Adsorption, desorption, and diffusion of nitrogen in a model nanoporous material. I. Surface limited desorption kinetics in amorphous solid water

The adsorption and desorption kinetics of N-2 on porous amorphous solid water (ASW) films were studied using molecular beam techniques, temperature programed desorption (TPD), and reflection-absorption infrared spectroscopy. The ASW films were grown on Pt(111) at 23 K by ballistic deposition from a collimated H2O beam at various incident angles to control the film porosity. The experimental results show that the N-2 condensation coefficient is essentially unity until near saturation, independent of the ASW film thickness indicating that N-2 transport within the porous films is rapid. The TPD results show that the desorption of a fixed dose of N-2 shifts to higher temperature with ASW film thickness. Kinetic analysis of the TPD spectra shows that a film thickness rescaling of the coverage-dependent activation energy curve results in a single master curve. Simulation of the TPD spectra using this master curve results in a quantitative fit to the experiments over a wide range of ASW thicknesses (up to 1000 layers, similar to 0.5 mu m). The success of the rescaling model indicates that N-2 transport within the porous film is rapid enough to maintain a uniform distribution throughout the film on a time scale faster than desorption.