Mobilities, Diffusion Coefficients, and Reaction Rates of Mass-Indentified Nitrogen Ions in Nitrogen

Abstract

Measurements at room temperature of the drift velocity, the longitudinal diffusion coefficient, and the transverse diffusion coefficient have been made for low-energy mass-identified nitrogen ions in nitrogen gas in a drift-tube mass spectrometer. These parameters were evaluated using an analysis described in the article immediately preceding this one. The mobilities of ${\mathrm{N}}^{+}$ and $\mathrm{N}_2^{}{}_{}{}^{+}$ were obtained over the $\frac{E}{N}$ range from 7 to 700 × ${10}^{-17\mathrm{}}$ V ${\mathrm{cm}}^2$, yielding zero-field reduced mobility values of 2.97 and 1.87 ${\mathrm{cm}}^2$/V sec, respectively. The mobilities of $\mathrm{N}_3^{}{}_{}{}^{+}$ and $\mathrm{N}_4^{}{}_{}{}^{+}$ were obtained over the $\frac{E}{N}$ range from 2 to 40 × ${10}^{-17\mathrm{}}$ V ${\mathrm{cm}}^2$, yielding zero-field reduced mobility values of 2.26 and 2.33 ${\mathrm{cm}}^2$/V sec, respectively. For ${\mathrm{N}}^{+}$ and $\mathrm{N}_2^{}{}_{}{}^{+}$, the longitudinal diffusion coefficients were determined from the widths of the experimental arrival time spectra, and the transverse diffusion coefficients were determined from the attenuation of the ion count rate as the drift distance was increased. For both ions the two diffusion coefficients were observed to be equal and in agreement with the Einstein relation at low $\frac{E}{N}$, but to behave quite differently as $\frac{E}{N}$ was increased. It has previously been reported that the longitudinal diffusion coefficients of both ions increase rapidly by more than an order of magnitude; the transverse diffusion coefficient of ${\mathrm{N}}^{+}$ has been found to increase in a similar fashion, although much less rapidly, while that of $\mathrm{N}_2^{}{}_{}{}^{+}$ remains nearly constant. Measurements were made up to an $\frac{E}{N}$ of 700 × ${10}^{-17\mathrm{}}$ V ${\mathrm{cm}}^2$. The rates of the two ion-molecule reactions, ${\mathrm{N}}^{+}$ + 2${\mathrm{N}}_2$ → $\mathrm{N}_3^{}{}_{}{}^{+}$ + ${\mathrm{N}}_2$ and $\mathrm{N}_2^{}{}_{}{}^{+}$ + 2${\mathrm{N}}_2$ → $\mathrm{N}_4^{}{}_{}{}^{+}$ + ${\mathrm{N}}_2$, were also measured by an attenuation technique over the $\frac{E}{N}$ range from thermal to 100 × ${10}^{-17\mathrm{}}$ V ${\mathrm{cm}}^2$. The reaction rates at thermal energy were determined to be 1.8 × ${10}^{-29\mathrm{}}$ ${\mathrm{cm}}^6$/sec and 5.0 × ${10}^{-29\mathrm{}}$ ${\mathrm{cm}}^6$/sec, respectively, and both rates were observed to decrease as $\frac{E}{N}$ was increased.