Self-Absorption in Arc Sources in Thermal Equilibrium

Abstract

An analytical determination of the self-absorption of a spectral line emitted by a source is not normally possible if account is taken of the spatial variations in the source of the excited atom concentrations and spectral line profiles. The problem is simpler for uniformly excited sources, for example sources in thermal equilibrium, but not for high-pressure arc sources normally assumed to be in thermal equilibrium, because of the existence of temperature gradients. In the paper an analysis is developed which shows that if such arcs are treated as uniformly excited, an error of less than 5% is introduced into the determination of the self-absorption of spectral lines, emitted by transitions between an upper state s and a lower absorbing state t, which satisfy the limits hνsq/kT0 gt-or-equal, slanted 10 and νst/νtq less-than-or-eq, slant 0.5, where q is the ground state and T0 the axial source temperature. The theory is then applied to the experimental data for a high-pressure mercury arc discharge and to new measurements of the line widths at 5461 and 4358 A. It is shown that the calculated value of the self-absorption at 5461 A agrees with the experimental value only when the line has a resonance profile The existence of this form of profile is also indicated by considerations of the broadening processes in the discharge