Evaluation of the computational methods for electron-impact total ionization cross sections: Fluoromethanes as benchmarks

Abstract

The experimental electron-impact total ionization cross sections ͑TICSs, ICSs͒ of CF 4 , CHF 3 , CH 2 F 2 , and CH 3 F fluoromethanes reported so far and a new set of data obtained with a linear double focusing time-of-flight mass spectrometer have been compared with the ab initio and ͑semi͒empirical based ICS available methods. TICSs computational methods include: two approximations of the binary-encounter dipole ͑BED͒ referred to hereafter as Kim ͑Kim-BEB͒ and Khare ͑Khare-BEB͒ methods, the Deutsch and Märk ͑DM͒ formalism, also requiring atomic and molecular ab initio information, the modified additivity rule ͑MAR͒, and the Harland and Vallance ͑HV͒ methods, both based on semiempirical or empirical correlations. The molecular ab initio information required by the Kim, Khare, and DM methods has been computed at a variety of quantum chemistry levels, with and without electron correlation and a comprehensive series of basis sets. The general conclusions are summarized as follows: the Kim method yields TICS in excellent agreement with the experimental method; the Khare method provides TICS very close to that of Kim at low electron-impact energies ͑Ͻ100 eV͒, but their Mott and Bethe contributions are noticeably different; in the Kim and Khare approximations the electron correlation methods improve the fittings to the experimental profiles in contrast with the large basis sets, that leads to poorer results; the DM formalism yields TICS profiles with shapes similar to the experimental and the BEB methods, but consistently lower and with the profiles maxima shifted towards lower incident electron energies; the MAR method supplies very good ICS profiles, between those of BEB and DM methods; finally, the empirical HV method provides rather poor fittings concomitant with the simplicity and the few empirical parameters used.

Electron-impact ionization of the methyl halides

“…Very few multiply charged ions are produced, however, and the two cross sections are thus essentially identical ͑see below͒. The only other pertinent experimental data for the methyl halides are those of Torres et al 10 for CH 3 F, which are seen to be in good accord with the present measurements. It is noteworthy that very good agreement between the work of this laboratory and that of Rapp and Englander-Golden 11 is observed for all thirteen atoms and molecules jointly studied by the two laboratories.…”

“…2 together with previously published data. 3,12 It should be noted that the measurements of Vallance et al, 2 Torres et al, 10 and Rapp and Englander-Golden 11 are for total charge production while the present data represent total ion production. 3,12 It should be noted that the measurements of Vallance et al, 2 Torres et al, 10 and Rapp and Englander-Golden 11 are for total charge production while the present data represent total ion production.…”

Electron-impact dissociative ionization of CClF3 and CCl3F

“…2. In both molecules, the lighter the ejected ions the higher their kinetic energies, a trend also observed for other halomethanes [6][7][8]. Figs.…”

“…In consequence, a study of the influence of the fragment size and the electronic structure on the KED and AE of the ions produced in the dissociative ionization is of fundamental interest. The paper also completes other electron-impact studies carried out on the family of halomethanes [6][7][8].…”

An (e, 2e+ ion) study of electron-impact ionization and fragmentation of tetrafluoromethane at low energies

“…Electron impact studies include measurements on electron energy loss [33][34][35][36][37][38][39][40][41], excitation [42], ionization observing total [43,44], total dissociation [45] and partial ionization cross sections using fragment ion mass analysis [42,[46][47][48][49][50][51][52]. Furthermore, binary (e, 2e) studies [53,54] and electron impact fragment ion-photon coincidences (FIPCO) [55] were performed.…”