Negative Ca- and Ba- ions of large radius on the surface and in the volumeof liquid helium

Negative Ca- and Ba- ions of large radii on the surface of and in bulk liqu id helium have been studied. Our results indicate that these ions are adsor bed on the helium surface. Ions on free liquid helium surfaces have not bee n studied previously because it was thought impossible to confine them on t he surface. Ca- and Ba- ions have very low binding energies, therefore, lik e electrons, they form a bubble of large radius in bulk helium, whose energ y is higher than on the surface. The behavior of ions on the surface exhibi ts a number of previously unknown features owing to their large masses and strong localization in the horizontal plane. Even in the absence of confini ng electric field, a hole is formed under an ion due to the polarization at traction between the liquid helium and the charged ion. This hole formation reduces the ion mobility by several orders of magnitude and increases its effective mass severalfold. The critical density of electrons and ions is a pproximately the same on the surfaces of thin and thick helium films. (C) 1 999 American Institute of Physics. [S1063-7761(99)01502-4].