Gas phase chemistry of pyrene and related cations with molecules and atoms of interstellar interest

Page, Valery Le; Keheyan, Yeghis; Snow, Theodore P.; Bierbaum, Veronica M.

doi:10.1016/s1387-3806(98)14217-3

Cited by 41 publications

(41 citation statements)

References 25 publications

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“…Because of their potential relevance in the interstellar medium, we have carried out comprehensive experimental studies of the reactions of aromatic cations and of carbon chain anions with H 2 , H, N, and O [12][13][14][15][16][17]. In these unusual systems, the polarizability of the neutral is small and that of the ion is large, which accentuates the effect of the V ␣ term on the overall potential.…”

Section: Resultsmentioning

confidence: 99%

Collision rate constants for polarizable ions

Eichelberger

Snow

Bierbaum

2003

J. Am. Soc. Mass Spectrom.

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Langevin described a model for the interaction between an ion and a neutral nearly a century ago and since then, many modifications have been introduced to adjust for specific circumstances. This work discusses the induced dipole-induced dipole interaction between an ion and a neutral without a permanent dipole and introduces an anisotropic adjustment. A point polarizable ion model (PPI) and an orientation dependent polarizable ion model (ODPI) are discussed and applied to systems where the ion is highly polarizable and the neutral is only weakly polarizable. with work outlining the attractive potential between a point charge and a point-polarizable neutral. The result was an expression for the collisional rate constant, k L , in terms of a few simple variables; e, the elementary charge, ␣ n , the polarizability of the neutral, and , the reduced mass of the ionneutral pair.The approach is appealing in its simplicity, and has been used as a standard for ion-molecule and ion-atom collisions. This method has been used extensively to calculate collision rate constants in many environments, including the interstellar medium, where atomic hydrogen, molecular hydrogen, and atomic oxygen and nitrogen are the most abundant species.Another major area of work involves the interaction of an ion and a neutral species with a permanent dipole moment. Several approaches have been taken, ranging from an orientation-fixed dipole approximation to orientation-dependent approaches which rigorously conserve angular momentum and energy. These methods have been discussed in detail [2,3]. Additional work in this area involves parameterizing the theory to obtain a solution that is easily accessible to experimentalists [4,5].Deviations from a point charge model have been described by Su et al. [9,10] and an induced dipoleinduced dipole adjustment has been discussed by Su and Bowers [11]. The possibility for large deviation is mentioned with respect to anionic systems, but due to the difficulty of acquiring ionic polarizabilities, only the Cl Ϫ and H Ϫ anions are discussed. These systems afford less than 5% deviation from the classical Langevin rate constants. In this paper we highlight the importance of the anisotropy of the induced dipole-induced dipole interaction and introduce an orientation dependent polarizable ion model. We demonstrate that these interactions, especially when anisotropy is included, have significant impact on the collision rate constants of large molecular cations and anions. TheoryThe Langevin rate constant is derived from a potential that assumes the ion is a point charge. Two factors, the long-range nature of the potential involving ions and the way in which the random orientation of an ion causes the location of the charge to average to the center, make this approximation accurate in most cases. Deviation can occur, however, in the second of these two factors. If the charge is mobile within the ion, the average location of the charge can shift from the center, effectively increasing the critical impact parameter...

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Section: Resultsmentioning

confidence: 99%

Collision rate constants for polarizable ions

Eichelberger

Snow

Bierbaum

2003

J. Am. Soc. Mass Spectrom.

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“…Other reactions involving O and N atoms will be discussed later. There are a few measurements of reactivity of the smaller PAH cations such as benzene (Ausloos et al 1989 ;Giles, Adams, & Smith 1989 ;Petrie, Javahery, & Bohme 1992 ;Scott et al 1997 ;Capron et al 1994), naphthalene (Le Page et al 1997Page et al , 1999bFeng & Lifshitz 1996), and pyrene (Le Page et al 1999a). In these experiments the major process is H addition, here shown for the naphthalene cation :…”

Section: Chemistry Involving Pah Cations and Atomic Andmentioning

confidence: 98%

“…The procedure was then compared to the results from Lifshitz and coworkers on the naphthalene and anthracene cations. We have not made a comparison for the pyrene cation, even though data are available, because we expect this ion to exhibit peculiar behavior due to the triplet nature of in its ground C 16 H 9 state, which is reÑected by a di †erent value for of *S 1000K t about 10 cal K~1 (Ling et al 1995 ;Le Page et al 1999a, 1999b. Reasonable agreement was found for naphthalene : eV)) \ 3.9 compared to 3.7 and log 10 (k(7 log 10 (k(10 eV)) \ 7.5 compared to 7.3 ; and for anthracene : log 10 (k(7.9 eV)) \ 2.2 compared to 2.0 and eV)) \ 7.6 comlog 10 (k(13.6 pared to 7.5.…”

Section: T He Rrkm Approachmentioning

confidence: 99%

Hydrogenation and Charge States of PAHsPAHs in Diffuse Clouds. I. Development of a Model

Page

Snow

Bierbaum

2001

ASTROPHYS J SUPPL S

131

158

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A model of the hydrogenation and charge states of polycyclic aromatic hydrocarbons (PAHs) in di †use clouds is presented. The main physical and chemical processes included in the model are ionization and photodissociation in the interstellar UV Ðeld, electron recombination with PAH cations, and chemistry between PAH cations and major interstellar species present in the di †use medium, such as H 2 , H, O, and N atoms. A statistical model of photodissociation is presented, which is a simpliÐed version of the Rice-Ramsperger-Kassel-Marcus theory. The predictions of this new approach have been successfully compared to experimental results for small PAH cations, justifying the application of the model to larger PAHs for which no experimental data are available. This simpliÐed statistical theory has also been used to estimate the importance of the dissociative recombination channel in the reaction between PAH cations and electrons. Recent experimental results obtained on the chemistry between PAH cations and H, O, and N atoms are discussed and included in the model. Finally, a discussion is presented on H 2 , other important processes which may a †ect the PAH distribution in the interstellar medium, such as electron attachment, photodetachment, photofragmentation with carbon loss, double ionization, and chemistry between PAH cations and minor species present in di †use clouds. The results obtained with this model for compact PAHs ranging from benzene to species bearing up to 200 carbon atoms are discussed in a separate paper.

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“…PAHs can also react with atomic oxygen but these reactions are also poorly known. Le Page et al (1999a) considered a reaction rate for PAH cations of k +O ∼ 10 −10 cm 3 s −1 for the addition of an O-atom, regardless of the size of PAH cations. Assuming a density n H = 10 4 cm −3 and an O/H ratio of 3 × 10 −4 , this leads to a reaction timescale of ∼100 years.…”

Section: Perspectivesmentioning

confidence: 99%

Evolution of polycyclic aromatic hydrocarbons in photodissociation regions

Montillaud

Joblin

Toublanc

2013

A&A

157

227

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Context. Various studies have emphasised variations in the charge state and composition of the interstellar polycyclic aromatic hydrocarbon (PAH) population in photodissociation regions (PDRs). These changes are expected to affect the energetics and chemistry in these regions, thereby calling for a quantitative description. Aims. We aim to model the spatial evolution of the charge and hydrogenation states of PAHs in PDRs. We focus on the specific case of the north-west (NW) PDR of NGC 7023, for which many observational constraints are available. We also discuss the case of the diffuse interstellar medium (ISM). Methods. We modelled the physical conditions in NGC 7023 NW using a state-of-the-art PDR code. We then used a new PAH chemical evolution model that includes recent experimental data on PAHs and describes multiphoton events. We considered a family of compact PAHs bearing up to 96 carbon atoms. Results. The calculated ionization ratio is in good agreement with the observed ratio in NGC 7023 NW. Within the PDR, PAHs evolve into three major populations. We find medium-sized PAHs (50 N C 90) to be normally hydrogenated, while larger PAHs (N C 90) can be superhydrogenated, and smaller species (N C 50) are fully dehydrogenated. In the more diffuse gas of the cavity, where the fullerene C 60 has recently been detected, all the studied PAHs are found to be quickly fully dehydrogenated. PAH chemical evolution exhibits a complex non-linear behaviour as a function of the UV radiation field because of multiphoton events. Steady state for hydrogenation is reached on timescales ranging from less than a year for small PAHs, up to 10 4 years for large PAHs at A V = 1. Critical reactions that would need more studies are the recombination of cations with electrons, the reactivity of cations with H 2 , and the reactivity of neutral PAHs with H. Conclusions. We have developed a new model of PAH chemical evolution based on the most recent available molecular data. This model allows us to rationalise the observational constraints without any fitting parameter. PAHs smaller than 50 carbon atoms are not expected to survive in the NGC 7023 NW PDR. A similar conclusion is obtained for the diffuse ISM. Carbon clusters turn out to be end products of PAH photodissociation, and the evolution of these clusters needs to be investigated further to evaluate their impact on the chemical and physical evolution of PDRs.

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Gas phase chemistry of pyrene and related cations with molecules and atoms of interstellar interest

Cited by 41 publications

References 25 publications

Collision rate constants for polarizable ions

Collision rate constants for polarizable ions

Hydrogenation and Charge States of PAHsPAHs in Diffuse Clouds. I. Development of a Model

Evolution of polycyclic aromatic hydrocarbons in photodissociation regions

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