Muonium formation via electron transport in solid nitrogen

Muon spin rotation (mu SR) techniques have been used to investigate the dia magnetic and paramagnetic states of energetic positive muons stopped in sol id molecular nitrogen. The paramagnetic signal arises from muonium (Mu = mu (+) + e(-)) atoms and reflects both "prompt" epithermal Mu formation and "d elayed" thermal Mu formation. The latter is shown to be due to convergence of the thermalized mu(+) With an electron liberated in its ionization track . Measurements in external electric fields of up to 10 kV/cm applied along and antiparallel to the initial muon momentum reveal a large anisotropy in the spatial distribution of muon-electron pairs: the mu(+) is shown to ther malize ''downstream'' of the ionization products of its track. The characte ristic muon-electron distances in alpha-N-2 and beta-N-2 and liquid nitroge n are estimated to be approximately 500 Angstrom, 250 Angstrom,and 300 Angs trom, respectively. The dependence of delayed Mu formation upon electron mo bility offers a method for determining such mobilities on a microscopic sca le. Electron drift mobilities are shown to differ by several orders of magn itude in the alpha and beta phases of solid nitrogen. Excess electrons from the muon track are apparently delocalized in orientationally ordered alpha -N-2; electron localization in orientationally disordered beta-N-2 is discu ssed in terms of the formation of a small polaron due to electron interacti on with the rotational degrees of freedom of N-2 molecules. The diamagnetic signal in condensed nitrogen is ascribed to the N(2)mu(+) molecular ion; i n beta-N-2 it consists Of two components, one relaxing slowly due to random fields from nuclear dipole moments and the other relaxing up to two orders of magnitude faster, due to very delayed Mu formation as the muon captures low-mobility electrons. [S0163-1829(99) 10815-4].