Single and Merged Beam Studies of the Reaction H2+ (v = 0,1; j = 0,4) + H2 → H3+ + H

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Abstract
Different approaches for studying low‐energy (meV) reactive ion‐molecule collisions in molecular beams are presented. In particular, differences in the photoionization (REsonant Multi‐Photon Ionization / REMPI, Pulsed Field Ionization / PFI) experiments using single molecular beams vs. merged and crossed beams are discussed. We address the question of the proper determination of cross sections and rate constants in single beam experiments and the factors that influence the width of the collision energy distributions. We identify space charge effects as the dominant contribution under our experimental conditions. Problems pertinent to state‐selected primary ion preparation with narrow energy distributions are highlighted.In light of the detailed data analysis, the derived relations are applied to the four‐center reaction H2+ + H2 → H3+ + H, which was studied in single and merged molecular beam experiments. We present merged beam data for the absolute reaction cross section for primary ions created in a storage ion source in the energy range from 5 meV up to 5 eV and relative state‐selected cross sections for different vibrational and rotational ion states (v = 0,1; j = 0–4) generated by (3+1) REMPI for a mean collision energy of about 10 meV.