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Physical Review Letters

Positive-Ion Trapping on Vortex Lines in Rotating He II

Gary A. Williams, Keith DeConde, R. E. Packard

Published Apr 14, 1975• 52 citations
Publisher

Abstract

The direct trapping of positive He ions on quantized vortex lines in rotating He II has been observed at temperatures below 0.7 K. Trapping lifetimes have been measured between 0.1 and 0.7 K, and agree with models of thermal escape and charge loss from vortex motion. An ion radius of 7.7\ifmmode\pm\else\textpm\fi{}0.1 \AA{} is found.
DOI
10.1103/physrevlett.34.924

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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.
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.
We have studied negative-ion absorption by rotating He II from 0.98\ifmmode^\circ\else\textdegree\fi{}K to 1.7\ifmmode^\circ\else\textdegree\fi{}K. Measurements of vortex-line capture cross sections are reported as a function of temperature (0.98\ifmmode^\circ\else\textdegree\fi{}K to 1.7\ifmmode^\circ\else\textdegree\fi{}K at three electric fields (16.3, 49.0, and 97.9 V/cm) and as a function of electric field (10 to 200 V/cm) at two temperatures. Great care was taken to insure that the reported cross-section values were not affected by space-charge effects. The results are found in good agreement with the ion-capture theories developed by Donnelly and Roberts. The decrease in cross section above 1.6\ifmmode^\circ\else\textdegree\fi{}K is in agreement with the escape calculation of Parks and Donnely and supports their value of 15.96 \AA{} for the negative-ion radius.