Helium Ions at 76 °K: Their Transport and Formation Properties

The time dependences of the ion wall currents in a 76 °K helium afterglow were measured with a quadrupole mass-filter system. The species observed were ${\mathrm{He}}^{+}$, ${\mathrm{He}}_2^{+}$, ${\mathrm{He}}_3^{+}$, ${\mathrm{He}}_4^{+}$, and metastable atoms. The results of the present work are a zero-field mobility, ${\mu }_0\left({\mathrm{He}}^{+}\right)$, of 14.4 ± 0.3 ${\mathrm{cm}}^2$ ${\mathrm{V}}^{-1\mathrm{}}$ ${\sec }^{-1\mathrm{}}$, and a three-body conversion frequency of ${\mathrm{He}}^{+}$ into ${\mathrm{He}}_2^{+}$ of $(1.44\mathrm{}\pm 0.14)\times {10}^{-31\mathrm{}}\times N^2$ ${\sec }^{-1\mathrm{}}$, where $N$ is the density in atoms/${\mathrm{cm}}^3$. The diffusion coefficient that describes the late-time behavior of ${\mathrm{He}}_3^{+}$ was found to be 95.3 ± 8.6 ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$ Torr. The metastable helium atoms at 76 °K were found to have a diffusion coefficient given by $D_m{p}_0=146\mathrm{}$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$ Torr. Evidence is presented for the presence of helium negative ions in the 76 °K helium afterglow.