Two aspects of the concept of local thermodynamic equilibrium are examined in mediumpressure, mercury arc discharges with thallium iodide additives. It is shown theoretically that the experimental intensity of the T 5350 Å line is reasonably consistent with a Boltzmann distribution of excited states. In fact, it is theoretically and experimentally shown that self-absorption is a major factor in limiting the intensity of the bright thallium 5350 Å line even though Ti is a minor constituent in the discharge and even though this line does not terminate on the ground state. The dramatic spectral inhomogeneity that develops in long, horizontal, ac discharges with excess TII in the presence of a longitudinal temperature gradient is examined in detail. Both emission and absorption studies show that the spectral inhomogeneity is caused by a large gradient in the thallium concentration. This inhomogeneous distribution of TlI vapors is caused by an inhomogeneous and unsymmetrical distribution of condensed THI along the tube walls which results from the temperature gradient. In turn, the temperature gradient is increased by the ionization of thallium which locally decreases the electrical resistivity, electric field, and power input to the arc. Thus, an equilibrium distribution of TlI vapors is attained only if the distribution of condensed TlI is reasonably uniform and the tube geometry and pressure are favorable for rapid diffusion of the vapors. The inhomogeneity also disappears if a sufficiently small quantity of solid TII is added to the lamp so that it is all vaporized.
C. F. Gallo, "On the Question of Equilibrium in Mercury + Thallium Iodide Arc Discharges," Appl. Opt. 5, 1285-1291 (1966)