Average diffusion-to-the-wall times for laser-tagged molecules in a long cylinder

An approximate closed-form expression is derived for the average wall-reach ing rate k(w) or time tau (w) = k(w)(-1) of laser-tagged molecules diluted in an inert carrier gas migrating to the wall of a long cylinder where they are removed with sticking coefficient eta (w) The exact equation for the s ticky wall problem requires a trial-and-error solution of a transcendental relation with Bessel-functions. With the aid of the Fermi-Amaldi albedo the ory, an explicit "compromise" relation is derived which approximates the ex act diffusion curve of kw versus eta (w) rather well. The result is applied to obtain curves' of k(w) versus eta (N) at different gas mix pressures fo r UF6 diluted in N-2.