ABSTRACT
In Section 1.1 we introduced the subject of atomic and molecular beam formation by stating that a beam with a cosine distribution is obtained when the beam-forming impedance is a circular orice and that a narrow slit has also been employed to produce an improved beam. Most relevant theoretical papers use the convention of referring to ‘molecules’ for both atoms and molecules, but we continue to retain the consistent use of ‘atoms’ to mean both atoms and molecules unless a distinction is necessary. Following the introduction of the basic equations of the kinetic theory of gases in Section 2.2, the relatively simple expressions for the beam formed by an orice were introduced in Section 2.3. It was also mentioned in Section 1.1 that when the impedance is a single tube a much narrower collimated beam is obtained and that by employing an array of tubes a greatly improved beam intensity is obtained. In this chapter we present the theory of beam formation to include the more difcult case of ow through a tube. We only briey mention calculations that only apply when Γ ≤ 10 where Γ is the ratio of length l to diameter d (Section 2.5). These are of little interest for atomic beam formation, but are needed for calculations of Knudsen cell operation, which are outside the scope of this work. Following the discussion of the basic theory, we present in the next chapter simplications that make the design of an atomic beam and the predictions of its performance easier. However, we warn that an extensive comparison of theoretical results with experiment in Chapter 11 points to an unsatisfactory disagreement between theory and experiment for the beam halfwidth, especially in the case of gaseous rather than vapour beams and also particularly when the beam-forming impedance is a focussing capillary array.
