Ionization processes in the positive column of the low-pressure Hg-Ar discharge

Abstract

Recent experiments and calculations have indicated that mutual collisions between excited 6 3P0, 6 3P1, and 6 3P2 atoms may be important for the ion formation in low-pressure Hg–noble-gas discharges. We calculate various rates for electron-impact and for excited-atom-collisional ionization for a range of conditions relevant to the Hg-Ar fluorescent lamp (pAR=3 Torr, R=1.8 cm, Twall=20–80 °C, and I=0.2–0.8 A). In the analysis use is made of experimentally determined values for particle densities and electron temperatures. Deviations from a Maxwellian electron-energy distribution, which substantially reduce the electron-impact ionization rates, are taken into account. We find that 6 3P–6 3P collisions become even more important for the ion formation than electron impact at the higher Hg vapor pressures. Both Hg+ and Hg+2 will be formed. The 6 3P–6 3P collisions are indicated to be mainly responsible for the well-known decrease in uv efficiency of the Hg–noble-gas discharge at temperatures above 40 °C.