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

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 CH₄⁺ or CH₃⁺ 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 CH₂⁺ 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 ²∑⁺ and ⁴Π. Vertical electron affinities corresponding to the formation of CH₄(³T₂), CH₃(²A ₁′), CH₂(³B₂), and CH( ²∑⁺) from their respective cations are 5.2±0.3, 3.9±0.1, 4.1±0.3, and 5.4±0.1 eV, respectively.