PHOTO-STABILITY OF SUPER-HYDROGENATED PAHs DETERMINED BY ACTION SPECTROSCOPY EXPERIMENTS

Abstract

We have investigated the photo-stability of pristine and super-hydrogenated pyrene cations (C 16 H + 10+m , m = 0, 6, or 16) by means of gas-phase action spectroscopy. Optical absorption spectra and photo-induced dissociation mass spectra are presented. By measuring the yield of mass-selected photo-fragment ions as a function of laser pulse intensity, the number of photons (and hence the energy) needed for fragmentation of the carbon backbone was determined. Backbone fragmentation of pristine pyrene ions (C 16 H + 10 ) requires absorption of three photons of energy just below 3 eV, whereas super-hydrogenated hexahydropyrene (C 16 H + 16 ) must absorb two such photons and fully hydrogenated hexadecahydropyrene (C 16 H + 26 ) only a single photon. These results are consistent with previously reported dissociation energies for these ions. Our experiments clearly demonstrate that the increased heat capacity from the additional hydrogen atoms does not compensate for the weakening of the carbon backbone when pyrene is hydrogenated. In photodissociation regions, super-hydrogenated Polycyclic Aromatic Hydrocarbons (PAHs) have been proposed to serve as catalysts for H 2 -formation. Our results indicate that carbon backbone fragmentation may be a serious competitor to H 2 -formation at least for small hydrogenated PAHs like pyrene.

Identification of stable configurations in the superhydrogenation sequence of polycyclic aromatic hydrocarbon molecules

“…These studies showed that it is possible to superhydrogenate PAH cations, with a clear preference for the addition of an odd number of H atoms, suggesting that PAH cations could act as a catalyst for H 2 formation. Hydrogenation of PAH cations has been suggested to protect PAHs from photoinduced fragmentation (Reitsma et al 2014), while other studies suggest that the weakening of the carbon skeleton as a result of H atom addition might actually lead to hydrogenated PAHs being more prone to fragmentation (Gatchell et al 2015;Wolf et al 2016). Sequential hydrogenation has also been demonstrated to lead to fragmentation as a result of the exothermic C-H bond formation process, although the low H-atom fluxes present in the ISM make this an unlikely scenario (Rapacioli et al 2018).…”

The global dust modelling framework THEMIS

“…Recent work by Wolf et al (2016) shows that the so-called super-hydrogenated polycyclic aromatic hydrocarbons 10 , which could be considered as analogues of a-C(:H) nano-particles, are less stable against photodissociation than their fully aromatic equivalents. Thus, and in the absence of laboratory data and models for a-C(:H) nanoparticle excitation and photo-dissociative processing, the work of Micelotta et al (2010bMicelotta et al ( ,a, 2011, which applies to fullerene and polycyclic aromatic hydrocarbons, can only be used to give an upper limit to the lifetime of a-C(:H) nano-grains with the same number of carbon atoms as fully aromatic particles.…”

“…This approach led us to a self-consistent explanation for the origin of the UV extinction bump in a-C nano-particles (a 1 nm, Jones 2012c) and also to viable routes to interstellar/circumstellar fullerene formation through UV 10 N.B. Small super-hydrogenated polycyclic aromatic hydrocarbons with less than 20 C atoms (e.g., Wolf et al 2016) cannot be aromatic and should therefore strictly be called polycyclic aliphatic hydrocarbons.…”

Superhydrogenated Polycyclic Aromatic Hydrocarbon Molecules: Vibrational Spectra in the Infrared

“…Experimentally, it has been demonstrated that coronene (C 24 H 12 ) could be fully superhydrogenated to form perhydrocoronene (C 24 H 36 ) in low UV flux regions (see Thrower et al 2012Thrower et al , 2014. Wolf et al (2016) explored experimentally the photo-stability of cationic pyrene (C 16 H 10 + ) with six (C 16 H 16 + ) or 16 extra H atoms (C 16 H 26 + ) and found superhydrogenated pryene cations would undergo backbone fragmentation upon absorption of two (for C 16 H 16 + ) or one (for C 16 H 26 + ) photons of energy just below 3 eV. On the other hand, by combining thermal desorption mass spectrometry measurements and density functional theory (DFT) calculations, Jensen et al (2019) have shown the existence of stable configurations of superhydrogenated neutral coronene.…”