ION TRAP STUDIES OF H– + H → H2 + e– BETWEEN 10 AND 135 K

Thermal rate coefficients for forming H2 via associative detachment in H– + H collisions were determined using the combination of a 22-pole ion trap (22PT) with a skimmed effusive beam of atomic hydrogen penetrating the ion cloud. The temperature of both reactants have been varied independently (ion trap: T22PT = 10-150 K, neutral beam accommodator TACC = 10, 50, 120 K). Using various combinations, the temperature range between 10 and 135 K has been accessed for the first time experimentally. The effective number density of H (typically some 108 cm–3) is determined in situ via chemical probing with CO+2 ions. With decreasing temperature, the measured thermal rate coefficients decrease slowly from 5.5 × 10–9 cm3 s–1 at 135 K to 4.1 × 10–9 cm3 s–1 at 10 K. The relative error is 10%, while the absolute values may deviate systematically by up to 40%, due to uncertainties in the calibration reaction. Significant improvements of the versatile and sensitive experiment are possible, e.g., by using electron transfer from H to D+ as calibration standard.