Radiative transport at the 184.9-nm Hg resonance line. I. Experiment and theory

Measurements are reported of the radiative decay rate of the Hg 6 ${\mathrm{}}^1$ $P_1$ population under conditions of strong radiation trapping at the 184.9-nm resonance line. The experiments were made in the afterglow of a low-pressure Hg discharge with a dye-laser absorption method. The radiative decay rate as a function of the Hg pressure shows a pronounced minimum. The value at minimum is lower than the limiting value at higher Hg densities by a factor of 2. Furthermore, the experimental rates are up to a factor of 8 lower than those calculated from theory with the usual assumption of complete redistribution. Indeed, complete redistribution is not expected in the present case, where the optical thickness is large and elastic collision rates are smaller than the single atom spontaneous emission rate. We derived an analytical expression for the fundamental-mode radiative decay rate in a homogeneous medium from a theory based on partial redistribution in frequency. A condition for the existence of a fundamental mode is discussed and is shown to be largely satisfied in our experiments. The expression for the decay rate was obtained for a resonance line with a hyperfine structure, and was elaborated for a cylindrical geometry. The resulting calculated decay rates agree with the experimental ones within 25%.