Study of Some of the Parameters Affecting Knudsen Effusion. VI. Monte Carlo Analyses of Channel Orifices

Abstract

Techniques for Monte Carlo simulation of real particle behavior have been extended to study the performance of channel orifices. Transmission coefficient calculations and wall-flux gradient behavior are considered. For the particular case L / R = 4.0, functions are derived to allow for variable orifice-to-collimator distances when the target collection method is used. A “state-of-the-art” discussion on orifice behavior is included to help delineate the purposes of this study. It is concluded that use of both knife-edged and channel orifices is sometimes necessary to establish the cell-orifice behavior of some systems; information derived from experimental studies, and from Monte Carlo analysis of flux relationships appropriate to the geometry of the system, seems to indicate that the channel orifice is a better choice for most effusion work.

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

“…The usual solution to obtain so small ratios is to decrease the orifice effusion diameter at constant cell diameter. This solution, as discussed by Ward and Fraser [19], presents the disadvantage to increase the relative proportion of surface diffusion contributions along the orifice walls and at the external surface (see our further observations) to the genuine effusion flow which is the useful one to characterize vaporization. Ward and Fraser [19] recommended the use of large and cylindrical orifices, this solution being adopted systematically for our mass spectrometric analysis.…”

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.…”

Knudsen cell mass spectrometry using restricted molecular beam collimation. I. Optimization of the beam from the vaporizing surface

“…Among these aspects, difficulty in producing a genuine molecular beam fully representative of the vaporization equilibrium of the samples in the Knudsen cells is partly related to the systematic surface evaporation or re‐vaporization contributions of the same molecules from the vicinity of the effusion orifice, as directly analyzed by mass spectrometry by Chatillon et al . The physicochemical processes leading to these contributions were investigated by many groups in the 1960s and were for the most part summarized by Winterbottom and co‐workers and Ward et al ., as relating to the surface diffusion of adsorbed vapor species along the effusion orifice walls and specular reflection on these walls. In order to definitively eliminate these spurious contributions and especially when determining the activity in mixtures, Martin‐Garin et al .…”