Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

J. A. Northby


In this research we have studied the structure of +He ions in He vapor at temperatures between 1.32K - 4.22K and at saturation ratios between 0.05 - 1. Classical macroscopic thermodynamics predicts formation of a liquid drop around the ion, and the drop radius is given by the Thomson equation. In the above temperature and pressure ranges the radius of the drop varies between 6A - 9A. An experimental verification of the Thomson equation shows the validity of the macroscopic thermodynamics when it is applied to microscopic systems and also gives information about the drop structure. To show the existence of the drops and to determine their sizes experimentally, we have measured the mobilities of +He ions in He vapor in the above temperature and pressure ranges. The mobility is related to the radius of the drop through the momentum transfer cross section. Hence the drop size can be determined from the mobility data if the interaction potential and the nature of coll is ions between the charged drop and the neutral vapor atom is known. We have assumed that the interaction potential is the sum of the polarization potential between the central ion and the neutral vapor atom, and the van der Waals interactions between the vapor atom and each of the liquid atoms in the drop. With the above potential the "experimental" drop radius is calculated in the elastic and "inelastic" models. Quantum corrections are made for the elastic model. The Thomson equation predictions were compared with the "experimental" radii and a good agreement was found. This comparison also showed the existence of a solid core within the liquid drop. The classical macroscopic thermodynamics was applied successfully to calculate the solid core radius. Finally, the temperature dependencies of the "experimental" radii showed slight variations from the predictions of the Thomson equation at T<2.3K. The deviations reach their maximum at ~1.9K. The existence of superfluid transition in the liquid helium layer of the ion-solid-liquid complex is suggested as an explanation of the temperature dependencies of the "experimental" radii. The proposed transition temperature is ~1.9K and it is broadened up to ~2.3K. The transition starts when the liquid thickness becomes more than a monoplayer.