Self-collimation in an atomic beam evaporated from a superfluid 4He film

Using a scaling Monte Carlo method, we have simulated the scattering in a pulsed atomic beam evaporated from a superfluid 4He film. The simulation assumes that the atoms leaving the surface have the equilibrium Maxwellian distribution at the temperature of the film T. This means that the initial particle flux varies as cos straight theta (Lambert's law). We find that the effect of atomic scattering just above the film is to bias the flux in favor of the forward (small straight theta) direction, in agreement with the experiment of Eckardt et al. The simulation predicts that the deviation from Lambert's law grows rapidly with increasing heat input Q and decreasing pulse length. At the same time, the average kinetic energy per particle in the forward direction is enhanced relative to the global average, 2k(B)T. The distribution with respect to speed is narrower than Maxwellian in the forward direction but broader at large angles. We find that a beam that has passed through a slit is slightly narrower than in a ballistic calculation with no collisions. This effect seems to saturate at values of Q that correspond to 30 or 40 collisions per atom.