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

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 ²A′₁ and 2p ²A″₂ 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 ²A″ ₂ electronic state. The cross section for the state-to-state process H₃⁺(X ¹A′₁, N=1, K=0)+K(g)→H₃*(2p ²A″₂, N=K=0)+K⁺ for a 6 keV ion beam is 7.0±1.0 Ų. Total ion beam attenuation cross sections for the species H₃ ⁺, H₂D⁺, D₂H⁺, and D ₃⁺ 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 ¹A′ ₁, N=1, K=0)+K(g)→K₊+H₃ (predissociative states). The branching ratio for the formation of metastable/predissociative states from the H₃⁺(X ¹A′₁, N=1, K=0)/K reaction is about 0.012. The relative intensity of metastable D ₃ 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.