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 3Πg→a 3Πu, e 3Πg→a 3Πu, C 1Πg→A 1Πu, and D 1∑+u→X 1∑−g of C2 radical, and A 2Δ→X 2Πr, B 2Σ−→X 2Πr, and C 2Σ+→X 2Πr 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.
Radiative dissociation of C2H2, C2HD, and C2D2 superexcited at 50–90 nm region
Toshio Ibuki, Yasuhiko Horie, Akira Kamiuchi, Yoshikazu Morimoto, Marcia C. K. Tinone, Kenichiro Tanaka, Kenji Honma; Radiative dissociation of C2H2, C2HD, and C2D2 superexcited at 50–90 nm region. J. Chem. Phys. 1 April 1995; 102 (13): 5301–5308. https://doi.org/10.1063/1.469256
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