The Mean Free Paths of Cesium Atoms in Helium, Nitrogen, and Cesium Vapor

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The mean free paths of cesium atoms in helium, nitrogen, and cesium vapor were measured with a molecular beam apparatus permitting the measurement of scattering deflection of about 5 seconds of arc. Utilizing the free fall of the beam atoms, the variation of the mean was found to be in agreement with the equations calculated on the basis of classical theory treating the atoms as elastic spheres. The measured values of the sums of the effective atomic radii are 12.0×{}10^-8 cm for Cs-He, 17.2×{}10^-8 for Cs-N₂, and 27.3×{}10^-8 cm for Cs-Cs collisions. These values correspond to mean free paths of 2.1 meters in He, 2.2 meters in N₂, and 1.36 meters in Cs-vapor of 10^-6-mm Hg pressure for cesium atoms with a velocity corresponding to the maximum intensity of the gravity deflection curve.