Mobility of Mercury Ions in Mercury Vapor

Measurements of mercury ion drift velocities and mobilities in mercury vapor by the Hornbeck technique have been made over a range of electric-field strength divided by reduced pressure (Ep₀) from 40 to 1500 V / (cm×{}mm Hg). Two stable mercury ions, believed to be Hg⁺ and Hg₂^_⁺, have been observed. The zero-field mobility of the two ions found by a long extrapolation gives the atomic ion Hg⁺ a zero-field mobility of 0.24±{}0.03 cm²/V-sec while the molecular ion mobility is estimated at 0.45±{}0.05 cm²/V-sec. The atomic ion value is in good agreement with theory; the molecular ion value is appreciably lower than the theoretical prediction. Charge exchange cross sections for Hg⁺ in Hg computed from present data are higher by a factor of nearly 1.5 than values measured directly but are qualitatively in line with rare-gas values measured by the present method. The secondary electron emission coefficient from a surface due to positive ion bombardment γᵢ has been calculated for the two cathodes used: γᵢ for molybdenum is estimated by 5×{}10^-5 electron per positive ion striking the cathode surface; γᵢ for an oxide-coated nickel surface is estimated at 2×{}10^-2 electron per positive ion striking the cathode.