Radiative dissociation of C₂H₂, C₂HD, and C₂D₂ superexcited at 50–90 nm region

Radiative dissociation of superexcited acetylene was studied at hν=13.8–24.8 eV (90–50 nm) by using C₂H₂, C₂HD, and C₂D₂. The observed emission bands were d ³Π_g→a ³Π_u, e ³Π_g→a ³Π_u, C ¹Π_g→A ¹Π_u, and D ¹∑⁺_u→X ¹∑⁻_g of C₂ radical, and A ²Δ→X ²Π_r, B ²Σ⁻→X ²Π_r, and C ²Σ⁺→X ²Π_r of CH and CD radicals. The fluorescence cross sections of the electronically excited C^*₂ radicals showed a hydrogen isotope effect, i.e., the cross sections were in order of σ_f[C₂(C₂H₂)]≳σ_f[C₂(C₂HD)]≳σ_f[C₂(C₂D₂)]. Reverse is true for the fluorescence cross sections of CH* and CD*, i.e., σ_f(C₂H₂)<σ_f(C₂HD)<σ_f(C₂D₂). These isotope effects were interpreted by the competition of some decay processes from the superexcited states. Hydrogen isotope effect in simple C–H and C–D bond dissociation is important for the C^*₂ formation. As a result of the competition with this C^*₂ formation, the ‘‘reverse’’ isotope effect emerges in the CH* and CD* formations. Another important competing process is the isomerization followed by formation of nonradiative fragments. Since H atom migrates more easily than D atom through a cyclic cavitated complex and the nonradiative fragmentation competes with the CH* and CD* formation, the radiative intensities of the CH* and CD* radicals inevitably show the apparent inverse hydrogen isotope effect. The isomerization seems to be specially important in the wavelength region, λ≳80 nm, where a trans‐bent superexcited state is formed.