First direct observation of Coulomb explosion during the formation of exotic atoms

In molecules, the emission of binding electrons is ultimately accompanied by an energetic fragmentation of the atoms, a phenomenon referred to as Coulomb explosion. The fragmentation is usually achieved by intense laser fields [1,2], heavy-ion bombardment [3,4], stripping in thin foils [5], or soft x rays [6]. Evidence for Coulomb explosion following the capture of heavy negatively charged particles was obtained from experiments with muonic atoms [7]. From a comparison of Lyman transitions of diatomic targets such as nitrogen with monoatomic gases such as neon at various gas pressures, it was found, that the Lyman yields in muonic nitrogen were similar to those in neon, but at much lower pressures [8]. The difference was interpreted to originate from the higher velocity of the muonic nitrogen system caused by Coulomb explosion, which results in an enhanced electron refilling. The refilling rate is given by Wref Nysref, with N being the density of the target material and y the velocity of the exotic atom relative to the surrounding atoms [9]. The cross section sref depends, in general, on the charge state and the velocity. The gross approximations made in [8], as independence of sref on y and similarity of refilling and of cascades in nitrogen and neon, did not allow a quantitative extraction of the charge state and y at the instant of the Coulomb explosion from the Lyman yields. The formation and the first steps of the deexcitation of an exotic atom proceed via Auger emission, excitation, and self-ionization first of the molecular [10] and, subsequently, of the atomic electron shell, which quickly leads to a high degree of ionization. In the case of muons and for noble gases with atomic numbers Z # 18, almost complete ionization has been demonstrated for pressures below 0.2 bar [11,12] showing that electron refilling from surrounding atoms or molecules can be excluded under such conditions. Moreover, the kinetic energy of the exotic atom