A new mergedhyphen;beam apparatus is described, which permits the measurement of the energy dependence of the absolute cross sections for ionndash;neutral and neutralndash;neutral reactions yielding ionic products over the energy range 0.010 to sim;10 eV. The apparatus has a somewhat different geometry than that of any previous mergedhyphen;beam instrument and incorporates several innovations which facilitate the rapid measurement of relative cross sections, the accurate determination of the beam overlap integral, and the highhyphen;resolution measurement of the product energy distribution. The performance of the instrument is illustrated with data on the reaction H+2+ H2rarr; H+3+ H.
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