State-selected ion–molecule reactions by a threshold electron–secondary ion coincidence (TESICO) technique. I. Apparatus and the reaction H2++H2→H3++H

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Abstract
A technique which enables the direct observation of state‐selected ion–molecule reactions through coincident measurements of secondary ions with threshold electrons has been developed. Using this technique, the reaction H2++H2→H3++H has been studied in detail at average collision energies between 0.11 and 0.93 eV. State‐selected reaction cross sections for individual vibrational states v=0–3 of the H2+ ion have been determined. It has been found that while the cross section decreases with increasing vibrational energy at low collision energies, this decreasing tendency becomes less and less prominent as the collision energy increases and the cross section is almost independent of vibrational energy at the highest energy studied. This feature qualitatively agrees with the previous study by a different technique. A detailed description of the apparatus and its performance is also given.