Photoionization of HCN: The Electron Affinity and Heat of Formation of CN

Abstract

A re‐examination of the photoionization of HCN has revealed two processes, ${\mathrm{HCN\hspace{0.17em}+\hspace{0.17em}h}}_{\nu }{\mathrm{\to H}}^{\mathrm{\hspace{0.17em}+\hspace{0.17em}}}{\mathrm{+CN}}^{-}$ , and ${\mathrm{HCN\hspace{0.17em}+\hspace{0.17em}h}}_{\nu }{\mathrm{\to H}}^{\mathrm{\hspace{0.17em}+\hspace{0.17em}}}\mathrm{+CN+e}$ , not previously reported. The difference in threshold values for the two processes leads directly to the electron affinity for CN, 3.82 ± 0.02 eV. The threshold for the second process leads to ${\mathrm{\Delta H}}_{f_0}\mathrm{°(}\mathrm{CN}\text{)\hspace{0.17em}=\hspace{0.17em}105.5\hspace{0.17em}±\hspace{0.17em}2}\mathrm{kcal}/\mathrm{mole}$ , but ${\mathrm{\Delta H}}_f\mathrm{°(}\mathrm{HCN})$ remains a major source of uncertainty.