Electron scattering cross sections from anthracene over a broad energy range (0.00001-10,000 eV).

We report a computational investigation of electron scattering by anthracene (C14H10) in the gas phase. Integral and differential cross sections have been calculated by employing two distinct ab-initio quantum scattering methods: the symmetry adapted-single centre expansion method (ePOLYSCAT) and a screening corrected form of the independent atom model (IAM-SCAR) at low and high energies, respectively. After a detailed evaluation of the current results, we present a complete set of integral scattering cross sections from 0.00001 to 10,000 eV.

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.

Electron scattering cross sections from HCN over a broad energy range (0.1-10,000 eV): Influence of the permanent dipole moment on the scattering process.

We report theoretical integral and differential cross sections for electron scattering from hydrogen cyanide derived from two ab initio scattering potential methods. For low energies (0.1-100 eV), we have used the symmetry adapted-single centre expansion method using a multichannel scattering formulation of the problem. For intermediate and high energies (10-10,000 eV), we have applied an optical potential method based on a screening corrected independent atom representation. Since HCN is a strong polar molecule, further dipole-induced excitations have been calculated in the framework of the first Born approximation and employing a transformation to a space-fixed reference frame of the calculated K-matrix elements. Results are compared with experimental data available in the literature and a complete set of recommended integral elastic, inelastic, and total scattering cross sections is provided from 0.1 to 10,000 eV.

Modeling chemical evolution in a cold molecular plasma: quantum dynamics of CF2(-) intermediates after electron attachment.

A dynamical study is presented for the chemical processes induced by electrons (with energies up to about 16 eV) on gaseous CF(2) (X(1)A(1) state), one of the important components of plasma etching molecular mixtures. The nuclear deformations from the C(2v) initial geometry are seen to lead to different anionic intermediates that suggest different chemical evolutions into final fragments. All nuclear motions are shown to be effective for the formation of a variety of resonances which could lead to different final fragments. The effect of the above vibrational activation of the transient anionic states initially formed at the equilibrium geometry is analyzed and discussed in detail, providing compelling evidence and physical reasons for the fragment observations given by various experiments on such plasmas.

Ring-breaking electron attachment to uracil: following bond dissociations via evolving resonances.

Calculations are carried out at various distinct energies to obtain both elastic cross sections and S-matrix resonance indicators (poles) from a quantum treatment of the electron scattering from gas-phase uracil. The low-energy region confirms the presence of pi(*) resonances as revealed by earlier calculations and experiments which are compared with the present findings. They turn out to be little affected by bond deformation, while the transient negative ions (TNIs) associated with sigma(*) resonances in the higher energy region ( approximately 8 eV) indeed show that ring deformations which allow vibrational redistribution of the excess electron energy into the molecular target strongly affect these shape resonances: They therefore evolve along different dissociative pathways and stabilize different fragment anions. The calculations further show that the occurrence of conical intersections between sigma(*) and pi(*)-type potential energy surfaces (real parts) is a very likely mechanism responsible for energy transfers between different TNIs. The excess electron wavefunctions for such scattering states, once mapped over the molecular space, provide nanoscopic reasons for the selective breaking of different bonds in the ring region.

Microsolvation of Li(+) in Small He Clusters. Li(+)Hen Species from Classical and Quantum Calculations.

A structural study of the smaller Li(+)Hen clusters with n ≤ 30 has been carried out using different theoretical methods. The structures and the energetics of the clusters have been obtained using both classical energy minimization methods and quantum Diffusion Monte Carlo. The total interaction acting within the clusters has been obtained as a sum of pairwise potentials: Li(+)-He and He-He. This approximation had been shown in our earlier study to give substantially correct results for energies and geometries once compared to full ab initio calculations. The general features of the spatial structures, and their energetics, are discussed in details for the clusters up to n = 30, and the first solvation shell is shown to be essentially completed by the first 8-10 helium atoms.

Ab initio quantum dynamics with very weak van der Waals interactions: Structure and stability of small Li2(1Sigmag+)-(He)n clusters.

The potential energy surface (PES) for the interaction between Li2(1Sigmag+) and 4He has been computed using an accurate, post-Hartree-Fock quantum calculation for its ground electronic state. The orientational anisotropy of the forces and the interplay between repulsive and attractive effects within the PES are analyzed to extract information on the possible existence of bound states in the triatomic system. The structures of a few of the Li2(He)n small clusters are examined by comparing a classical approach with a full quantum one to generate bound configurations and to extract information on the possible spatial arrangements of the smaller clusters via à vis the location of the Li2 dopant. Some significant consequences on the Li2 behavior in larger clusters and droplets are drawn from the above findings.

Replacement equivalence of H- and argon in small (Ar)nH- clusters from optimized structure calculations.

The structural properties of some of the smaller ionic clusters of argon atoms containing the atomic impurity H-, ArnH- with n from 2 up to 7, are examined using different modeling for the interactions within each cluster and by employing different theoretical treatments, both classical and quantum, for the energetics. The same calculations are also carried out for the corresponding neutral homogeneous clusters Ar(n+1). The results of the calculations, the physical reliability of the interactions modeling, and the similarities and the difference between the anionic and the neutral complexes are discussed in some detail. The emerging picture shows that, due to specific features of the employed atom-atom potentials, the ArnH- and Ar(n+1) clusters present very similar structures, where the H- dopant substitutes for one of the outer Ar atoms but does not undergo as yet solvation within such small clusters.

Rotational quenching in ionic systems at ultracold temperatures.

The behavior of the rotational quenching for a molecular ion in collision with closed-shell neutral gases is investigated. We confirm that Wigner's threshold law for inelastic scattering holds in the presence of a long-range interaction due to polarization forces decreasing as the inverse fourth power of the distance but find that, because of the contributions of the higher angular momenta, its range of applicability is markedly reduced when compared to the scattering by neutral species. The calculations of the quenching cross sections make evident the special features of ionic systems at ultralow collision energies and yield rate coefficients of the order of 10(-9) x cm(3) x s(-1), much larger than those found for the quenching of neutral molecules.