F. Carelli

Rotational state-changing cold collisions of hydroxyl ions with helium

Understanding low-temperature molecular collisions is challenging, but using non-resonant photodetachment makes it possible to study the state-resolved dynamics of the inelastic collisions between hydroxyl ions and cold helium buffer gas.

CARBON-RICH MOLECULAR CHAINS IN PROTOPLANETARY AND PLANETARY ATMOSPHERES: QUANTUM MECHANISMS AND ELECTRON ATTACHMENT RATES FOR ANION FORMATION

The elementary mechanisms through which molecular polyynes could form stable negative ions after interacting with free electrons in planetary atmospheres (e.g., Titans) are analyzed using quantum scattering calculations and quantum structure methods. The case of radical species and of nonpolar partners are analyzed via specific examples for both the C n H and HC n H series, with n values from 4 to 12. We show that attachment processes to polar radicals are dominating the anionic production and that the mediating role of dipolar scattering states is crucial to their formation. The corresponding attachment rates are presented as calculated upper limits to their likely values and are obtained down to the low temperatures of interest. The effects of the computed rates, when used in simple evolutionary models, are also investigated and presented in detail.

Formation of cyanopolyyne anions in the interstellar medium: the possible role of permanent dipoles.

The possibility of attaching near-threshold electrons to N-terminated carbon chains, like those observed in the outer envelopes of carbon-rich stars, is examined via accurate quantum chemistry orbital structures evaluation and quantum scattering analysis of the corresponding extra-electron wavefunctions at meV energies. It is shown that the differences in the signs and sizes of the permanent dipole moments which exist for both the neutral and anionic species of the C(n)N series of molecules play a significant role in suggesting or excluding possible energy paths to permanent anion formations of cyanopolyynes, for which the cases with n from 1 to 7 are examined in more detail.

ELECTRON-DRIVEN REACTIONS IN PROTO-PLANETARY ATMOSPHERES: METASTABLE ANIONS OF GASEOUS o-BENZYNE

In this paper, we present an investigation into low-energy electron scattering (E < 15 eV) processes from a specific benzene-like polyatomic target such as ortho-benzyne, o-C6H4(1?), in order to gain a better understanding of the effects that possible low-lying metastable electron-attachment states could have on its nuclear fragmentation dynamics. The current importance of the dynamical evolution of this molecule lies in the fact that o-C6H4 is considered to be relevant for the circumstellar synthesis of large polycyclic aromatic hydrocarbons (PAHs), as a precursor for C6H6 production via ion-based ring closure reaction from C2H2. Our parameter-free scattering calculations are performed within the molecular reference frame, where we obtain the metastable anionic states for the nuclear equilibrium configuration and further characterize the properties of such transient anions with respect to those found earlier for the benzene molecule. Our quantum studies indicate that o-C6H4 is a more efficient producer of compact, fairly long-lived anionic intermediates than benzene itself; hence, this should more rapidly enter the chemical reaction cycles of PAHs formation, thereby disappearing from possible direct observation as a stable anion.

A Quantum Study of the Chemical Formation of Cyano Anions in Inner Cores and Diffuse Regions of Interstellar Molecular Clouds

The barrierless, exothermic reactions of H – with HCnN cyanopolyynes, with n = 1 and 3, are analyzed using ab initio calculations of the interaction forces. The shape of the reactive potential energy surface suggests the most efficient approach of H – to be on a nearly collinear arrangement on the H-side of HCnN. Using simple transition state formulation of the reaction rates, which are obtained via calculation of the partition functions of each transition state configuration, provides a new non-Langevin behavior of the reaction which can help explain the unexpectedly large density of CN – formation found in observations. A similar procedure is also employed for the reaction of H – with HC 3 N and the differences in the results, indicating a lower efficiency of the latter reactivity compared with that for CN –, are discussed in this paper.

Electron scattering cross section calculations for polar molecules over a broad energy range

We report computational integral and differential cross sections for electron scattering by two different polar molecules, HCN and pyrimidine, over a broad energy range. We employ, for low energies, either the single-centre expansion (ePOLYSCAT) or the R-matrix method, while for the higher energies we select a corrected form of the independent-atom representation (IAM-SCAR). We provide complete sets of integral electron scattering cross sections from low energies up to 10,000 eV. Our present calculated data agree well with prior experimental results.

Dynamics of formation of anthracene anions in molecular clouds and protoplanetary atmospheres

Quantum mechanical scattering calculations are carried out for gas-phase molecules of anthracene, an aromatic system with three condensed aromatic carbon rings, in collision with low-energy electrons. They reveal the presence of various metastable (resonant) anionic states of this prototype species belonging to the larger polycondensed aromatic hydrocarbons deemed to be present in various regions of the interstellar medium. These resonances are analysed in terms of their most likely paths to stabilization into bound anionic molecules or aromatic fragments, while the important role of threshold virtual states is also discussed within the same context.

Electron-attachment rates for carbon-rich molecules in protoplanetary atmospheres: the role of chemical differences

The formation of anionic species in the interstellar medium from interaction of linear molecules containing carbon, nitrogen and hydrogen as atomic components (polyynes) with free electrons in the environment is modelled via a quantum treatment of the collision dynamics. The ensuing integral cross sections are employed to obtain the corresponding attachment rates over a broad range of temperatures for the electrons. The calculations unequivocally show that a parametrization form often employed for such rates yields a broad range of values that turn out to be specific for each molecular species considered, thus excluding using a unique set for the whole class of polyynes.

Electron attachment rates for PAH anions in the ISM and dark molecular clouds: dependence on their chemical properties

CONTEXT: The attachment of free electrons to polycondensed aromatic ring molecules (PAHs) is studied for the variety of these molecules with different numbers of condensed rings and over a broad range of electron temperatures, using a multichannel quantum scattering approach. The calculations of the relevant cross sections are used in turn to model the corresponding attachment rates for each of the systems under study, and these rates are parametrized as a function of temperature using a commonly employed expression for two-body processes in the interstellar medium (ISM). AIM: The scope of this work is to use first principles to establish the influence of chemical properties on the efficiency of the electron-attachment process for PAHs. METHODS: Quantum multichannel scattering methods are employed to generate the relevant cross sections, hence the attachment rates, using integral elastic cross sections computed over a broad range of relevant energies, from threshold up to 1000 K and linking the attachment to low-energy resonant collisions. RESULTS: The rates obtained for the present molecules are found to markedly vary within the test ensemble of the present work and to be lower than the earlier values used for the entire class of PAHs anions, when modelling their evolutions in ISM environments. The effects of such differences on the evolutions of chemical networks that include both PAH and PAH- species are analysed in some detail and related to previous calculations.

Nucleophilic substitution with two reactive centers: The CN− + CH3I case

The nucleophilic substitution reaction CN(-) + CH3I allows for two possible reactive approaches of the reactant ion onto the methyl halide, which lead to two different product isomers. Stationary point calculations predict a similar shape of the potential and a dominant collinear approach for both attacks. In addition, an H-bonded pre-reaction complex is identified as a possible intermediate structure. Submerged potential energy barriers hint at a statistical formation process of both CNCH3 and NCCH3 isomers at the experimental collision energies. Experimental angle- and energy differential cross sections show dominant direct rebound dynamics and high internal excitation of the neutral product. No distinct bimodal distributions can be extracted from the velocity images, which impedes the indication of a specific preference towards any of the product isomers. A forward scattering simulation based on the experimental parameters describes accurately the experimental outcome and shows how the possibility to discriminate between the two isomers is mainly hindered by the large product internal excitation.