Threshold Dissociation of the 1-ethynylpyrene Cation at Internal Energies Relevant to H i Regions

Abstract

Abstract Photoelectron–photoion coincidence spectroscopy has been used to measure the threshold photoelectron (TPE) spectrum of 1-ethynylpyrene and to obtain the breakdown graph describing the dissociation of the 1-ethynylpyrene cation. The TPE measurement has allowed us to improve the determination of the ionization energy of 1-ethynylpyrene at 7.391 ± 0.005 eV. Concerning the main dissociation channels, the analysis of the breakdown graph has given 3.70 ± 0.60 eV as the activation energy for the loss of one H atom and 2.98 ± 1.80 eV for the loss of a second independent H atom. The corresponding entropies of activation are affected by large errors as observed in similar studies of other polycyclic aromatic hydrocarbon (PAH) cations. Minor dissociation channels were also detected and identified as the loss of the C2H group and the loss of a C2H2 unit and/or that of an H atom plus the C2H group. The activation energies and the entropies of activation of these minor pathways could not be derived from the measurements. It is found that the cation of 1-ethynylpyrene behaves like that of pyrene and is consequently more photostable than that of 1-methylpyrene. We conclude that photodissociation is not the leading cause of the low abundance, if not the absence, of ethynyl-substituted PAH species in the interstellar medium.