Structure‐dependent Photostability of Polycyclic Aromatic Hydrocarbon Cations: Laboratory Studies and Astrophysical Implications

Abstract

Following a previous experimental study of the photostability of a series of polycyclic aromatic hydrocarbons (PAHs) this work extends to irregularly shaped PAHs, methyl-substituted PAHs, and related compounds. A photostability index R has been devised, which measures the propensity for H-loss of PAH monocations relative to that for regular catacondensed or pericondensed PAHs containing the same number of atoms. This index is also valid, to a good approximation, for neutral PAHs. Structural e †ects (presence of a methyl group ; dihydro groups ; H-H steric hindrance ; degree of n conjugation ; isomers) and size-dependent e †ects on R are demonstrated and discussed. Astrophysical applications of the propensity of PAH photodissociation include an estimation of the maximum amount of energy that can be deposited in the nascent parent PAH ion in H I regions, and an evaluation of the lower limit to the size of photostable PAHs in these regions, for various types of PAHs and their derivatives. The results provide a possible explanation for the existence of a limited number of PAHs in the ISM as required by the interstellar carbon budget determined by Snow & Witt. The information obtained is also relevant to interpretation of the presence and formation of PAHs and their methyl derivatives in meteoritic, micrometeoritic, cometary, and interplanetary particle materials.

Dissociative Photoionization of Polycyclic Aromatic Hydrocarbon Molecules Carrying an Ethynyl Group

“…In contrast, Jochims et al (1999) had observed in their experiments that the minimum photon energy causing the loss of a H atom by a photoionized PAH molecule was at least 2 eV lower when the molecule carried a methyl, a vinyl, or a phenyl group. Prior to the latter study, and Gotkis et al (1993) had found in TPIMS experiments that the minimum photon energy for H atom loss upon photoionization was also at least 2 eV lower in 1-and 2-methylnaphthalene than in naphthalene, for any time scale.…”

“…Non-statistical processes such as direct dissociation from an excited repulsive state might happen although previous works have favored the statistical path considered here (see Jochims et al 1994;Allain et al 1996a;Le Page et al 2001). Figure 6 presents for comparison some of the photostability index values determined by Jochims et al (1999) for various regular PAH molecules and derivatives along with those we have derived by considering that the internal thermal energy of the molecules reflected their translational temperature. It also reports the E E c, H * --differences.…”

“…The different behaviors of the diverse PAH derivatives with respect to dissociation are explained by the various degrees of conjugation of their π electronic system (Jochims et al 1999). Presently, the CC triple bond of an ethynyl side group yields a stronger conjugation of the π electronic orbitals over the whole molecule in comparison with the CC double bond of a vinyl group, the aromatic ring of a phenyl group, and the saturated C atom of a methyl group.…”

“…We present here laboratory measurements on the dissociative photoionization of two ethynyl-substituted PAH species, 9-ethynylphenanthrene (C 16 H 10 ) and 1-ethynylpyrene (C 18 H 10 ). The photostability index R as defined by Jochims et al (1999) can be derived from few experimental data, namely, the adiabatic ionization energy, the appearance energy of the singly dehydrogenated fragment ion, and the internal thermal energy of the parent ion. Their values have been determined in photoelectron photoion coincidence (PEPICO) spectroscopy experiments and, after deriving the value of R for both ethynyl-substituted PAH species, we conclude that they are as photostable as the nonsubstituted molecules.…”

Commission 14: Atomic and Molecular Data: (Donnees Atomiques et Moleculaires)

“…The photoionization quantum yield of PAHs is an important parameter in astrophysical modeling of their formation and destruction, and has been measured for a series of PAHs in the 5-25 eV range by Jochims, Baumgartel & Leach (1996), who also established 'rules of thump' for other species. Another important parameter is the structure-dependent photostability of PAH cations, studied by Allain et al (1996a,b), Allain & Leach (1997) and Jochims, Baumgartel & Leach (1999). Work has also been carried out on the dissociative ionization of PAHs in the 8-35 eV photon energy range (Jochims et al 1997).…”