Isotope Effects in Dissociative Excitation of C_2H_2 and C_2D_2 by Collisions with Metastable Ne(3^P_0,2) Atoms
概要
Isotope effects in dissociative excitation of C2H2 and C2D2 by collisions with metastable Ne(3P0,2) atoms have been studied in the Ne flowing afterglow. The CD(A2Δ−X2Πr, B2Σ− −X2Πr, and C2Σ+−X2Πr) and C2(d3Πg−a3Πu, C1Πg−A1Πu, e3Πg−a3Πu, and D1Σu+−X1Σg+) emission systems have been identified in the UV and visible region. The emission rate constants of CD(A,B,C) and C2(d,C,e,D) were determined to be 1.7, 0.25, 0.014, and 5.4, 0.39, 0.12, 0.0046 × 10-13 cm3 molecule-1 s-1, respectively. The isotopic ratios of kH(CH*)/kD(CD*) and kH(C2*)/kD(C2*) were determined to be 0.85 and 1.73, respectively. Although the reduction of emission rate constant of C2* from C2D2 is consistent with a simple dissociation model from the RRKM theory, the increase in the emission rate constant of CD* does not agree with it. The inverse isotope effect for CD* is explained by the fact that the formation of C2* and CH* or CD* occurs competitively, so that faster dissociation of C–H bonds suppresses the formation of CH*. Other possible mechanisms to explain the inverse isotope effect for CD* are discussed. The nascent vibrational and rotational distributions of CD(A:𝑣'=0–2,B:𝑣'=0) and C2(d:𝑣'=0–6, C:𝑣'=0–3, e:𝑣'=0–6, D:𝑣'=0–2) were determined and compared with those obtained in the Ne(3P0,2)/C2H2 reaction.