A study of the unimolecular decomposition of the (C2H2)2+ complex

The internal energy effects and the energetics of the ion–molecule reactions ${\mathrm{C}}_2{\mathrm{H}}_2^{+}+{\mathrm{C}}_2{\mathrm{H}}_2\mathrm{\hspace{0.17em}\to \hspace{0.17em}}{\mathrm{C}}_4{\mathrm{H}}_3^{+}+\mathrm{H},$ ${\mathrm{C}}_4{\mathrm{H}}_2^{+}+{\mathrm{H}}_2,$ and ${\mathrm{C}}_4{\mathrm{H}}_2^{+}\text{+2}\mathrm{H}$ have been studied by photoionization of neutral van der Waals acetylene dimers. The appearance energies for ${\mathrm{C}}_4{\mathrm{H}}_3^{+}$ and ${\mathrm{C}}_4{\mathrm{H}}_2^{+}$ from $({\mathrm{C}}_2{\mathrm{H}}_2{)}_2$ were found to be identical and have the value of 10.90±0.05 eV (1137±5 Å). These measurements suggest activation energies of ∼0 and ∼12 kcal/mol for the ${\mathrm{C}}_4{\mathrm{H}}_3^{+}+\mathrm{H}$ and the ${\mathrm{C}}_4{\mathrm{H}}_2^{+}+{\mathrm{H}}_2$ channels, respectively. The ratio of the fragmentation probabilities for the formation of ${\mathrm{C}}_4{\mathrm{H}}_2^{+}+{\mathrm{H}}_2$ and ${\mathrm{C}}_4{\mathrm{H}}_3^{+}+\mathrm{H}$ from ${\mathrm{C}}_2{\mathrm{H}}_2^{+}{\mathrm{(X̄\hspace{0.17em}}}^2{\Pi }_u\mathrm{,v)\hspace{0.17em}\cdot \hspace{0.17em}}{\mathrm{C}}_2{\mathrm{H}}_2$ was found to decrease from approximately 0.70 to 0.45 as the ionization photon energy was increased from the ionization threshold of ${\mathrm{C}}_2{\mathrm{H}}_2\text{\hspace{0.17em}(11.40)\hspace{0.17em}}\mathrm{to}\text{\hspace{0.17em}∼14.60\hspace{0.17em}}\mathrm{eV},$ an observation in qualitative agreement with the prediction by the quasiequilibrium theory. The energy dependence measurements for the relative fragmentation probabilities of ${\mathrm{C}}_4{\mathrm{H}}_2^{+}$ and ${\mathrm{C}}_4{\mathrm{H}}_3^{+}$ from $({\mathrm{C}}_2{\mathrm{H}}_2{)}_2^{+}$ support the conclusions that the ${\mathrm{C}}_4{\mathrm{H}}_3^{+}$ ions fragment further to form ${\mathrm{C}}_4{\mathrm{H}}_2^{+}\mathrm{\hspace{0.17em}+\hspace{0.17em}}\mathrm{H}$ with little kinetic shift effects and that the ${\mathrm{C}}_4{\mathrm{H}}_2^{+}\text{\hspace{0.17em}+\hspace{0.17em}2}\mathrm{H}$ channel is mainly responsible for the substantial increase in the intensity of ${\mathrm{C}}_4{\mathrm{H}}_2^{+}$ observed previously in electron impact and charge transfer experiments at energies $\text{⩾16\hspace{0.17em}}\mathrm{eV}.$ This experiment also reveals evidence that the high vibrationally excited levels of the ${\mathrm{X̄\hspace{0.17em}}}^2{\Pi }_u$ state are as effective as the ${\mathrm{Ā\hspace{0.17em}}}^2{\Sigma }_g^{+}$ state in the formation of ${\mathrm{C}}_4{\mathrm{H}}_2^{+}\text{+2}\mathrm{H}$ provided the reaction is energetically allowed. The pressure dependence measurements for the relative intensities of ${\mathrm{C}}_4{\mathrm{H}}_2^{+}$ and ${\mathrm{C}}_4{\mathrm{H}}_3^{+}$ show that the solvation effects do not affect the fragmentation probabilities for the formations of ${\mathrm{C}}_4{\mathrm{H}}_2^{+}+{\mathrm{H}}_2$ and ${\mathrm{C}}_4{\mathrm{H}}_3^{+}+\mathrm{H}$ at energies below the thermochemical threshold $\text{(∼14.93\hspace{0.17em}}\mathrm{eV})$ for the ${\mathrm{C}}_4{\mathrm{H}}_3^{+}\text{+2}\mathrm{H}$ channel, whereas at energies higher than 14.93 eV the association of additional acetylene molecules to dimer ions will strongly favor the ${\mathrm{C}}_4{\mathrm{H}}_3^{+}+\mathrm{H}$ formation at the expense of the ${\mathrm{C}}_4{\mathrm{H}}_2^{+}\text{+2}\mathrm{H}$ channel.