Electron impact dissociative ionization of the CH₂F₂molecule: cross sections, appearance potentials, nascent kinetic energy distributions and dissociation pathways

Abstract

Ions produced by crossed-beam collisions of pulsed monoenergetic electrons and supersonic expansion molecules have been analysed by time-of-flight mass spectroscopy (TOF-MS) in order to determine the appearance potentials, absolute total, dissociative and parent ionization cross sections and nascent ion kinetic energy distributions. The electron impact study was conducted at incident electron energies up to 100 eV on the parent CH 2 F 2 molecule (and Ar/CH 2 F 2 mixtures), a fluoromethane where the CH 2 F + ion is produced at higher yields (1:18) than the parent molecule ion, CH 2 F + 2 . TOF-MS band profiles analysis has enabled us to determine the ions' nascent kinetic energy distributions, information that combined with the dissociative ionization appearance potentials, calculated molecular orbital energies and orbital bond characters, leads to improved identification of the electron impact dissociative channels.

Cations of halogenated methanes: adiabatic ionization energies, potential energy surfaces, and ion fragment appearance energies

“…Experimentally, IE a s of halomethanes and radicals and AEs of ion fragments from halomethanes have been measured by electron impact ionization (EI) [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], photoionization mass spectrometry (PIMS) or photoionization efficiency spectroscopy (PIES) , photoelectron spectroscopy (PE) [41][42][43][44][45][46][47][48][49][50][51][52][53], and (threshold) photoelectronphotoion coincidence (TPEPICO) techniques [54][55][56][57][58][59][60][61][62][63][64][65][66][67]. The early results, mainly from EI, PI, and PE studies, have been collected by NIST [68].…”

Electron-impact dissociative ionization of CClF3 and CCl3F

“…Details of the experimental setup have been given elsewhere [3]. In the ionization region of the TOF-MS, a pulsed supersonic molecular beam at 25 Hz collides at right angles with a narrow energy electron beam (±0.5 eV, FWHM).…”

“…The TOF-MS band shape depends on both the KED and the ion position with respect to the extraction plate [3]. The latter induces an asymmetry in the TOF band wing associated with the short times; however, the wing related to the higher time-of-flight is unaffected and hence used to derive the ion KED.…”

“…High concentrations of some of the reactive ions and/or neutrals are indispensable for plasma processing and other applications, such as etching in the semiconductor manufacture [2]. The ions created in the electron-molecule collisions are released with characteristic kinetic energy distributions (KEDs), the knowledge of which is of substantial assistance in the understanding of the dissociative ionization mechanisms [3] and their control influences further the rate of the processes. Other fields of practical concern include the appearance of atomic and molecular (inter)halogen fragments produced by low energy electrons formed by solar winds that in turn efficiently deplete the earth's ozone layer [4].…”

“…Studies of electron-impact collisions on CClF 3 and CCl 3 F have already been reported focusing on a variety of properties and experimental methods, namely, light emission from neutral and ion fragments [9][10][11][12], electron energyloss spectroscopy, oscillator strengths [13], dissociation into metastable products [14], and determination of the total and partial ionization cross-sections (ICSs) [3,15]. The consistency of the ICS measurements has been validated by contrasting the experimental results with those computed with the most reliable simulation methods: binary-encounter-Bethe, Deutsch-Märk, and modified additivity rule [16].…”