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

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

doi:10.1086/318952

Cited by 131 publications

(164 citation statements)

References 76 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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“…Notes. E 0 is the energy necessary for the process and ΔS is the energy difference between the transition state and the initial molecule (Le Page et al 2001). Values for E 0 and ΔS are taken from Le Page et al (2001) and Paris et al (2014).…”

Section: Discussionmentioning

confidence: 99%

H₂formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen

Boschman

Cazaux

Spaans

et al. 2015

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Aims. Molecular hydrogen is the most abundant molecule in the Universe. It is thought that a large portion of H 2 forms by association of hydrogen atoms to polycyclic aromatic hydrocarbons (PAHs). We model the influence of PAHs on total H 2 formation rates in photodissociation regions (PDRs) and assess the effect of these formation rates on the total cloud structure. Methods. We set up a chemical kinetic model at steady state in a PDR environment and included radiative transfer to calculate the chemistry at different depths in the PDR. This model includes known dust grain chemistry for the formation of H 2 and a H 2 formation mechanism on PAHs. Since H 2 formation on PAHs is impeded by thermal barriers, this pathway is only efficient at higher temperatures (T > 200 K). At these temperatures the conventional route of H 2 formation via H atoms physisorbed on dust grains is no longer feasible, so the PAH mechanism enlarges the region where H 2 formation is possible. Results. We find that PAHs have a significant influence on the structure of PDRs. The extinction at which the transition from atomic to molecular hydrogen occurs strongly depends on the presence of PAHs, especially for PDRs with a strong external radiation field. A sharp spatial transition between fully dehydrogenated PAHs on the outside of the cloud and normally hydrogenated PAHs on the inside is found. As a proof of concept, we use coronene to show that H 2 forms very efficiently on PAHs, and that this process can reproduce the high H 2 formation rates derived in several PDRs.

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“…Observations of spatial correlations between vibrationally excited H 2 and PAHs in photo dissociation regions provide indications of PAHs acting as catalysts for H 2 formation (Habart et al 2003;Habart et al 2004). PAHs in the ISM are thought to exhibit a range of hydrogenation and charge states depending on the environment, especially the UV flux (Le Page et al 2001;Le Page et al 2003;Montillaud et al 2011). At high UV fluxes small PAHs will be dissociated, whilst at intermediate fluxes H loss and ionization dominate, resulting in dehydrogenated and cationic PAHs.…”

Section: Introductionmentioning

confidence: 99%

Superhydrogenated PAHs: Catalytic formation of H₂

Thrower

Nilsson

Jørgensen

et al. 2011

EAS Publications Series

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Abstract. The possible role of neutral PAHs as catalysts for H2 formation in the interstellar medium is investigated by a combined experimental and density function theory study of the superhydrogenation of coronene (C24H12). The calculations suggest efficient hydrogenation of both edge and centre sites, along with competing abstraction reactions to form H2 in a series of catalytic cycles. Scanning tunneling microscopy and thermal desorption measurements have been used to provide direct evidence of the formation of superhydrogenated coronene as a result of exposure to D atoms. Lower limit estimates for the crosssections of 1.8×10 −17 , 5.5×10 −18 and 1.1×10 −18 cm 2 for the formation of singly, doubly and triply hydrogenated coronene are derived. The results suggest that superhydrogenated PAHs may play an important role in H2 formation in the ISM.

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

Cited by 131 publications

References 76 publications

Collision rate constants for polarizable ions

Collision rate constants for polarizable ions

H₂formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen

Superhydrogenated PAHs: Catalytic formation of H₂

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

Cited by 131 publications

References 76 publications

Collision rate constants for polarizable ions

Collision rate constants for polarizable ions

H2formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen

Superhydrogenated PAHs: Catalytic formation of H2

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Product

Resources

About

H₂formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen

Superhydrogenated PAHs: Catalytic formation of H₂