Mass-Spectrometric Study of Ions at Near Atmospheric Pressures. I. The Ionic Polymerization of Ethylene

Abstract

An apparatus is described which allows the mass‐spectrometric analysis of ions produced by the alpha‐particle irradiation of gases in the pressure range 5–200 Torr. The ions emerge from the field free ionization chamber by effusion. This method of sampling should give an (approximate) picture of the steady state ionic concentration. A simplified kinetic treatment based on the steady state assumption can explain the experimental results. The mass spectrum of ethylene at 40 Torr pressure is very complex. It extends beyond the highest mass (215) measured. The major ions are C_nH_2n−1⁺. A simplification of the reaction system is achieved with a dilution—charge‐exchange technique. The ethylene spectrum obtained from 1% ethylene in xenon gives the ionic polymerization initiated by C₂H₄⁺. The most abundant series is (C₂H₄)_n⁺. Xenon is deactivating the excited addition products. Low ionization potential impurities or reaction products, present in less than parts‐per‐thousand concentration, can still change the ionic population profoundly. This is of significance to conventional radiolysis work. Temperature increase from 20°—60°C has little effect on the ethylene spectra. Other gases or gas mixtures show large changes because of cluster rearrangement.