Multiple ionization of helium and krypton by electron impact close to threshold: appearance energies and Wannier exponents

Abstract

We determined appearance energy (AE) values AE(Xn+/X) for the formation of singly (He+) and doubly charged (He2+) He ions and multiply charged Kr ions Krn+ up to n = 6 following electron impact on He and Kr atoms using a high-resolution electron impact ionization mass spectrometer. The data analysis employs an iterative, non-linear least-squares fitting routine, the Marquart–Levenberg algorithm, in conjunction with either a 2-function or a 3-function fit based on a power threshold law. This allows us to extract the relevant AEs and also the corresponding exponents for a Wannier-type power law from the measured near-threshold data. The values of the AEs determined in this work are compared with other available experimental and with spectroscopic AE values and the extracted exponents p are compared with other available experimental data and with the predictions of the various Wannier-type power law models. One observation is particularly noteworthy, namely the fact that none of the available experimental data seem to support the large values of ‘p’ predicted by the Wannier–Geltman and the generalized Wannier law for n > 3.

Electron-impact multiple ionization of Ne, Ar, Kr and Xe

“…In general, the MI measurements are relative multiple to single. Absolute values were obtained by measuring total or single [46,47]. Experimental data: for electron-impact, Schram et al [9], Nagy et al [12], Krishnakumar et al [13], Syage [14], ♦ Rejoub et al [15], McCallion et al [16], Kobayashi et al [17], hollow Straub et al [18], Almeida et al [19], Liebius et al [20], cross-circle 10 keV value by Singh et al [21]; for proton-impact, DuBois [89]; DuBois et al [90], • Andersen et al [91], Cavalcanti et al [33,92], Haugen et al [93], Gonzalez et al [94], for Ne, Ar and Kr Sarkadi et al [95], for Xe Manson and DuBois [96].…”

“…In that respect, it is known that electron-electron correlation is important at low energies and the IPM cannot describe the vertical drop of experimental values in the energy region close to the appearance energy, AE, (the lowest possible energy for each k-fold MI). The k-fold MI cross sections close to the AE can be approximated as a power law of the excess electron energy above the threshold, with certain exponent p, which is expected to be around the number of ionized electrons [46,47]. We will note later in this work that the low experimental values for this exponent are reasonable considering the PCI.…”

Improving accuracy and precision of ice core δD(CH<sub>4</sub>) analyses using methane pre-pyrolysis and hydrogen post-pyrolysis trapping and subsequent chromatographic separation

“…Specifically, we find decreased δD(H 2 ) values for higher water levels (generated in the ion source). Performing the same measurements at lower electron voltages (from 90 to 70 eV) reduces the effect and is generally recommended for δD(H 2 ) analysis due to the formation of 4 He 2+ at voltages >79 eV (Denifl et al, 2002) In any case, it is possible to create and maintain stable conditions for our complete system enabling robust δD(CH 4 ) measurements, that is, strictly following the "identical treatment" (IT) principle of samples and references (Werner and Brand, 2001). To provide each CH 4 -derived H 2 peak with identical background conditions, we stick to the previously described regular injections of either pure CH 4 in He or sample/reference air-derived CH 4 every 20 min (Bock et al, 2010a) and leave the open split inserted over the course of the day.…”

On the interference of Kr during carbon isotope analysis of methane using continuous-flow combustion–isotope ratio mass spectrometry

“…The following experiment uses the fact that, for the electron energies used in the ion source of the IRMS (e.g. 80 eV for the Bern system), ions with two (and sometimes three) electrons removed are also produced (Denifl et al, 2002;King and Price, 2008 peak. This focusing test has the advantage that it can be run on any system with any natural-air sample without modifications to the CH 4 separation system.…”