Using a merged-beam apparatus, we have measured the absolute cross section for the associative ionization process H(1s)+H(2s)→${\mathrm{H}}_2^{+}$+${\mathit{e}}^{\mathrm{-}}$ between 0.5 and 10 eV. The total cross section rises until a maximum of 2.4×${10}^{\mathrm{-}17}$ ${\mathrm{cm}}^2$ at the ${\mathrm{H}}_2^{+}$ dissociation threshold (3.4 eV) and exhibits a minimum around 2 eV. We attribute this feature to interferences between two reaction paths. Calculations combining a quantum-defect treatment of the multichannel ionization with a close-coupling treatment of the atom-atom half collision yield results in good agreement with experiment.

• Received 22 October 1990

DOI:https://doi.org/10.1103/PhysRevLett.66.1685