Atomic and Molecular Surface and Volume Processes in the Analysis of Negative Hydrogen Discharges

Abstract

The generation of hydrogen negative ions in hydrogen discharges is due to a combination of electronic excitation and wall relaxation processes. Principal processes are energetic electron excitation to form H 2(v″), followed by low-energy electron dissociative attachment to form H - +H. Wall recombination of H 2 +, H 3 + ions are also a source of H 2(v″).We consider the formation of H 2(v″) and H - by H 2 + and H 3 + ions incident upon metal surfaces.1 A four-step model for incident H 2 + (v)recombination and dissociation proceeds via: (1) Electric dissociation of high v ions in the image field ultimately producing atomic dissociation fragments; (2) resonant capture to form H 2(b 3 Σu) and H 2(X 1 Σg, v ″), with b dominant over X in ratio 69:31; (3) Auger relaxation of b to X to contribute to the H 2(X 1 Σg, v″), yield; (4) H 2(v “) drift to surface to experience ”hard“ nuclear collision and form final population distribution, H 2(v″), and final dissociation products.1 Experimental H 2/H yields are consistent with four-step ratio, not consistent with single-step singlet-model ratio, and imply rapid Auger relaxation following b-state capture. Final vibrational distributions reported here differ markedly from earlier calculated distributions. Level shifts due to image effects cause H 2 n =2-parentage capture to be marginally accessible for barium surfaces (ø = 2.7 eV) but quite possible for cesium (ø = 2.14 eV) and Cs/Mo (ø ≈1.6 eV) surfaces. Predissociation of H 2(c3IIu) competes with Auger relaxation but predissociation times are too long to allow significant H 2 dissociation.Opposing image shifts of H 2 -(2Σu), H 2(X1 Σg) allow direct H - formation from H 2 (v″) by rebounds from Ba, Cs, Cs/Mo surfaces. H - yields are evaluated explicitly for Ba surfaces.For H 3 + incident: (1) Small (2%) electric dissociation to H + + H 2 (2) resonant capture to 2p 2 E’→2 A 1, 2 B 2 states; (3) drifting H 3(v“’) in 2 A 1 ground electronic state dissociates into H 2+H to provide H 2(v ″) distribution; (4) hard collision of drifting H 2(v ″) produces final distribution H 2 (v ″). Final vibrational distributions from H 2 + and H 3 + are compared. Low work function surfaces allow capture into H 3(2s 2 A’1) and H 3(2p 2 A 2“) states. Vibrational and rotational couplings, respectively, coupleKeywordsVibrational LevelGround Electronic StateSurface RecombinationLawrence Livermore National LaboratoryBarium SurfaceThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.