The Production and Measurement of Molecular Beams

A new method for studying molecular beams has been developed in which the beam is detected and its intensity measured by the increase in pressure which is produced in an ionization gauge when the gauge is moved to allow the beam to enter it through a narrow slit. This method has the advantage over other methods in use in that it is applicable to beams of molecules of non-condensable and chemically inactive gases. Many details of the construction of the new beam detector have been described and the characteristics of its operation have been studied both experimentally and theoretically. Simple kinetic theory fails to explain the observed pressure changes unless adsorption on the inner walls of the gauge is taken into account. A satisfactory hypothesis for interpreting the observations is that gas is adsorbed by the walls at a rate which is proportional to the molecular density in the volume of the gauge and liberated at a rate proportional to the number of adsorbed molecules. A value of ${10}^{-4\mathrm{}}$ per collision is found for the probability of adsorption of a mercury molecule on glass. The sensitivity of the gauge method of detecting a beam, although seriously impaired by adsorption, still compares favorably with that of other methods in use. A study of the relation between the beam intensity and the pressure in the source chamber has been made in which the ionization gauge method has been used for the measurements. The results show that beams of far greater intensity can be produced than were thought possible by Knauer and Stern who have made a similar investigation. The results of these investigators, however, are consistent with the results of the present investigation if an interpretation, different from that of Knauer and Stern, is adopted.