Formation of long-lived gas-phase antiprotonic helium atoms and quenching by H₂

IN 1964 Condo¹ suggested that the decay characteristics of negative (π^- and K^-) mesons in helium bubble chambers could be explained by the capture of these particles in large-angular-momentum meta-stable orbitals of exotic helium atoms. Russell² predicted that similar "atoms' might be formed by antiprotons in liquid helium. Nearly two decades later the postulated metastability of K^- and π^- mesons in liquid helium was observed experimentally ^3-5. We recently observed⁶ that about 3% of the antiprotons stopped in liquid helium survive for several microseconds before annihilating in the helium nuclei. This is more than a million times longer than the typical (picosecond) lifetimes of antiprotons that come to rest in matter, and it represents the signature of the formation of metastable antiprotonic atoms. Here we show that the same phenomenon is observed in gas-phase helium, but that surprisingly the lifetime of the 'atoms' is the same as in the liquid phase, despite the reduction in collisional de-excitation. In addition, we show that the presence of trace amounts of hydrogen gas greatly reduces the lifetime, suggesting that a single collision with H₂ is sufficient to destroy the metastability.