Determination of mobility and diffusion coefficients of Ar + and Ar 2+ ions in argon gas
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ORIGINAL RESEARCH
Determination of mobility and diffusion coefficients of Ar+ and Ar2+ ions in argon gas Jamiyanaa Dashdorj 1 & William C. Pfalzgraff 2 Larry A. Viehland 2
&
Aaron Trout 3 & Delenn Fingerlow 2 & Michelle Cordier 3 &
Received: 20 November 2019 / Revised: 11 January 2020 / Accepted: 14 February 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract It has recently been shown that accurate theoretical calculations can be used to calibrate a drift-tube mass spectrometer (DTMS) to measure gaseous ion mobilities accurate to within 0.6%. Here we present a new method for calibrating a DTMS instrument to obtain diffusion coefficients parallel to the electric field which are accurate to within 8%. This method is developed and verified by consideration of He+ (2S1/2) ions in He. We apply these techniques to determine transport coefficients for Ar+(2P3/2) and Ar2+ (3P2,1,0) ions in Ar gas at 300 K, with results given as a function of E/N, the ratio of electrostatic field strength to gas number density, in the range 30–210 Td. The measured mobilities are accurate within 0.8%; for Ar+ they agree within 1.5% with Monte Carlo simulations, and for both the cations and dications they are in excellent agreement with previous measurements. Our method gives new diffusion coefficients that agree within 5% with quantum Monte Carlo calculations. Keywords Gaseous ion mobility . Argon ions . Ion diffusion coefficients
Introduction An accurate method to determine the mobilities and diffusion coefficients of gaseous ions is important not only for understanding ion-neutral interactions and charge transfer processes [1], but also for further improvements in ion mobility spectrometry, a technique used to detect chemicals like food contaminants, illegal drugs, and explosives, as well as in other applications like defining protein structures [2–5].
Jamiyanaa Dashdorj and William C. Pfalzgraff contributed equally to this work. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s12127-020-00258-z) contains supplementary material, which is available to authorized users. * William C. Pfalzgraff [email protected] 1
Department of Physics, Chatham University, Pittsburgh, PA 15232, USA
2
Department of Chemistry, Chatham University, Pittsburgh, PA 15232, USA
3
Department of Mathematics, Chatham University, Pittsburgh, PA 15232, USA
Recent work has shown that a drift-tube mass spectrometer (DTMS) can be calibrated to measure gaseous ion mobilities that are accurate to within 0.8% using highly accurate theoretical values [6]. In principle, accurate diffusion coefficient measurements provide an even more sensitive probe of the ion-neutral interaction potential. However, experimental values of gaseous ion diffusion coefficients still have estimated errors of 10–20% [6, 7]. Qualitatively, the longitudinal diffusion coefficient (along the direction of the electric field), DL, is related to the widths of the arrival time spectra measured with the DTMS. This makes di
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