Observations on hot bands in the molecular and dissociative photoionization of acetylene and the heat of formation of the ethynyl ion

Photoion yield curves in the vicinity of threshold are obtained for the molecular and the ethynyl ions of acetylene and acetylene‐d₂ at ion source temperatures of 360, 298, and 130 °K. Weak ionization below the adiabatic threshold for $C2H2+$ and $C2D2+$ is ascribed to the ionization of molecules excited by one quantum of the bending vibrations, ν₄ and ν₅. Consideration of selection rules suggests a change in symmetry from the linear ground state molecule to a bent gound state ion. The 0 °K curves for C₂H⁺ and C₂D⁺ are estimated from the observed 130 °K data. Satisfactory agreement is obtained when the 298 °K data are compared with a curve calculated from the 0 °K curve by convolution with vibrational and rotational distributions. The 0 °K thresholds corrected for kinetic energy are used to calculate $Δ H f0°(C2H+)=17.47 ± 0.01 eV (402.8 ± 0.2 kcal mol−1)$ and ΔH f °₀(C₂D⁺) = 17.43 ± 0.02 eV (402.0 ± 0.5 kcal mol⁻¹). The ionization energy of the ethynyl radical is estimated to be 11.96 ± 0.05 eV.