Molecular Beams Formed by Arrays of 3–40 μ Diameter Tubes

Abstract

The results of an experimental investigation of molecular beams formed by a closely packed array of glass tubes of uniform diameter are presented. The sets of tubes used (which are commercially available) ranged in diameter from 3 to 40 μ with length‐to‐diameter ratios of 12 to 100, and were driven by pressures extending from 0.025 to 15 Torr. The experimentally determined functional variations for flux density, on‐axis intensity, and beam half‐width (conditions L≥λ and a «L) are: $$\begin{array}{c}\mathrm{Q/A=}\frac{3}{2}(\sqrt{2}{\text{/3π)(ν̄/σ}}^2\mathrm{)(L/\lambda )(\epsilon /L)(a/L),}\\ {\text{Ic/A=(1/8π}}^{\frac{3}{2}}{\mathrm{)(\nu ̄/\sigma }}^2{\mathrm{)(L/\lambda )}}^{\frac{1}{2}}{\mathrm{(\epsilon /L)(a/a}}_0{)}^{\frac{1}{3}},\\ {\theta }_{\frac{1}{2}}{\text{=(4/√π)(L/λ)}}^{\frac{1}{2}}\mathrm{(a/L).}\end{array}$$ L is the length of the tubes, a is the tube radius, and ε is the percent transparency of the area A of the array. It is found experimentally that the flux is approximately ⅔ of that predicted theoretically, the beam width is twice that predicted theoretically, and the prediction for center line intensity is modified by (a/a.)^⅓ The dependence of the beam intensity on the tube radius is attributed to self‐scattering due to the dense packing of many tubes.