On the design of capillary and effusive gas dosers for surface science

Abstract

We present a theoretical analysis of flux distributions and enhancement factors for dosers of two types: capillary arrays and arrays of effusive sources. Our analysis introduces an important distinction between the flux per solid angle and the flux per unit area of the sample, and encompasses array diameters both smaller and larger than the sample. We find, contrary to earlier assertions, that a single effusive source gives unacceptably large flux gradients unless it is located so far from the sample that its enhancement factor is negligible. Arrays of a small number of effusive sources located near the sample edge, however, are competitive with capillary arrays for doser-to-sample distances comparable to the sample radius, and are readily adapted to large or unusually shaped samples. Capillary arrays outperform effusive arrays at all doser-to-sample distances, and are particularly effective when placed very close to the sample. If flux uniformity is a concern, a capillary doser should generally be at least 20% larger than the sample. Additional effects due to trapping by cold surfaces in the chamber and multiple collisions of the emitted molecules are discussed qualitatively.