Two-way traffic between molecules and moving surfaces: evaporation-rates and rotor-propelled beams

Abstract

Prompt evaporation of molecules swept up from a localized gas-cloud is important for efficiency in the generation of molecular beams by rapidly rotating shafts. It is easy to show that, for many substances of interest at experimentally convenient temperatures, the mean lifetime Ƭ on the condensed state lies between 1 and 10 μs divided by the vapour-pressure P at the surface temperature T , with P measured in torrs (1 Torr ≈ 133 Pa). Reasons are given why, for the actual rotor surface Ƭ may often be shorter and need seldom be much longer. In experimental situations designed for high intensity, P may be a few torrs and the input gas pressure p a few millitorrs; the input gas flow is likely to be a few litres per second and a substantial proportion could be put into a 2 km s -1 beam of angular divergence 0.2 rad and intrinsic intensity approaching 10 18 sr -1 s -1 . A converse connection between surface physics and molecular beam technique lies in the possibility of deducing surface lifetimes from the azimuthal distributions of rotor-generated beams.