Capture cross sections for circular Rydberg states have been measured
as a function of the angle, phi, between ion velocity and angular mome
ntum of the circular orbital. The system studied is 2.5 keV Na-23+ on
Li(1s2,nlm) with n=25, l=n-1, and m=+l, where m is defined relative to
a weak, external magnetic field. A strong dependence on phi is found.
It is expected that studies such as the present will lead to an impro
ved understanding of the three-body problem in the region of sufficien
tly large quantum numbers and impact velocities for classical physics
to be accurate.