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Physical Review Letters

Effect of He3 Impurities on the Lifetime of Ions Trapped on Quantized Vortex Lines

Gary A. Williams, R. E. Packard

Published Jul 28, 1975• 44 citations
Publisher

Abstract

The lifetimes of both positive and negative ions trapped on quantized vortex lines in rotating $^{3}\mathrm{He}$-$^{4}\mathrm{He}$ mixtures have been measured at temperatures down to 0.1 K. Evidence is found that the $^{3}\mathrm{He}$ condenses at the vortex core, and possibly at the surface of the electron bubble.

DOI
10.1103/physrevlett.35.237

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The lifetimes of both positive and negative ions trapped on quantized vortex lines in rotating 3He^{3}\mathrm{He}3He-4He^{4}\mathrm{He}4He mixtures have been measured at temperatures down to 0.1 K. Evidence is found that the 3He^{3}\mathrm{He}3He condenses at the vortex core, and possibly at the surface of the electron bubble.
The mobility of negative ions trapped on linear vortices in rotating superfluid helium has been measured by a time-of-flight technique in the temperature range $0.8\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}<T<1.6\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}$. Measurements could not be extended below 0.8\ifmmode^\circ\else\textdegree\fi{}K because, contrary to the present theory, the ion escape rate became too large. No evidence of direct trapping of positive ions by vortex lines was observed down to 0.4\ifmmode^\circ\else\textdegree\fi{}K. The trapped-ion mobility is interpreted in terms of scattering of the ion by thermal distributions of quantized vortex waves and rotons localized about the vortex axis.
The motion of negative ions trapped in quantized vortex lines in rotating helium II has been measured down to 0.8\ifmmode^\circ\else\textdegree\fi{}K by means of a time-of-flight apparatus. Trapped ions always move more slowly than free ions at the same temperature. The contribution of the extra retardation due to the vortex line appears to vary as $\mathrm{exp}(\ensuremath{-}\frac{\ensuremath{\Delta}}{\mathrm{kT}})$, where $\frac{\ensuremath{\Delta}}{k}=1.87\ifmmode^\circ\else\textdegree\fi{}$K between 0.8 and 1.5\ifmmode^\circ\else\textdegree\fi{}K.
The decay of quantum turbulence in superfluid (_^4)He in the zero temperature limit wasinvestigated. Quantum turbulence was created by impulsive spin-down from a specific angular velocity to rest. A measurement of the vortex line density was performed using the scattering of charged vortex rings. The vortex line density decayed as L ? t^(-3/2) which is the characteristic of quasiclassical turbulence forced at large length scales. The interaction of metastable spin-triplet ?He?_2^* molecules with quantized vortex lines in superfluid (_^4)He at temperatures below 200 mK was studied too. The molecules were generated during an injection of electrons from a sharp metal tip at high voltage. They were detected as a current into a metal collector after ionization upon colliding with the collector surface. The detected current was suppressed by even a small rotation indicating the trapping of the molecules on quantized vortices. The presence of (_^3)He impurities at the level of 0.3 ppb strongly suppressed the detected signal. The temperature dependence of the detected signal showed a sharp peak, most probably associated with the condensation of (_^3)He atoms on vortex cores. The time of flight of the molecules as a function of temperature was measured. It was observed that there are three regimes for ?He?_2^* molecules transportation in superfluid (_^4)He : ballistic regime for T 200 mK, and an intermediate regime between them. The vortex lines were created by either rotation or ion injection. The trapping diameter of the molecules on quantized vortices was found to be 96 � 6 nm at pressure of 0.1 bar and 27 � 5 nm at 5.0 bar. It was also demonstrated that a tangle of vortices moving in superfluid (_^4)He are capable of conveying the ?He?_2^* molecules through the drift region.
Measurements have been made of the lifetime for the escape of negative ions trapped in rotating helium II as a function of temperature and pressure. These measurements concern lifetimes in the range from 10 to 1000 sec, and have been made between the temperatures of 1.09 and 1.68\ifmmode^\circ\else\textdegree\fi{}K from the vaporization pressure to the solidification pressure. At constant pressure the lifetime decreases rapidly with increasing temperature. The curves of constant lifetime in the pressure-versus-temperature plane have negative slopes dPdT\frac{\mathrm{dP}}{\mathrm{dT}}dTdP​ and positive second derivatives d2PdT2\frac{{d}^{2}P}{d{T}^{2}}dT2d2P​. The results have been interpreted in terms of the bubble model of the negative ion and the vortex-line model of the rotating superfluid. The interpretation suggests that the ion radius equals approximately 19.5 A at saturated vapor pressure and decreases to a value between 12 and 14 \AA{} at the solidification pressure. However, a significant discrepancy between theory and experiment casts some doubt on the radius values obtained.