Scattering experiments are often not performed with the ideal set-up consisting of a monochromatic beam and target particles at rest. This is, for instance, never the case in molecular beam scattering, which is of primary interest in this paper. Two arrangements are normally employed, one consisting of a gas chamber as target (case a), the other of a crossed beam, generally at right angles to the incoming beam (case b). In both cases the scattering particles have a Maxwellian velocity distribution. This means that in any such experiment an "effective cross section" is measured, which is an integral over cross sections for different collision velocities. Other similar integrals are needed, if one works with polarized beams and takes into account the fact that the cross section is angular dependent. A third type of integral is required, when the primary beam is not monochromatized, and one has to average over its velocity distribution too. The functions needed to interpret scattering experiments of all the types mentioned are given in this paper, as well as a table of values for the 24 most important of these functions.

As is well known, van der Waals forces between two atoms (not both in an S-state) :Rave 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 substates m 7 = 89 and { of the upper (~P~) 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 -~ is 94"9 4-4-3 for Xe, 79"4-u 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 sealers, the sequence of alternate measurements could be automatized. In that way a comparison could be made every second, thus l~rgely 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 elementsums 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 discnssed.

Vor einigen Jahren wies RAMSAY 1 darauf hin, daß der totale Streuquerschnitt von Atomen von ihrer gegenseitigen Orientierung abhängen sollte. Ein Atomstrahl, der durch eine Streukammer geschickt wird, sollte daher nicht nur abgeschwächt sondern auch teilweise ausgerichtet werden (collision alignment). Dabei ist es notwendig, daß die