Impurity ions in liquid helium

Abstract

A technique for introducing a wide variety of positive ions into liquid helium is described and used to investigate positive atomic ions of K, Rb, Cs, Ca, Sr, and Ba. The zero-field mobilities of the impurity ions were measured in He II in the temperature range 1.2 < T < 1.4 K. The dependence of mobility on ion mass alone was found directly for Ca + ions by investigating two isotopes. Measurements were made of the ion drift velocities in high electric fields, where nucleation of quantized vortex rings occurs. All the data and its analysis are consistent with the assumptions thai ull the impurity ions are singly charged, that the size of the cluster s~,-rounding the ion is significantly different for the different ions, and that the ion-roton cross section is approximately geometric. A possible alternative model for the structure of the ion complex is discussed and a mechanism is suggested which may account for the observed variations in ionic radii.

Atomic bubbles in impurity-stabilized solid He4

“…The sample is prepared in three steps: solidification of pure 4 He, doping the 4 He crystal with Cs or Ba, and then melting the pure solid He surrounding the doped part of the crystal. The experimental setup is described in detail in Ref.…”

“…The helium crystal is grown in a copper cell with an inner volume of 170 cm 3 by applying pressure from an external He reservoir. The cell is immersed in superfluid 4 He cooled to 1.5 K by pumping on its surface.…”

“…We stop the release of He pressure when the liquid-solid interface drops below the cell windows. The experiments described in the present paper were performed with such a sample, with the temperature and pressure varied along the melting curve of pure 4 He. The results are compared with those obtained in bulk solid He.…”

“…In order to check whether the crystalline structure of the iceberg is always bcc or whether it corresponds to the structure of bulk pure 4 He under the same pressure and temperature, we have varied the temperature of the sample, moving along the melting line in the phase diagram of pure 4 He (line DE in Fig. 1).…”

“…In liquid helium it results in the formation of a solid helium shell around the positive ion, a structure called snowball. The optical absorption and fluorescence spectra of alkali atoms isolated in liquid and solid 4 He matrices are described with high accuracy by the atomic bubble model [5,6]. The spectra depend on specific macroscopic matrix properties, such as its molar volume and (anisotropic) elasticity constants, and provide thus information about the quantum matrix.…”