Threshold electron impact ionization of carbon tetrafluoride, trifluoromethane, methane and propane

Abstract

We have studied the ionization behaviour close to threshold for the fragment ion production from CF 4 , CHF 3 , CH 4 and C 3 H 8 . In particular, we use a reliable method to extrapolate weak ion signals towards the appearance energy. Our evaluation is based on an extension of Wannier's law for the ionization of atoms towards the case of polyatomic molecules. In the experiment we use a hemispherical electron monochromator with an energy resolution of ≈135 meV. The vibrationally cold target molecules are prepared in a supersonic expansion and the resulting ions are mass analysed using a quadrupole mass spectrometer. We observe several disagreements with recommended ionization energy values, reaching up to 5 eV in the CHF 3 case. This is in contrast to a number of recent reliability tests that we have performed on Ne, Ar, Kr, Xe, H 2 , D 2 , N 2 , O 2 and N 2 O and that have reproduced the tabulated literature data within the resolution of the monochromator.

Electron-impact-ionization dynamics of five C2to C4perfluorocarbons

“…Comparison of the data with the zero-KER thresholds for CF 4 [37], and the synchrotron measurements on C 2 F 6 , C 3 F 8 , and C 3 F 6 measured by Jarvis et al [16,17], indicate that G4 theory underestimates the results from photoionization experiments by, at worst, around 3%. However, thresholds for CF 3 + , CF 2 + , and CF + measured in EI-ionization experiments on CF 4 can be significantly higher than the adiabatic values calculated via G4 theory by amounts up to several electron volts [55]. This is most probably a consequence of the fact that vertical excitation and impulsive dissociation often form cations at highly nonequilibrium geometries.…”

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

“…The parent ion CF 4 + is not observed due to its instability [37,39,48]. According to Stephan et al [46], Brehm et al [22] and Fiegele et al [50], the life time of CF 4 + ion is below 10 µs. On the other hand, some studies found indications of the existence of the CF 4 + ion with very small relative intensity [64][65][66][67].…”

“…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.…”

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].…”