The quenching cross sections deduced therefrom were found to be strongly state dependent, some of them being of the order of the geometrical cross section for pHe⁺-H₂ collisions (2· 10⁻¹⁵ cm²).

[1] B. Ketzer et al., Phys. Rev. Lett. 78, 1671 (1997).