No abstract

Data are presented for effusion probabilities and cell-wall flux gradients calculated dynamically by means of Monte Carlo computer studies. Typical effusion cell dimensions were programmed for both knife-edged and Clausing orifices; all calculations apply to a flat sample. The influence of the size and the length-to-radius ratio of the orifice and the effects of different condensation coefficients are described. An analytical expression relating the equilibrium sample flux to the flux at steady state has been derived. It is shown that for a given cell design, the ratio of these fluxes is constant and independent of the nature of the vaporizing species. Because the Monte Carlo method requires no mathematical approximations or simplifying assumptions, results analogous to real cell behavior are to be expected. Comparison of these results with the treatment derived by Motzfeldt shows that his formula is applicable and quite accurate over a wide range of experimentally encountered conditions. 7 J. W. Ward and M. V. Fraser, "Study of Some of the Parameters Affecting Knudsen Effusion. VI. Monte Carlo Analyses of Channel Orifices," J. Cbem. Phys. (to be published).

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.

Computer simulation of real experiment by Monte Carlo techniques has been extended to include the study of particulate fluxes in divergent conical Knudsen cell orifices. The effects of specular reflection and surface diffusion have been considered, in terms of the cone angle and length-to-radius ratio of the orifice. Large statistical sample sizes resulted in precise steady-state values for transmission coefficients, spatial distributions of effusate, and evaporation flux gradients. The results of other studies have been analyzed and interpreted in light of the Monte Carlo data.