L α emission from collisional excitation of H Atoms by threshold energy Ar+ ions

In a specially designed collision chamber, H atoms produced by a microwave discharge were excited into the 2p state by impact of Ar⁺ ions in a beam. A degree of dissociation up toη=94% was measured at the exit of the microwave source, although in the center of the collision cellη is smaller, with a lower limit of 33 %. The effect of vibrational excitation of H₂ by the discharge and the resulting difficulties of measuringη are discussed. The cross section forL _α emission increases linearly from the threshold, at 10.2 eV_CM, to 17.1 eV_CM. The absolute cross section at this energy is 1.5·10^?17 cm² forη=33% (with a minimum of 0.6·10^?17 cm², for the maximum possibleη). The absence of an activation barrier rules out some features shown by an earlier ab initio calculation.