Regime Change and Transitional Regime Behavior of C 60 Molecular Beams

Abstract We measure the flow regime change of C 60 molecular beams, produced by a Knudsen oven with an orifice (diameter, 1 mm; thickness, 0.5 mm), from effusive to transitional. We determine the change temperature to be about 800 K and find an inefficient translational cooling of C 60 molecules in the transitional regime. Such an inefficient cooling leads us to modify the velocity distribution function assumed standardly in directed molecular beams. The determined change temperature could be used as a benchmark for estimating the beam regime in experiments involving C 60 molecular beams prepared by a Knudsen oven with a thin millimeter-scale orifice.