A thermodynamic model for ${{\rm{O}}}_{2}^{-}$ mobility in neon gas over broad density and temperature ranges

Abstract

We report new measurements of the mobility µ of O − 2 ions in supercritical neon in the range 45 K ≤ T ≤ 334 K for number density N ≥ 0.5 nm −3 . We rationalize the experimental data of all isotherms with the Stokes-Cunningham formula by computing the ion hydrodynamic radius as a function of T and N with the thermodynamic free volume model developed for the ion mobility in superfluid He. The model parameters are determined by re-analyzing published data for T = 45 K for N up to N ≈ 1.65N c (N c ≈ 14.4 nm −3 is the critical number density), which roughly span four orders of magnitude of the Knudsen number (0.1 ≤ K n ≤ 1000), covering the transition from the kinetic-to the hydrodynamic transport regime. These parameters provide an excellent description of the dependence of µ on N for all higher isotherms and yield a strict test of the model validity, thereby bridging the gap between the kinetic-and the hydrodynamic transport regimes.

O2 − ion mobility in dense Ne gas: The free volume model

“…Density dependence of the effective hydrodynamic radius for T=45 K. Experiment (point) [15]. Lines: prediction of the free model with (solid line) or without (dashed line) the slip correction factor [22]. independent.…”

“…We successfully adopted the free volume model to describe the mobility of oxygen ions in dense supercritical Ne gas [22]. We now present here new measurements in dense supercritical Ne covering a temperature range from just above the critical one to well above room temperature in order to complete the assessment of the validity of the free volume description of the crossover region between the kinetic and the hydrodynamic transport regimes.…”

“…Dashdotted line: hydrodynamic mobility in a nonuniform fluid[15]. Dotted line: ionic bubble model[22]. Open circles: Molecular Dynamics simulations[25].…”

Resonant low-energy electron attachment to O2 impurities in dense neon gas

“…in which k s and k d are the rates of collisional stabilization and dissociation, respectively. (k s + k d ) can be estimated from the measurements of the O − 2 ion mobility µ in dense neon gas [52][53][54] as (k s + k d ) = (e/MµN) where M is the atomic mass of neon. At the typical density of the 1st peak, N ≈ 40 × 10 26 m −3 and τ a (k s + k d ) N 8.…”

Thermodynamic Model for the Mobility of Oxygen Anions in Dense Neon Gas

“…The recent development of a thermodynamic model aimed at predicting the electron bubble radius in superfluid and normal liquid helium and in dense cold helium gas [25]- [29] in conjunction with the use of the Millikan-Cunningham interpolation formula to bridge the hydrodynamic regime behavior to the dilute gas behavior has allowed us to have a very nice agreement of the theoretical prediction with the arXiv:2202.03741v2 [physics.chem-ph] 9 Feb 2022 experimental data of the O − 2 ion mobility in dense neon gas even at temperatures close to the critical one [30,31], though paying the price of introducing some adjustable parameters. We want, however, to stress the fact that, if the physical picture underlying the mathematical formulation of the model were uncorrect, the adjustable parameters of the theory, to which we attach physical significance, would not show the coherent and, in some way, predictable behavior we observe.…”