228 cases would be to compare 7p with yy at a somewhat different value of the excitation energy, but in other cases, it may be more plausible that 6) ¥ 64 . Of course, whenever 6p ¥ dy , Figure 17 is not applicable, and the (ap/tu)o Values in Figure 170 are not necessarily the maximum 7p/7yq ratios. Another difficulty arises when the absorption cross sections, c, are different for the two isotopes (gq * gp). Then it is not clear whether one should use 7p/y ratios or ratios of the photoionization cross sections, ¢;(¢; = no), and one should compare the results ealeulated using both the 7p/ny ratio and the oip/oin ratio. If multiple ionization occurs, then the simple Platzman mechanism may be inappropriate. Furthermore, there are alternative explanations”' ” of isotope effects in ionization that do not use the kinetics proposed by Platzman. REFERENCES 0.2 03 04 O35 0.6 oO? 0.8 0.9 1.0 R Fig. 171—The maximumfraction of the ionization produced by direct ionization for the protonated molecule (¢/yn) as a function of &. See equation (6) for the definition of R. The nine separate curves are each labelled with the ya value for that curve. 1. Platzman, R. L. J. Phys. Radium 21, 853 (1960); Radiat. Res. 17, 419 (1962); 7. Chem. Phys. 38, 2775 (1963). . Plataman, R. L. Vortex 23, 372 (1962). ob Ww be ol . Jesse, W. P. and Platzman, R. L. Nature 195, 790 (1962). . Person, J.C. and Nicole, P. P. J. Chem. Phys. 49, 5421 (1968). . Johnson, P. M. and Riee, 8S. A. J. Chem. Phys. 49, 2734 (1968). 6. Krauss, M., Walker, J. A., and Dibeler, V. H. J. Res. Natl. Bur. Std. 724, 281 (1968). THE EFFECT OF PRESSURE UPON IONIZATION N PURE RARE GASES H.A. Schultz Components of the ion chambers designed to measure the effect of pressure upon the ionziation of pure rare gases were cleaned, assembled, and checked for satisfactory operation. Several portions of the apparatus were modified because they were unsatisfactory in their original form. A fast particle traversing a medium produces ioni- zation and excitation within the material. If the medium is a pure rare gas, the value of W, the average energy lost by the particle per ion pair produced, can be expected to vary with gas pressure; this is because rare gas atoms that are excited to certain high energy levels can release their excess energy through two competing processes: photon emission and assoclative ionization. The latter is a collision process in which an excited atom combines with an atom in the ground state to yield a diatomic positive ion and a free electron. This reaction is often called the Hornbeck-Molnar effect. At sufficiently high pressures the collision process will be more probable than the emis- sion process, and at lower pressures the reverse will be true; this implies that W must decrease with increasing pressure. The previous report) described briefly an appara- tus designed to utilize two ionization chambers of similar geometry but different dimensions to measure the effect. of pessure upon W for neon and argon. Af- ter the stainless steel chambers had been fabricated in the shops, they were disassembled in the laboratory, cleaned carefully with solvents to remove foreign materials that had been picked up on the surfaces, then reassembled. It is possible that trace quantities of gaseous impurities desorbed by surfaces inside the chambers will produce relatively few spurious ions, or a relatively great number; it will depend upon the ionization potential of the impurity and, to a very great extent, upon the decay scheme associated with the photon emission processes of the rare gas atoms. The effect of small traces of impurities can be small if all the excitation energy of a rare gas atom is lost in