Photoionization and ion cyclotron resonance studies of the reaction of vibrationally excited C2H2+ ions with H2

The reaction ${\mathrm{C}}_2{\mathrm{H}}_2^{+}+{\mathrm{H}}_2\to {\mathrm{C}}_2{\mathrm{H}}_3^{+}+\mathrm{H}$ has been studied using ion cyclotron resonance and photoionization mass spectrometric techniques. At thermal kinetic energies, the cross section for the reaction is near zero for ground state reactants but increases rapidly with increasing vibrational energy in the ${\mathrm{C}}_2{\mathrm{H}}_2^{+}$ ions. The ratio of the cross sections for ions in the ν₂ = 1 and ν₂ = 2 states is approximately 1:2, respectively. Some reaction is observed at photoionization energies below the theshold for formation of ν₂ = 1 ions owing to reaction of thermally excited ions and neutrals. For the distribution of vibrational energies in ${\mathrm{C}}_2{\mathrm{H}}_2^{+}$ ions formed by electron impact at 15 eV, the rate constant for the reaction is 6.3±1.8×10⁻¹¹ cm³/sec. Exchange of hydrogen atoms in nonreactive collisions between ${\mathrm{C}}_2{\mathrm{H}}_2^{+}$ ions and D₂ molecules has also been observed. An upper limit of ${\mathrm{\Delta \; H}}_f({\mathrm{C}}_2{\mathrm{H}}_3^{+}\text{)=267\hspace{0.17em}}\mathrm{kcal/mole}$ is obtained.