Messung der Anisotropie der van der Waals-Kraft

Abstract

As is well known, van der Waals forces between two atoms (not both in an S-state) have no spherical symmetry. We report on an experiment, which verifies this anisotropy by measuring it.

The experiment consists in comparing the total scattering cross sections of Ga atoms in selected magnetic substatesm _J=1/2 and 3/2 of the upper (² P _3/2) level of the ground state doublet of Ga. Rare gases are used as the scatterer. The relative total cross section difference in units of 10⁻⁴ is 94.9±4.3 for Xe, 79.4±6.6 for Ar, and 195±5 for He as the scattering gas.

In addition, measurements of the total scattering cross sections of Na and Ga on various gases have been made.

The apparatus uses an atomic beam with magnetic selection, a common geometry, and a common detecting scheme for both states, which are compared in short intervalls by merely switching the selecting magnet's current. By converting the beam intensity to a pulse rate and summing the numbers of pulses in scalers, the sequence of alternate measurements could be automatized. In that way a comparison could be made every second, thus largely eliminating most drifts, which otherwise would have made impossible an intensity measurement of that accuracy.

A theoretical estimate of the experimental results has been made on the following basis: 1. Only van der Waals forces are taken into account. 2. Matrix element sums in the expression for the van der Waals potential are approximately reduced to terms involving only the polarizability of the rare gases and matrix elements of Ga, which can be calculated from Hartree-Fock functions. 3. The Schiff approximation is used to calculate the scattering cross section. The calculations are in rough agreement with experiments, and errors are discussed.