The Aging of Ions in Air and Nitrogen

Previous work on the mobility of aged ions in air has been extended to the measurement in air and in nitrogen of mobilities of ions at different short ages. The ions were produced by alpha-rays directly in the field used for their measurement and the average age was obtained from the time taken by the ions to cross between the two electrodes. In air containing 4 mg of water per liter, both kinds of ions of short age had the same mobility of 2.0 ${\mathrm{cm}}^2$/volt·sec while at an average age of 0.18 sec the negative ions moved 8 percent faster than the positive. In air partially dried by KOH the mobility of the negative ions decreased from 2.3 (above units) at 0.004 sec to 2.1 at 0.34 sec while the positive ion mobility decreased from 2.0 at 0.004 sec to 1.7 at 0.26 sec. In air dried by aid of liquid air traps the negative ion mobility decreased from 2.44 at 0.004 sec to 2.35 at 0.28 sec. The positive ions in this dry air were found to be very sensitive to minute changes in conditions that had no effect on negative ions. These changes are supposed to arise from small traces of some unknown impurity. Positive mobilities as high as 2.3 at 0.05 sec were obtained. The new information is applied to the explanation of the previous results for older ions. No transformation of one type of positive ion into another single type as observed by Erikson was found under the conditions of these measurements. The ions were all found to have the same mobility at the shortest ages but with increase of time the ion mobilities became spread over a range of values, the numbers given above corresponding to the peak of the distribution curves. The nitrogen used contained 0.3 percent of oxygen. The negative ions of short age all had a very high mobility which could only be roughly estimated as of the order of 100. With increase of age these fast ions gradually transformed into slower ions which were not all alike and had a peak mobility that decreased from 3.0 at 0.04 sec to 2.5 at 0.25 sec. The fast negative ions presumably remain free electrons for a large portion of their path and since their mobility does not appear to change with time it is necessary to suppose that these ions alternate frequently between being free electrons and monomolecular ions, but after more molecules become attached, the ion can no longer revert to the electron stage. The positive ions in nitrogen were always contained in one group only, the peak mobility of which ranged from 2.2 at 0.007 sec to 1.8 at 0.17 sec. The slow decrease of mobility of the ions with age and the large effect of slight changes apparently of gas composition upon the rate of this decrease for positive ions especially, indicate that the ions are gradually undergoing change of size owing to the accretion of scarce molecules or the exchange of such molecules for those which first gather about the central charge to form an ion cluster.