Threshold dissociation of the 1-ethynylpyrene cation at internal energies relevant to H I regions

TL;DR

This study measures the ionization energy and dissociation energies of 1-ethynylpyrene cation using photoelectron spectroscopy, revealing its stability and dissociation pathways relevant to interstellar conditions.

Contribution

It provides the first detailed breakdown graph and dissociation energies for 1-ethynylpyrene cation, enhancing understanding of its stability in space environments.

Findings

Ionization energy of 1-ethynylpyrene at 7.391 eV

Activation energy for H loss is 3.70 eV

Minor dissociation channels identified

Abstract

Photoelectron photoion coincidence spectroscopy has been used to measure the threshold photoelectron spectrum of 1-ethynylpyrene and to obtain the breakdown graph describing the dissociation of the 1-ethynylpyrene cation. The threshold photoelectron measurement has allowed us to improve the determination of the ionization energy of 1-ethynylpyrene at 7.391 $\pm$ 0.005 eV. Concerning the main dissociation channels, the analysis of the breakdown graph has given 3.70 $\pm$ 0.60 eV as the activation energy for the loss of one H atom and 2.98 $\pm$ 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 cations. Minor dissociation channels were also detected and identified as the loss of the C$_2$H group and the loss of a C$_2$H$_2$ unit and/or that of an H atom plus the C$_2$H 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 the cation of pyrene and is consequently more photostable than the cation of 1-methylpyrene. We conclude that photodissociation is not the leading cause of the low abundance, if not the absence, of ethynyl-substituted polycyclic aromatic hydrocarbon species in the interstellar medium.