Selective Excitation of Acetylene by Ion-Impact

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Energy-loss spectra of H⁺, He⁺ and H₂⁺ ions scattered by acetylene (C₂H₂) molecules have been studied in the collision energies between 1 and 2.5 keV at a scattering angle of 0°C. It was observed that, in accordance with the Wigner spin rule, the H⁺-impact on acetylene induced only the singlet-singlet (S–S) transitions of acetylene while the He⁺-impact and the H₂⁺-impact induced both the S–S and the singlet-triplet (S–T) transitions of acetylene. A difference was found between the He⁺-impact and the H₂⁺-impact excitation of the S–T transitions of acetylene in that the lowest-lying triplet state \ ildea ³Σ_u⁺ was preferentially excited by the He⁺-impact but both the lowest and second lowest triplet states, the \ ildea and the \ ildeb ³Δ_u, were excited by the H₂⁺-impact.