DETERMINATION OF THE PARTIAL PRESSURE OF THALLIUM IN HIGH-PRESSURE LAMP ARCS: A COMPARATIVE STUDY.

The partial pressure of thallium has been measured in high-pressure Hg-TlI discharges with different mercury, thallium, and electron pressures. The partial pressure at the arc axis was measured using the optically thin Tl 655-nm line. The effective partial pressure was measured from the self-reverse Tl 535-nm line on the basis of the multiparameter method. The effective partial pressure was calculated from the known axis pressure of thallium and the calculation of its radial variation by taking the chemical reactions into account. The experimental results confirm the dispersion character of the blue wing of the Tl 535-nm line. The systematic difference obtained between the measured and calculated effective pressure, particularly at the moment of minimum electron density, may be interpreted as deviations from the local thermodynamic equilbrium caused by overpopulation of the upper level of the Tl 535-nm line.