Abstract
Thermal rate coefficients for forming H-2 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: T-22PT = 10-150 K, neutral beam accommodator T-ACC = 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 10(8) cm(-3)) is determined in situ via chemical probing with CO2+ ions.
With decreasing temperature, the measured thermal rate coefficients decrease slowly from 5.5 x 10(-9) cm(3) s(-1) at 135 K to 4.1 x 10(-9) cm(3) 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.