VELOCITY DISTRIBUTIONS IN K AND TI BEAMS 1321 close to the value found by Mais and Rosenbergl4 for the scattering of potassium beams by nitrogen. cath the property that a non-Maxwellian distribution using through a nonideal aperture gives a distribution The probability that an atom will travel from the experimentally indistinguishable from that arising ven slit to the detector wire without a collision under our assumptions. An ideal aperture is one in which and the detector wire, 60 cm. Then Pn can be calculated precisely to meet this condition within the range of from Eqs. (9)and (10). If Eq.(5)is multiplied by the experimental feasibility. The agreement between theore- appropriate value of Pu when account is taken of the tical prediction and observed distributions is there- variation of the pressure with rotor velocity, a dis- fore, not fortuitous and the result of the present work tribution is obtained whose shape is only trivially demonstrates within relatively narrow limits of error distorted on the scale of Figs. 4 and 5. In view of the that a gas in thermal equilibrium with its surroundings uncertainty in the composition of the gas, the gas possesses a Maxwellian velocity distribution temperature, and the related cross sections, the result In view of the success of this work, a study of the is subject to a corresponding uncertainty. However, the velocity distributions of alkali halide molecular beams excellent results for low and moderate beams of potas- has been made, in which it has been assumed that the sium and thallium make it clear that any distortion due experimental velocity spectrum is the superposition to scattering by the residual gas in the apparatus must of the Maxwellian velocity distribution of the molecular components which are in thermal and chemical equil brium within the oven chamber. The results of this DISCUSSION AND CONCLUSIONS work will be published elsewhere The results presented in this work demonstrate that he experimental velocity distribution can be predicted ACKNOWLEDGMENTS velocity spectrum in the oven is Maxwellian and that many helpful discussions. The authors also take this effusion occurs through an ideal aperture. It may be opportunity to express their appreciation to Mr Joseph thought that the excellent agreement is somewhat Haas of the Columbia Radiation Laboratory for the fortuitous, i. e, that an oven slit design was discovered construction of the cylindrical velocity selector witl 16W. H. Mais, Phys, Rev. 45, 773(1934) the 702 perfectly milled slots