HYDROGEN ISOTOPE EFFECTS IN THE PHOTODISSOCIATION OF C2H2 AND C2D2 MOLECULES SUPEREXCITED AT 16.85 AND 21.22 EV

In the photodissociative excitations of C2H2 and C2D2 at 16.85 and 21.22 eV, the electronic transitions observed are A 2-DELTA --> X 2-PI(r), B 2-SIGMA- --> X 2-PI(r) and C 2-SIGMA+ --> X 2-PI(r) or CH and CD radicals, d 3-PI(g) --> a 3-PI(u), e 3-PI-(g) --> a 3-PI(u), C 1-PI(g) --> A PI(u) and D 1-SIGMA(u)+ --> X 1-SIGMA(g)+ of C2 radical, and the hydrogen Balmer series. The isotopic ratios of sigam(f)(C2*(H)) /sigma(f)(C2*(D)) = 1.34 and 1.4, sigma(f)(CH*)/simga(f)(CD*) = 0.67 and 0.9 are obtained from the fluorescence cross sections (sigma(f)) determined at 16.85 and 21.22 eV, respectively, where C2(H) and C2*(D) represent the electronically excited C2* radicals produced from the superexcited C2H2** and C2D2**. At the photon energy of 21.22 eV the ratio of sigma(f)(H*)/sigma(f)(D*) = 1.3 is deduced from the fluorescence cross sections of the H and D Balmer lines. The values of the sigma(f)(C2*(H))/sigma(f)(C2(D)) and sigma(f)(H*)/sigma(f)(D) ratios show a normal primary hydrogen isotope effect. The obtained sigma(f)(CH*)/sigma(f)(CD*) ratios imply the inverse secondary hydrogen isotope effect, which may be due to the tunneling effect to form molecular hydrogen through a cis-bent transition state. The rates of relaxation of the nonionizing superexcited acetylene through radiation are estimated to be about 40% and 60% at 16.85 and 21.22 eV, respectively.