Radiative transport at the 184.9-nm Hg resonance line. II. Extensive experiments

Measurements are reported of the fundamental mode radiative decay rate of the Hg 6 ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ population under conditions of low collision rate and large optical thickness of the 184.9-nm resonance line. The measured data extend over a wide range of experimental conditions. The measurements were made using two different experimental methods, both employing low-pressure mercury–noble-gas discharges. In the first method the decay rate of the 6 ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ population was measured in the afterglow of the discharge using a dye-laser absorption technique. In the second one the decay was measured of the laser-induced fluorescence at the 184.9-nm line created by pulsed optical pumping. Radiative decay rates obtained with the two independent experimental methods for nominally the same conditions agreed within the experimental error of 5%. For zero perturber gas density the radiative decay rate as a function of the Hg density shows a pronounced minimum. This minimum disappears with the addition of an argon density of more than about 3×${10}^{22}$ ${\mathrm{m}}^{\mathrm{-}3}$, which corresponds with an extra collisional broadening of ∼15 MHz. The experimental decay rates differ largely from rates calculated with a complete frequency redistribution theory. Comparison with rates calculated from the partial redistribution theory described in paper I (preceding paper) shows agreement within 25%. The behavior of the experimental rates at the lowest mercury densities could be interpreted as due to a deviation from the natural abundance in the 6 ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ excited-state population.