Formation of CHn molecules in excited electronic states by electron capture collisions of CH+n ions with metal atoms

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Abstract
Neutralized ion beam experiments with 5 keV CH+n ions have provided information on the nature of the electronic state of the neutrals formed by electron capture from metal atom targets (Na, K, Mg, and Zn). A general trend from formation of excited dissociative states toward stable electronic states is noted as the ionization potential of the target metal is increased from 4.4 eV (K) to 9.6 eV (Zn). Within experimental uncertainties fragmentation energies of the dissociating state formed in electron capture collisions of CH+4 or CH+3 with Na, K, or Mg atoms are independent of the ionization potential of the target. Results of our measurements show that CH4 and CH3 are formed exclusively in these reactions in the 3T2 and 2A1′ states, respectively. The dissociating state of CH2 formed by electron transfer to CH+2 from Na or K atoms is identified as the nonoptical 3B2 state which is known only from theoretical calculations. Analysis of neutralized CH+ beam profiles indicates that CH+ ions formed by electron impact of CH4 or CHCl3 are a mixture of singlet and triplet states. Analyses based on fragmentation energies indicate that when CH+ ions are neutralized by electron transfer from Na or K, the resulting states of CH are most probably 2Σ+ and 4Π. Vertical electron affinities corresponding to the formation of CH4(3T2), CH3(2A1′), CH2(3B2), and CH(2Σ+) from their respective cations are 5.2±0.3, 3.9±0.1, 4.1±0.3, and 5.4±0.1 eV, respectively.