Ionization Cross Sections Of Molecules Research Articles

A database of electron-impact ionization cross sections of molecules composed of H, C, N, O, and F

Electron-impact ionization cross sections are essential parameters required in various plasma models. To provide self-consistent ionization cross sections in plasma modeling, an open database with tabular fitting parameters and an online web system are established in this work. There are 88 species composed of H, C, N, O, and F in this database. The Binary-Encounter-Bethe (BEB) model, the Deutsch-Märk (DM) model, and the combined model which consolidates the BEB and DM models are considered to obtain the atomic and molecular ionization cross sections. The input parameters of the above models for all the species are calculated according to the quantum chemical computation with the hybrid density functional theory method with the Austin–Petersson–Frisch functional including dispersion. The same methods and basis set ensure consistent ionization cross sections in the database. The average relative error between the calculated and experimental ionization potential is 1.84%. The average root-mean-square error of the fitted ionization cross sections is 1.81% with respect to the exact values predicted by the combined model. The online database can be accessed from the website http://plasma.mathboylinlin.com/db/cs/ion/.

The efficient calculation of electron impact ionization cross sections with effective core potentials.

A black box Binary Encounter Bethe (BEB) with an effective core potential (ECP) procedure is implemented, which facilitates the efficient calculation of electron impact ionization cross sections for molecules that include heavy atoms. This is available in the Quantemol electron collisions software, a user friendly graphical user interface to the UKRMol+ codes. Tests were performed for the following series of molecules: CF4, CCl4, CBr4, CI4, and CAt4; CH4, SiH4, GeH4, and SnH4; PH3, PF3, and PCl3; SiCl4 and BCl3; and CH3Br and CF3I. Use of an ECP generally raises the predicted ionization cross section at lower energies leading to improved agreement with experiment compared to all electron calculations for BEB cross sections. Scaling BEB cross sections by the polarizability of the target molecule is shown to give somewhat erratic results, which do not always provide closer agreement with the measured cross sections.

Ionization of biological molecules by multicharged ions using the stoichiometric model

In the present work, we investigate the ionization of molecules of biological interest by the impact of multicharged ions in the intermediate to high energy range. We performed full non-perturbative distorted-wave calculations (CDW) for thirty-six collisional systems composed by six atomic targets: H, C, N, O, F, and S—which are the constituents of most of the DNA and biological molecules—and six charged projectiles (antiprotons, H, He, B, C, and O). On account of the radiation damage caused by secondary electrons, we inspect the energy and angular distributions of the emitted electrons from the atomic targets. We examine seventeen molecules: DNA and RNA bases, DNA backbone, pyrimidines, tetrahydrofuran (THF), and C n H n compounds. We show that the simple stoichiometric model (SSM), which approximates the molecular ionization cross sections as a linear combination of the atomic ones, gives reasonably good results for complex molecules. We also inspect the extensively used Toburen scaling of the total ionization cross sections of molecules with the number of weakly bound electrons. Based on the atomic CDW results, we propose new active electron numbers, which leads to a better universal scaling for all the targets and ions studied here in the intermediate to the high energy region. The new scaling describes well the available experimental data for proton impact, including small molecules. We perform full molecular calculations for five nucleobases and test a modified stoichiometric formula based on the Mulliken charge of the composite atoms. The difference introduced by the new stoichiometric formula is less than 3%, which indicates the reliability of the SSM to deal with this type of molecules. The results of the extensive ion-target examination included in the present study allow us to assert that the SSM and the CDW-based scaling will be useful tools in this area.

Evaluation of Electron-Impact Ionization Cross Sections for Molecules.

We describe the recent progress in the development of the semiempirical approach developed by Jain and Khare for the calculations of ionization cross sections for molecules by electron impact. Along with the state-of-the-art description of this approach, the emphasis will be on the evaluation of cross sections for a carbon dimer, C2, and trimer, C3. Single-differential ionization cross sections as a function of secondary or ejected electron energy and averaged secondary electron energy for C2 and C3 are calculated at fixed impinging electron energies of 100 and 200 eV. The integral ionization cross sections are also derived from ionization threshold to 2000 eV. Extensive comparisons of the presently evaluated direct ionization cross sections with the only available theoretical binary-encounter-dipole model calculations of Kim and Rudd, spherical-complex-optical-potential model calculations of Nagama and Antony, DM model calculations of Deutsch and Märk, and the calculations of Michelin et al. based on the combination of the Schwinger variational iterative method and distorted wave approximation revealed a satisfactory agreement. The present study also establishes the validity of the semiempirical approaches and so provides a solid foundation for further applications to the larger molecules.

Total electron ionization cross-sections for molecules of astrochemical interest

ABSTRACTWe describe a newly upgraded instrument for measuring absolute total electron ionization cross-sections over the energy range from 0 to 300 eV, and present cross-sections for nine previously unstudied molecules, as well as several small molecules for which comparison data is available. The measured cross-sections are compared with the predictions of the BEB model, and show reasonable agreement with the model, albeit peaking at higher electron energies than predicted by the model. We show that the maxima in the cross-sections follow an additivity model, such that the molecular cross-sections can be expressed as a sum over contributions from the constituent atoms. These contributions have been determined from a global fit to the data for all molecules studied, and allow maximum cross-sections to be predicted for molecules that have not been studied to date. We demonstrate the expected correlation between the maximum ionization cross-section and the molecular polarisability, and show that the atomic contributions to the cross-section show a similar dependence on the atomic polarisability. The observed correlation can be used as an alternative method for predicting unknown maximum cross-sections.

Molecular orbital composition and its effect on electron-impact ionization cross sections of molecules: A comparative study

The composition of molecular orbitals (MO) is the key parameter in the calculation of electron-impact ionization cross sections (Qion) of a molecule using the Deutsch-Märk method. There are a few theoretical approaches for determining the MO composition, such as Mulliken-like methods [including Mulliken analysis, Stout-Politzer (SP) analysis, and c-squared population analysis] and natural atomic orbital (NAO) method. However, almost all the previous works chose the Mulliken analysis without explanation, which is arbitrary and can lead to meaningless results in some cases. In order to give a guide on how to select an appropriate method for the MO composition and reveal the effect of the MO composition on Qion, a comparative study is presented in this work based on the results of quantum chemical computation. It is found that the Mulliken-like and NAO methods output similar MO compositions for occupied orbitals and much different MO compositions for virtual orbitals. The results by the Mulliken and SP methods are not always in the range of 0%–100% for virtual orbitals. Moreover, the Mulliken-like methods are more dependent on basis sets than the NAO method. This basis-set dependence exists not only in the MO composition but also in ionization cross sections of molecular shells. As a result, we conclude that the NAO method is the best choice for the MO composition in calculating the Qion of a molecule.

Absolute multiple ionization cross sections of water and carbon monoxide molecules induced by very high-q fast projectiles (q/v > 1)

Molecular properties, including the orientation of molecular axis, electron population numbers of atoms and ions in molecular ions and ionization energies at every ionization stage, are considered to generate a relatively simple model to calculate the absolute multiple ionization cross sections of molecules impacted by the very high-q fast projectiles (q/v>1). The multiple ionization probabilities are obtained under the framework of independent event model (IEVM). Calculation results are consistent with available experimental data. According to the picture of sequential release process, the high-order ionization of CO will be significantly influenced by molecular orientation, while the multiple ionization of H2O will be quite different and totally isotropic.

Electron impact ionization cross sections of the CO2 clusters

The modified Jain–Khare semi-empirical formalism for the evaluation of differential and integral electron impact ionization cross sections for molecules has been extended to the evaluation of cross sections for the electron ionization of CO2 clusters: (CO2)240 and (CO2)1700. The energy dependent differential cross sections are evaluated at the incident electron energies of 50, 100 and 200eV. The integral total ionization cross sections have been calculated in the energy range varying from ionization thresholds to 1000eV which revealed a good agreement with the available experimental and the theoretical data. The ionization rate coefficients have also been evaluated using the presently calculated ionization cross sections and Maxwell–Boltzmann energy distributions.

Electron impact ionization cross-section of C2, C3, Si2, Si3, SiC, SiC2 and Si2C

Total ionization cross-sections for C2, C3, Si2, Si3, SiC, SiC2 and Si2C molecules have been calculated by electron impact. Spherical complex optical potential formalism has been employed for obtaining the inelastic cross-sections for these molecules. Then by applying complex scattering potential-ionization contribution method, total ionization cross-sections are derived. These cross-sections are calculated in the energy range from ionization threshold to 2 keV. There are no measurements available in the literature to the best of our knowledge with which our results can be compared. The results show a linear relationship between maximum ionization cross-section and square root of the ratio of polarizability to ionization potential, depending on its atomicity. This gives a confirmation for the consistency of the data reported here. Present work is a maiden attempt to find electron impact ionization cross-section for these molecules, except for C2 and C3.

Electron impact total ionization cross sections for H 2S, PH 3, HCHO and HCOOH

This paper reports electron impact total ionization cross sections for molecules H 2S, PH 3, HCHO and HCOOH at energies circa threshold to 2 keV. Spherical Complex Optical Potential (SCOP) is the key ingredient in the Schrodinger equation to generate complex phase shifts and hence the total inelastic cross sections. Total inelastic cross section takes care of total loss of the flux into two dominant channels viz. total electronic excitation (discrete) and total ionization (continuum). We have employed Improved Complex Scattering Potential-ionization contribution (ICSP-ic) a semi empirical method to filter out ionization cross sections from total inelastic cross sections. The results are found to be in overall good agreement with most of the data available in the literature for all the presented targets.

An elementary method for calculating orientation-averaged fully differential electron-impact ionization cross sections for molecules

Currently there are no reliable theoretical approaches for calculating fully differential cross sections (FDCS) for low-energy electron-impact ionization of large molecules. We have recently introduced the distorted-wave impulse approximation as a first step in developing improved theoretical approaches. One significant obstacle to evaluating improved theoretical approaches which require significant computer resources lies in the fact that the existing experimental data require taking averages over all molecular orientations. To circumvent this problem, it has been proposed to approximate the orientation-average by using an orientation-averaged molecular orbital in the calculation of the FDCS. The theoretical justification and expected range of validity for the approximation is given in this paper. Examples are presented for electron-impact ionization of H(2) and N(2).

Performance of binary‐encounter‐Bethe (BEB) theory for electron‐impact ionization cross sections of molecules containing heavy elements (Z > 10)

AbstractThe binary‐encounter‐Bethe (BEB) theory developed by Kim and coworkers has been successful for computing electron‐impact ionization cross sections of many molecules. However, some recent publications have stated that BEB theory performs poorly for molecules that contain heavier elements such as chlorine and sulfur. We have found that the BEB calculations in those publications were performed incorrectly. When performed correctly, BEB predictions are as good for heavy‐element molecules as for light‐element molecules. We recommended recently that an alternative, less‐confusing procedure be used for molecules that contain heavier elements. The alternative procedure, based upon effective core potentials (ECPs), does not require explicit kinetic energy corrections. For peak cross sections of a group of 18 molecules, the root‐mean‐square difference between BEB predictions and experimental values is 13%. Results are presented for CCl3CN, C2Cl6, C2HCl5, C2Cl4, both isomers of C2H2Cl4, CCl4, TiCl4, CBr4, CHBr3, CH2Br2, GaCl, CS2, H2S, CH3I, Al(CH3)3, Ga(CH3)3, and hexamethyldisiloxane. Incorrect BEB calculations have been reported in the literature for several of these molecules. Published in 2005 John Wiley & Sons, Ltd.

Exterior Characteristics of Molecular Orbitals and Molecular Surfaces as Studied by Atomic Probes

Abstract This is a brief survey of studies on exterior characteristics of molecules conducted by means of atomic probes, mostly by metastable states of rare gas atoms in collisional ionization experiments and partly by atoms from H to Ar in quantum chemical calculations. Spatial distributions of molecular orbitals (MO) outside the molecular surfaces have been studied to determine stereoelectronic properties related with partial ionization cross sections of molecules due to electrophilic attacks of metastable atoms. A newly developed collision-energy/electron-energy resolved two-dimensional electron spectroscopy has disclosed anisotropic and dynamic characteristics of molecular surfaces (MS) via collision-energy dependence of partial ionization cross sections. Starting from a simple theoretical model of exterior electron densities (EED), researchers have developed quantum chemical treatments of the ionization reactions to a level of trajectory calculations based on potential energy surfaces of both entrance and exit reaction channels together with electronic transition rates between the surfaces. The present stage of techniques can elucidate one of the two aspects, MO or MS, assuming knowledge of the other. Since experimental aspects may contain various features of MO and MS, a simultaneous determination of both MO and MS by experiments will allow further developments in experimental techniques.

Comment on the accuracy of absolute electron-impact ionization cross sections for molecules

Electron impact ionization of molecules has been studied extensively in many laboratories and it is perhaps surprising therefore that uncertainty in the values of many cross sections has persisted. In an effort to rectify this situation absolute partial and total electron-impact ionization cross sections have been measured in this laboratory for a range of molecular targets, including H2, N2, O2, CO, NO, CO2, CH4, and SF6. These total cross sections for these molecules are compared here to those measured by Rapp and Englander-Golden, which have long been widely adopted as the de facto standard. The excellent agreement observed between these two independent data sets would seem to warrant a high level of confidence in them, and in the corresponding partial cross sections determined in this laboratory.

New model for electron-impact ionization cross sections of molecules

A theoretical model for electron-impact ionization cross sections, which has been developed primarily for atoms and atomic ions, is applied to neutral molecules. The new model combines the binary-encounter theory and the Bethe theory for electron-impact ionization, and uses minimal theoretical data for the ground state of the target molecule, which are readily available from public-domain molecular structure codes such as GAMESS. The theory is called the binary-encounter Bethe (BEB) model, and does not, in principle, involve any adjustable parameters. Applications to 19 molecules, including H2, NO, CH2, C6H6, and SF6, are presented, demonstrating that the BEB model provides total ionization cross sections by electron impact from threshold to several keV with an average accuracy of 15% or better at the cross section peak, except for SiF3. The BEB model can be applied to stable molecules as well as to transient radicals.

Electron impact ionization cross sections of molecules: Part II. Theoretical determination of total (counting) ionization cross sections of molecules: a new approach

Cross sections for single ionization of molecules by electrons (threshold ⩽ E ⩽ 200 eV) have been calculated using a newly developed semiclassical formula. The formula is a combination of the classical binary encounter approximation, the Born-Bethe approximation and the additivity rule. This formula has been applied to H 2, N 2, O 2, H 2O, CO 2, C 2H 2, NH 3, CH 4, CF 4, C 2H 4, CH 3OH, SF 6, UF 6, C 2H 6 and C 6H 6 and the cross sections obtained have been compared with results from the classical binary encounter approximation (Gryzinski formula), the semiclassical Jain-Khare approach, and reliable experimental data. The main advantage of the present treatment is that ionization cross sections can be described by a simple analytical formula (depending only on basic atomic properties), which yields results in better agreement with experimental data than the classical formulae.

Cross Sections for Ionization by Electrons. I. Absolute Ionization Cross Sections of Zn, Cd, and Te2. II. Comparison of Theoretical with Experimental Values for Atoms and Molecules

Ionization cross sections for Zn, Cd, and Te2 vapors have been measured by collection of the total positive ion currents produced from electron impact by 50-eV electrons in the specially designed ion source of a mass spectrometer. The results: σi(Zn) = 5.03±0.45×10−16 cm2, σi(Cd) = 8.54±0.33×10−16 cm2, σi(Te2) = 17.46±0.48×10−16 cm2. These results are combined with available data to set up a compilation of absolute ionization cross sections for 27 atoms. Very limited agreement is found with theoretical ionization cross sections: It is shown that ionization cross sections of molecules cannot be calculated reliably by summation of the ionization cross sections of the constituent atoms.