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Physical Review

On the Concentration of Metastable Mercury Atoms

Enrique Gaviola

Published May 15, 1930• 5 citations
Publisher

Abstract

The concentration of metastable atoms in presence of 3 mm nitrogen obtained using the measurements of Stuart seems to be in contradiction with a calculation given here using observations of Wood. The contradiction disappears if the different statistical weights of the levels $2^{3}P_{1}$ and $2^{3}P_{0}$ are taken in account. Corrected curves for the number of metastable atoms as functions of the foreign gas pressure are given for CO, ${\mathrm{H}}_{2}$O, ${\mathrm{N}}_{2}$, A and He which are to replace the corresponding curves of the previous paper. It is shown that the assumption of the author that by high primary light intensities the number of metastable atoms increases only with the square root of the intensity is strikingly supported by measurements of Wood.

DOI
10.1103/physrev.35.1226

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Physical Review•Nov 1, 1953•32 citations
The concentration of metastable atoms in presence of 3 mm nitrogen obtained using the measurements of Stuart seems to be in contradiction with a calculation given here using observations of Wood. The contradiction disappears if the different statistical weights of the levels 23P12^{3}P_{1}23P1​ and 23P02^{3}P_{0}23P0​ are taken in account. Corrected curves for the number of metastable atoms as functions of the foreign gas pressure are given for CO, H2{\mathrm{H}}_{2}H2​O, N2{\mathrm{N}}_{2}N2​, A and He which are to replace the corresponding curves of the previous paper. It is shown that the assumption of the author that by high primary light intensities the number of metastable atoms increases only with the square root of the intensity is strikingly supported by measurements of Wood.
A complete and general study of the dependence of life and concentration of metastable mercury atoms, optically excited, on experimental conditions, on foreign gas-pressures, and on intensity of illumination is given. This study explains why the life can not be larger than ${10}^{\ensuremath{-}2}$ second under laboratory conditions and why foreign gases act so differently in regard to the accumulation of excited atoms. The theoretical results are in good agreement with all experimental data available. An improved and more general formula for the quenching of mercury resonance radiation is obtained. The efficiencies of collisions of the second kind of metastable mercury atoms with N2{\mathrm{N}}_{2}N2​, A, and He molecules are calculated numerically. It is shown that the concentration of metastable atoms increases only with the square root of the intensity of the exciting light and not with the exciting light itself as generally assumed.
Effect of nitrogen and of mixtures of nitrogen and carbon dioxide upon the fluorescence of mercury vapor. By "stepwise" optical excitation, unexcited mercury atoms are raised to the 23S12^{3}S_{1}23S1​ state. They may leave this level upon the emission of radiation by four paths---the first to the 21P12^{1}P_{1}21P1​ state, a transition of relatively low probability, and the other three to the 23P22^{3}P_{2}23P2​, 23P12^{3}P_{1}23P1​, and 23P02^{3}P_{0}23P0​ states with the emission of 5461A, 4358A and 4046A, respectively. Measurements, by totally independent methods, of the mean life of the 23S12^{3}S_{1}23S1​ state yielded values, when measured by the line 5461A, equal to approximately four times those when measured by either 4358A or 4046A. Upon the addition of foreign gas, then, the intensity ratio of 5461A to either 4358A or 4046A should decrease with increasing pressure while that of 4046A to 4358A should remain constant. However, these ratios were all found to be invariant with pressure, up to 400 mm in the case of nitrogen and up to 100 mm in the case of the nitrogen-carbon dioxide mixtures.
The evidence in favor and against the assumption of Foote that every collision of a foreign gas molecule with an excited mercury atom is efficient in quenching the resonance radiation is discussed, and a new calculation of the efficiency of collisions is given, based on Stuart's measurements, which shows that the efficiency can be assumed to be equal to one in the case of CO, H2{\mathrm{H}}_{2}H2​, and perhaps O2{\mathrm{O}}_{2}O2​, but that it is undoubtedly smaller than one for H2{\mathrm{H}}_{2}H2​O, N2{\mathrm{N}}_{2}N2​, A, and He. CO has actually a greater quenching efficiency than H2{\mathrm{H}}_{2}H2​. The radius of the excited mercury atom is calculated using an improved value for the amount of resonance-radiation re-absorbed in the resonance vessel and found to be ${r}_{{\mathrm{Hg}}^{\ensuremath{'}}}=2.91\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ cm, or 1.62 times larger than the radius of the normal atom for the case of H2{\mathrm{H}}_{2}H2​, and ${r}_{{\mathrm{Hg}}^{\ensuremath{'}}}=5.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ cm or three-fold normal for the case of CO. The apparent higher quenching efficiency of oxygen than hydrogen is explained by the partial oxidation of the mercury vapor and consequent decrease of the density of the last. It is shown that the life of metastable atoms increases with the admission of certain foreign gases into the fluorescence vessel.
The experiments of Wood and of Gaviola show that when mercury vapor is illuminated by light from a mercury arc, and nitrogen is added in increasing amounts, the concentration of atoms in the 3P2^{3}P_{2}3P2​ state passes through a minimum. No simple theory accounts for this result. An explanation involving excitation of atoms in the 3P1^{3}P_{1}3P1​ state by vibrating nitrogen molecules is discussed. The results obtained when a filter of bromine vapor is used indicate a transfer of excited mercury atoms from one to another 3D^{3}D3D state upon collision with nitrogen.
Half value time for the 23{2}^{3}23P0{\mathrm{P}}_{0}P0​ state of Hg for various pressures of admixed N2{\mathrm{N}}_{2}N2​--- A high speed rotating disk has been used to interrupt the total radiation from a water-cooled and magnetically controlled quartz mercury arc before optically exciting mercury vapor in a quartz resonance tube at room temperature. In the neighborhood of ${10}^{\ensuremath{-}4}$ sec. after the optical excitation a flash of visible radiation from another water-cooled and magnetically controlled mercury arc was passed through the resonance tube and the absorption spectrum photographed. The line 4047 ($2^{3}P_{0}\ensuremath{-}2^{3}S_{1}$) showed strong absorption when a few millimeters of nitrogen were introduced into the resonance tube. The lines 4077, 4358, 5461, 5770 and 5790 showed no measurable absorption under similar conditions. The rate of decay of the amount of 4047 absorbed with respect to the time-wait after excitation of the mercury vapor was taken as the rate of decay of the metastable 23P02^{3}P_{0}23P0​ state. The life or half-value time of the state was found to vary markedly with the pressure of admixed nitrogen. The maximum life 4.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ sec. occurred for 6.8 mm of nitrogen. No absorption of 4047 was observed upon the introduction of hydrogen, argon, neon, or helium into the resonance tube.
When a mercury atom in the resonance 63P16^{3}P_{1}63P1​ state collides with a nitrogen molecule, it may be transferred to the metastable 63P06^{3}P_{0}63P0​ state. The cross section for this reaction has been measured by a new method. Mercury vapor, in the presence of a known pressure of nitrogen gas, is excited by 2537A radiation. After the source of the excitation is cut off, the decay time of the imprisoned radiation is observed. The quenching collision cross section ${\ensuremath{\sigma}}_{Q}$ is calculated from the measured decay time and the known value of the imprisonment time in the absence of the quenching gas. Measurements have been taken over the temperature range 325-525\ifmmode^\circ\else\textdegree\fi{}K; ${\ensuremath{\sigma}}_{Q}$ is found to increase from 5.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2 at the lower temperature to 1.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2 at the upper temperature.