Dissociative Photoionization of Polycyclic Aromatic Hydrocarbon Molecules Carrying an Ethynyl Group

Abstract

The life cycle of the population of interstellar polycyclic aromatic hydrocarbon (PAH) molecules depends partly on the photostability of the individual species. We have studied the dissociative photoionization of two ethynylsubstituted PAH species, namely, 9-ethynylphenanthrene and 1-ethynylpyrene. Their adiabatic ionization energy and the appearance energy of fragment ions have been measured with the photoelectron photoion coincidence spectroscopy technique. The adiabatic ionization energy has been found at 7.84 ± 0.02 eV for 9-ethynylphenanthrene and at 7.41 ± 0.02 eV for 1-ethynylpyrene. These values are similar to those determined for the corresponding non-substituted PAH molecules phenanthrene and pyrene. The appearance energy of the fragment ion indicative of the loss of a H atom following photoionization is also similar for either ethynyl-substituted PAH molecule and its non-substituted counterpart. The measurements are used to estimate the critical energy for the loss of a H atom by the PAH cations and the stability of ethynyl-substituted PAH molecules upon photoionization. We conclude that these PAH derivatives are as photostable as the non-substituted species in H I regions. If present in the interstellar medium, they may play an important role in the growth of interstellar PAH molecules.

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

“…Two minor peaks were found at m/z values 200 and 201 as demonstrated with Interestingly, only [1EtyPyr − H] + fragment ions were observed in our previous study (Rouillé et al 2015). The present detection of supplementary fragments is attributed to a longer residence time of the excited ions in the acceleration zone, which is a consequence to using acceleration fields of 80 and 160 V cm −1 instead of 355 V cm −1 .…”

“…The destruction of interstellar molecules proceeds mainly through photodissociation triggered by impinging VUV photons. For that reason, we have evaluated experimentally the photostability of two ethynylsubstituted PAH cations (Rouillé et al 2015). We have concluded that ethynyl-substituted PAH species are as photostable as the corresponding regular species, whereas it had been previously observed that methylated and hydrogenated PAH molecules are less photostable (Jochims et al 1999).…”

Selective identification of cyclopentaring-fused PAHs and side-substituted PAHs in a low pressure premixed sooting flame by photoelectron photoion coincidence spectroscopy

“…Moreover, we also considered the 9-ethynylphenanthrene (orange dotted line), also likely to form in this zone. We used for this species the referenced spectrum previously determined by Rouillé et al 74 .…”

“…226 m/z is often attributed to CP-PAHs likely involved in the soot nucleation process 11,12,101 Among the other potential candidates, we also considered the 1-ethynyl-pyrene , the spectrum of which has been recorded by Rouillé et al 74 As for previous mass analyses, we report in fig. 10 As seen in fig.…”

“…IE ad values are important as they are widely used to identify structural isomers in VUV-photoionization mass spectrometry experiments. In this work, the experimental IE ad have been measured by fitting a Gaussian peak to the first transition of each spectrum, which is also the most intense band, meaning that there is little geometric change of the molecular structure upon ionization (favorable FC factors), which is typical of PAHs 38,73,74 . The determined values, from the maximum of the Gaussian fitting function, are reported in the first column of table 3 and compared to measured values provided by the literature and referenced in the NIST Webbook.…”