Real-time measurement of residence times of gas molecules on solid surfaces

A new technique for measuring the surface residence times of gases under molecular flow conditions is described. A pulse of molecules is passed through a cylindrical tube where the molecules undergo multiple collisions with the wall. The transit time, measured using a mass spectrometer, is governed by gas kinetic theory and the magnitude of the gas-surface interaction per collision or surface residence time. Monte Carlo trajectory calculations of molecules diffusing across the tube were in excellent agreement with experimental results on non-interacting gases. Potentially sticky gases, such as H2O, HCl and NO2, as well as a number of ideal gases were investigated on three materials commonly used in vacuum science: pyrex glass, stainless steel and teflon. No deviation from ideal gas behaviour was observed for H2O, HCl and NO2 on teflon whereas we had to invoke a two-site adsorption model for both H2O and HCl interacting with pyrex and stainless steel.