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