Hydrogen isotope effects in the photodissociation of C 2H 2 and C 2D 2 molecules superexcited at 16.85 and 21.22 eV

Abstract

In the photodissociative excitations of C 2H 2 and C 2D 2 at 16.85 and 21.22 eV, the electronic transitions observed are A 2Δ→X 2Π r, B 2Σ −→X 2Π r and C 2Σ +→X 2Π r or CH and CD radicals, d 3Π g→a 3Π u, e 3Π g→a 3Π u, C 1Π g→A 1Π u and D 1Σ + u→X 1Σ + g of C 2 radical, and the hydrogen Balmer series. The isotopic ratios of σ f(C * 2(H))/σ f(C * 2(D))=1.34 and 1.4, σ f(CH*)/σ f(CD*)=0.67 and 0.9 are obtained from the fluorescence cross sections (σ f) determined at 16.85 and 21.22 eV, respectively, where C * 2(H) and C * 2(D) represent the electronically excited C * 2 radicals produced from the superexcited C 2H ** 2 and C 2D ** 2. At the photon energy of 21.22 eV the ratio of σ f(H*)/σ f(D*)=1.3 is deduced from the fluorescence cross sections of the H and D Balmer lines. The values of the σ f(C * 2(H))/σ f(C * 2(D)) and σ f(H*)/σ f(D*) ratios show a normal primary hydrogen isotope effect. The obtained σ f(CH*)/σ 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.