The Mobility of NA+ Ions in N2 and H2 as a Function of Time

Abstract

The mobilities of ${\mathrm{Na}}^{+}$ ions from a Kunsman source have been measured in ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$ using gases of considerable purity and time intervals ranging from ${10}^{-5\mathrm{}}$ to ${10}^{-1\mathrm{}}$ seconds. The method used was the Rutherford A.C. method using both sinusoidal and square wave form oscillations. With corrections for temperature and pressure included the method gives reduced absolute mobility constants for the ions good to about 10 percent. The results indicated three distinct classes of ions in ${\mathrm{H}}_2$ and probably three in ${\mathrm{N}}_2$. In ${\mathrm{H}}_2$ there is a fast ion of mobility 17.5 cm/sec per volt/cm which is presumably the atom ion ${\mathrm{Na}}^{+}$ which exists up to time intervals of ${10}^{-4\mathrm{}}$ seconds. It then undergoes an abrupt transition to an ion of mobility of about 13.5 cm/sec per volt/cm whence, after some ${10}^{-3\mathrm{}}$ seconds, it gradually transforms to the normal ion in ${\mathrm{H}}_2$ which has an absolute mobility of 8.4 cm/sec. per volt/cm. The intermediate ion is presumably a single molecular addition product, (possibly water vapor) to the initial ${\mathrm{Na}}^{+}$ atom ion which retains its charge. The final so called normal ion may result from a gradual growth of the ion cluster by further addition of molecules, or by a gradual interchange of one or two primarily attached molecules to molecules of impurity that are strongly attracted but less prevalent, gradually making a stable ion. In ${\mathrm{N}}_2$ the initial ion observed up to 5×${10}^{-3\mathrm{}}$ seconds has a mobility of 3.75 cm/sec. per volt/cm. and probably changes abruptly to an ion of about 3.0 cm/sec. per volt/cm at the end of the interval. At the end of about ${10}^{-2\mathrm{}}$ seconds it has transformed to the normal positive ion of 1.6 cm/sec. per volt/cm mobility. The interpretation is similar to that suggested for ${\mathrm{H}}_2$. The mobilities of 17.5 and 3.75 for ${\mathrm{Na}}^{+}$ ions in ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$ give the first opportunity to check the theoretical mobility equations since in these cases the nature of the ion is known. They show that the Langevin theory assuming forces of dielectric polarization of an inverse fifth power type yields values of the mobility that are about 20 to 40 percent too low. This result was to be expected since it seems unlikely that ordinary forces of dielectric polarization observed in weak homogeneous fields should be capable of extrapolation to the huge inhomogeneous fields existing within a molecular diameter or two of an ion. The amount of deviation is however gratifyingly small and should prove of value in extending the theory.