Effects of Temperature and Nitrogen Pressure on the Afterglow of Mercury Resonance Radiation

Abstract

Measurements have been made of the rate of decay of the $\ensuremath{\lambda}2537$ afterglow from an optically excited quartz cell containing 0.00236 mm of mercury vapor, and pressures of nitrogen ranging from 0.5 mm to 100 mm, at temperatures of 301\ifmmode^\circ\else\textdegree\fi{}, 374\ifmmode^\circ\else\textdegree\fi{}, and 486\ifmmode^\circ\else\textdegree\fi{}K. The work was done with a rotating disk phosphoroscope so designed that two traces representing the decay of the afterglow were recorded on a photographic plate. Very pure nitrogen was necessary in order to obtain consistent results. The exponential decay constants ranged from 300 to 12,900 ${\mathrm{sec}.}^{\ensuremath{-}1}$. A striking result was a very rapid increase of the decay coefficient with rise of temperature at high pressures. The rate of decay depends on the rates at which collisions with nitrogen molecules cause transitions of the mercury atom back and forth between the $2^{3}P_{0}$ and $2^{3}P_{1}$ states and possibly from either of these states to the normal state, on the rate of diffusion of metastable $2^{3}P_{0}$ atoms to the walls of the vessel where they are reduced, and on the imprisonment of $\ensuremath{\lambda}2537$ radiation by successive absorptions and re-emissions of quanta before they escape from the cell. This latter factor depends on the frequency shape of the radiation emission and absorption coefficients and the geometry of the cell. It introduces such great mathematical difficulties into the theory that a rigorous solution for the decay constant in terms of conditions and atomic properties is not attempted here, but an approximate solution is given which, it is believed, considers all the important processes, is in excellent accord with the experimental results, and renders possible their interpretation in terms of effective cross-sections. From this theory and data the effective temperature cross-sections of excited mercury atoms for collisions with nitrogen molecules which give rise to energy transitions in the mercury atom were found to be, in the sense in which they are defined in this paper, independent of temperature in the range from 300\ifmmode^\circ\else\textdegree\fi{}K to 500\ifmmode^\circ\else\textdegree\fi{}K, and their mean values are: for $^{3}P_{0}\ensuremath{\rightarrow}^{3}P_{1} {{\ensuremath{\sigma}}_{0}}^{2}=6.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18} {\mathrm{cm}}^{2}$ $^{3}P_{1}\ensuremath{\rightarrow}^{3}P_{0} {{\ensuremath{\sigma}}_{1}}^{2}=3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17} {\mathrm{cm}}^{2}$ $^{3}P_{1}\ensuremath{\rightarrow}^{1}S_{0} {{\ensuremath{\Sigma}}_{1}}^{2}\ensuremath{\leqq}2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18} {\mathrm{cm}}^{2}$ $^{3}P_{0}\ensuremath{\rightarrow}^{1}S_{0} {{\ensuremath{\Sigma}}_{0}}^{2}\ensuremath{\leqq}2.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22} {\mathrm{cm}}^{2} \mathrm{roughly}.$The diffusion cross section for metastable atoms increases very slowly with temperature. The values found are 15.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$, 17.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$, and 18.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ respectively at 301\ifmmode^\circ\else\textdegree\fi{}, 374\ifmmode^\circ\else\textdegree\fi{} and 486\ifmmode^\circ\else\textdegree\fi{}K.