Monte Carlo Simulation of Specular and Surface Diffusional Perturbations to Flow from Knudsen Cells

Thermodynamic study of the CsOH(s,l) vaporization by high temperature mass spectrometry

“…Using different positioning of the effusion orifice in a plane perpendicular to the ionization chamber axis (restricted collimation axis), it is possible to evaluate the percentage of the diffusion contribution. We know, according to Winterbottom and Hirth [20] and Ward et al [21] that the emission of molecules decreases sharply when starting from the orifice edge (as depicted in figure 4B, position b) and we can postulate that with our large observed zone (our penumbra zone) we detect all the surface contribution: more simply the whole surface diffusion flow is evaporated before reaching the external limit of the observed penumbra zone. This is readily checked by comparing the penumbra diameter from the orifice edge with the extinction of the signal at about x = 3 mm (see figure 4A).…”

“…Figure 4A) showed systematically that the profile becomes sharper when temperature increases, meaning that the contribution of the flow out decreases relatively to genuine effusion at high temperature. This is typically related to a surface diffusion phenomenon occurring along the effusion orifice walls: the total contribution according to such a phenomenon was analytically resolved by Winterbottom and Hirth [20] and numerically (Monte-Carlo method) analyzed by Ward et al [21]. In figure 4B, position (a) depicts the observation by our restricted collimation of the orifice of a Knudsen effusion cell with a saturated concentration n o (eff) for evaporated molecules i.e.…”

Monte Carlo simulation of a Knudsen effusion mass spectrometer sampling system

“…As noted, many investigators have written Monte Carlo codes to simulate Knudsen flow. [12][13][14][15][16][17][18][19] Molecule-molecule collisions are minimal and hence the linear trajectory of each molecule can be traced individually until it either exits the upper orifice or goes out of the beam before that. The details of the code are discussed in the companion report to this paper.…”

“…[12] Since then, many investigators have further extended this Monte Carlo approach. [12][13][14][15][16][17][18][19] Today, because of the wide availability of high-speed desktop computers with multicore processors, Monte Carlo simulation is one of the easiest and most flexible ways to describe Knudsen flow.…”

The Surface Diffusion of High Temperature Vapors in Porous Alumina

“…However, Winterbottom and Hirth used data for silver vapor on nickel and molybdenum surfaces to support their theoretical conclusion that the greatest surface diffusion contributions will be found with orifices of small diameters and small length-to-radius ratios or knife-edges. Ward et al [8,9] There has been little experimental data with which to compare this model. Boyer and Meadowcroft [10] have measured the free energy of vaporization of silver under conditions which Winterbottom and Hirth predict will promote surface diffusion.…”