The Band Fluorescence of Mercury Vapor

Abstract

The band fluorescence of mercury vapor excited by the 2537A atomic line has been studied during the afterglow. Time sampling techniques were employed to overcome difficulties attending the measurement of very low-intensity radiations. The two main band constituents (centered at 4850A and 3350A) were found to decay simultaneously under all conditions investigated. The temporal structure of the decaying fluorescence was found to be characterized by two time constants and has the form associated with chain types reactions. The two time constants and the relative intensities of the 4850A and 3350A bands were measured for a range of vapor densities at a fixed vapor temperature. In order to evaluate the role of diffusion in the lifetime of the excited particles involved, measurements were made in vessels of two different sizes.The experimental results have led to an interpretation according to which the optically excited $\mathrm{Hg}(^{3}P_{1})$ atoms are converted into metastable $\mathrm{Hg}(^{3}P_{0})$ atoms by inelastic collisions. The $\mathrm{Hg}(^{3}P_{0})$ atoms are then converted in a three-body collision reaction to metastable ${\mathrm{Hg}}_{2}({^{3}0_{u}}^{\ensuremath{-}})$ molecules which, in turn, may be destroyed in the vapor by either one of two processes, i.e., by spontaneous radiation to the ground state, giving rise to the 3350A band, or by a second three-body collision through which the emission of the 4850A band is induced. An analysis based upon this picture has permitted the quantitative evaluation of several properties of the particles involved. The diffusion coefficient of the $\mathrm{Hg}(^{3}P_{0})$ atom was found to be 210 ${\mathrm{cm}}^{2}$/sec for a density of ${10}^{16}$ atoms/cc and a temperature of 200\ifmmode^\circ\else\textdegree\fi{}C; the diffusion coefficient of the ${\mathrm{Hg}}_{2}({^{3}0_{u}}^{\ensuremath{-}})$ molecules is 88 ${\mathrm{cm}}^{2}$/sec under the same conditions. The spontaneous radiation rate of the ${\mathrm{Hg}}_{2}({^{3}0_{u}}^{\ensuremath{-}})$ molecules is 20 ${\mathrm{sec}}^{\ensuremath{-}1}$ and the three-body collision induced radiation rate is 21\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}32}$ ${(\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{c})}^{\ensuremath{-}2}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. The three-body destruction rate for the $\mathrm{Hg}(^{3}P_{0})$ atoms is 100\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}32}$ ${(\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{c})}^{\ensuremath{-}2}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. It was also found that the ${\mathrm{Hg}}_{2}({^{3}0_{u}}^{\ensuremath{-}})$ molecules are approximately 90 percent reflected by the glass walls of the enclosure. The diffusion coefficient obtained for $\mathrm{Hg}(^{3}P_{0})$ atoms is in agreement with the results of other work and the three-body collision rates are comparable with those for similar processes investigated elsewhere.