Modeling ionic mobilities by scattering on electronic density isosurfaces: Application to silicon cluster anions

Abstract

We have developed a new formalism to evaluate the gas-phase mobility of an ion based on elastic scattering on an electronic density isosurface ͑SEDI͒. In this method, the ion is represented by a surface of arbitrary shape defined as a set of points in space where the total electron density assumes a certain value. This value is the only adjustable parameter in the model. Conceptually, this treatment emulates the interaction between a drifting ion and the buffer gas atoms closer than the previously described methods, the exact hard spheres scattering ͑EHSS͒ model and trajectory calculations, where the scattering occurs in potentials centered on the nuclei. We have employed EHSS, trajectory calculations, and SEDI to compute the room temperature mobilities for low-energy isomers of Si n (nр20) cations and anions optimized by density functional theory ͑DFT͒ in the local density approximation and generalized gradient approximation. The results produced by SEDI are in excellent agreement with the measurements for both charge states, while other methods can fit the mobilities for cations only. Using SEDI, we have confirmed the structural differences between Si n ϩ and Si n Ϫ predicted by DFT calculations, including the major rearrangements for nϭ9, 15, 16, and 18. We have also assigned the multiple isomers observed in recent high-resolution mobility measurements for Si n ϩ with nϭ17-19, some of them to near-spherical cage-like geometries.

Structures of small bismuth cluster cations

“…Here the "scattering on electron density isosurface" (SEDI) (Ref. 24) model has been used, where the collision cross section is calculated on the basis of the electron density distribution of each cluster. The SEDI model requires the electron density threshold as adjustable parameter which has to be calibrated against the experimental cross section for one cluster size.…”

A Comparison of Different Orthogonal Tight-Binding Molecular Dynamics Simulation Methods for Silicon Clusters

“…First principles quantum mechanical calculations such as the Car-Parrinello method within the generalized gradient approximation (CP-GGA) is successful in predicting the structures of Si clusters up to twenty atoms by Shvartsburg et al (2000). While the CP-GGA method is very accurate in yielding valuable informations about the structure and electronic properties of these clusters, the demand on computational time to optimize medium sized clusters for longer time scales is enormously high.…”

Singly and doubly lithium doped silicon clusters: Geometrical and electronic structures and ionization energies

“…The theoretical investigation is carried out in concert with the experimental threshold photoionization measurements. However, among the investigated species only Si 6 Li 2 , Si 7 Li, Si 10 Li, and Si 11 Li have sufficiently low ionization thresholds allowing experimental determination of their ionization energies. Our ultimate goal is to identify the ground state structure of the clusters and to derive the role of the lithium atoms upon investigation of their growth mechanisms in comparison with the bare silicon clusters.…”

“…the center of a Si 8 cube, while the Li − anion is not. 19 Using ab initio molecular dynamics simulations Majumder and Kulshreshtha 20 showed that Be and B atoms are encapsulated in a bicapped tetragonal prism Si 10 , while Li, Na, Mg, and Al prefer to expose on the surface of the clusters.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12] However, the determination of the ground state structure of clusters is not straightforward and remains a challenging subject. Structures of small neutral and positively charged silicon clusters, predicted earlier by theory, have only recently been confirmed or clarified by far-infrared spectroscopic experiments.…”