Experimental observations of excited dissociative and metastable states of H3 in neutralized ion beams

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Abstract
Electron transfer reactions for a fast beam of H3+ ions with Mg and K atoms have been investigated by neutral beam scattering techniques. Reactions with Mg and K targets form H3 molecules in the dissociative 2p 2E′ ground state and predissociative 2s 2A1′ and 2p 2A2″ excited states, respectively. Fragmentation energies, obtained from beam scattering measurements, allow the scaling of these electronic states of H3 with respect to their dissociation products. A metastable form of H3 observed in the H3+/K reaction is identified as the nonpredissociating, nonrotating molecule in the 2p 2A2″ electronic state. The cross section for the state-to-state process H3+(X 1A1′, N=1, K=0)+K(g)→H3*(2p 2A2″, N=K=0)+K+ for a 6 keV ion beam is 7.0±1.0 Å2. Total ion beam attenuation cross sections for the species H3+, H2D+, D2H+, and D3+ with K targets are in the relative order 1.0, 0.59, 0.58, 0.53. The higher cross section observed for the H3+/K reaction is partially accounted for by an usually high cross section for the near resonant process H3+(X 1A1′, N=1, K=0)+K(g)→K++H3 (predissociative states). The branching ratio for the formation of metastable/predissociative states from the H3+(X 1A1′, N=1, K=0)/K reaction is about 0.012. The relative intensity of metastable D3 neutrals is at least an order of magnitude lower than that of H3 for identical experimental conditions. This effect is discussed in reference to the Pauli principle and the statistical weights of nuclear spin states of D3+(D3) and H3+(H3). The importance of angular momentum restrictions on electron transfer cross sections is also discussed.