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A new mass-spectrometric method for the study of solvation of ions and of ion complexes is applied to the ammonium ion. The major ion in the α-particle radiolysis of NH3 is the clustered ammonium ion NH4+·nNH3. A study of the distribution of these clusters is made at room temperature and in the pressure range 0.05 to 200 Torr ammonia. Clusters up to n=20 can be observed. The intensity distribution of the ions shows that the inner solvation shell contains four molecules. Approximate free-energy changes for inner- and outer-shell solvation are obtained. Studies of competitive solvation of the NH4+ ion by water and ammonia molecules confirm the inner four shell. Ammonia is preferentially taken up in the inner shell and water in the outer shell. The higher stability of the ammoniated inner shell requires a very small effective NH4+ radius. This requirement leads to an examination of the structure of the NH4+·4NH3 ion.

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 CnH2n−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.

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