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 d3Πga3Πu, e3Πga3Πu, C1ΠgA1Πu, and D1+uX1g of C2 radical, and A2Δ→X2Πr, B2ΣX2Πr, and C2Σ+X2Πr 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 σf[C2(C2H2)]≳σf[C2(C2HD)]≳σf[C2(C2D2)]. Reverse is true for the fluorescence cross sections of CH* and CD*, i.e., σf(C2H2)<σf(C2HD)<σf(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.

This content is only available via PDF.
You do not currently have access to this content.