On the absolute photoionization cross section and dissociative photoionization of cyclopropenylidene

Abstract

We report the determination of the absolute photoionization cross section of cyclopropenylidene, c-C₃H₂, and the heat of formation of the C₃H radical and ion derived by the dissociative ionization of the carbene. Vacuum ultraviolet (VUV) synchrotron radiation as provided by the Swiss Light Source and imaging photoelectron photoion coincidence (iPEPICO) were employed. Cyclopropenylidene was generated by pyrolysis of a quadricyclane precursor in a 1 : 1 ratio with benzene, which enabled us to derive the carbene’s near threshold absolute photoionization cross section from the photoionization yield of the two pyrolysis products and the known cross section of benzene. The cross section at 9.5 eV, for example, was determined to be 4.5 ± 1.4 Mb. Upon dissociative ionization the carbene decomposes by hydrogen atom loss to the linear isomer of C₃H⁺. The appearance energy for this process was determined to be AE_0K(c-C₃H₂; l-C₃H⁺) = 13.67 ± 0.10 eV. The heat of formation of neutral and cationic C₃H was derived from this value via a thermochemical cycle as Δ_fH_0K(C₃H) = 725 ± 25 kJ mol⁻¹ and Δ_fH_0K(C₃H⁺) = 1604 ± 19 kJ mol⁻¹, using a previously reported ionization energy of C₃H.