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Physical Review

The Metastable State in Mercury Vapor

Harold W. Webb

Published Aug 1, 1924• 62 citations
Publisher

Abstract

Metastable state produced in mercury vapor at low pressures by electron impacts of 4.9 volts.---The persistence of the "radiation" produced by 4.9 volt impacts in mercury vapor was measured in a four electrode tube of the type usually employed in resonance potential measurements, consisting of an equipotential cathode of simple design, an inner grid $G$ for controlling the velocity of the impacting electrons, an outer photo-electric grid $H$ and a photo-electric plate $P$. A.c. and d.c. voltages were applied to the two grids in such a way that the electrons had sufficient velocity to excite the radiation only in alternate half-cycles and that the photo-electric current to the electrometer alternated in direction simultaneously. As a result of the persistence of the radiation there resulted a decrease in the current with increasing frequency, reaching a minimum at 1800 cycles for a distance between grids of 17 mm and at 3800 cycles for a distance of 8.5 mm. These results did not vary much with pressure,.003 to.032 mm. They show a lapse of time between the excitation and the arrival of the first radiation at the plate $P$ of about 1/3600 and 1/7600 sec. respectively. A mathematical discussion shows that the diffusion of the radiation by repeated emission and re-absorption (the "imprisonment" of radiation theory) cannot account for these results. A calculation, based on the assumption that the excited atoms remain in a metastable state and carry the energy of excitation to the photo-electric surfaces and there give it up, gives results in very close agreement with the observations. The conclusion is that a metastable state is formed by the atoms excited by the 4.9 volt electron impacts and that these are the effective ones in producing the photo-electric response in the tubes. In these experiments the $2{p}_{2}$ state is the only one excited by the impacts in appreciable amount. How these results can be reconciled with the Bohr theory, according to which the $2{p}_{2}$ state is not metastable, is not clear.

DOI
10.1103/physrev.24.113

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Measurement of the Life of the Metastable Mercury Atom

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Half value time for the ${2}^{3}$${\mathrm{P}}_{0}$ state of Hg for various pressures of admixed ${\mathrm{N}}_{2}$--- 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 $2^{3}P_{0}$ 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.

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Mean Lives of Mercury Lines λ2537 and λ1849

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The mean life of the mercury line $\ensuremath{\lambda}2537$ was measured by the alternating voltage method previously described by Webb. The radiation was excited in a quartz tube by impact with electrons from a hot cathode. The photoelectric system was contained in a separate tube also of quartz. The photoelectric surface was zinc evaporated on a nickel plate. Measurements of the apparent life were made with diminishing mercury vapor pressures until further reduction of pressure produced no change in the measured rate of decay of the radiation. At these low pressures (below 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ mm, for the geometry used) the absorption and reemission of the radiation was negligible and the true life of a single excitation process was measured. This was found to be 1.08\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ sec. with an estimated precision of one percent.Absorption coefficient of $\ensuremath{\lambda}1849$. An absorption cell with a one mm thick absorbing layer of mercury vapor was mounted so as to move in and out between the excitation and photoelectric tubes used in the above measurements. The absorption of $\ensuremath{\lambda}1849$ was measured at pressures such that the absorption of $\ensuremath{\lambda}2537$ was negligible. From this the atomic absorption coefficient for the main component of the line was found to be 6.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}11}$ from which the mean life of the $2^{1}P_{1}$ state was calculated to be 2.9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec. with an estimated precision of 25 percent.

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The Metastable 2 p 3 -State of Mercury Atoms

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Let us consider a mixture of mercury vapor, thallium vapor and nitrogen in a closed quartz tube under well defined conditions of pressure and temperature, illuminated by λ 2537A of constant intensity. \n \nDue to the absorption of the illuminating radiation a constant number N of mercury atoms will be activated in unit time. Those activated atoms persist in the 2p2-state for a finite time t which is known to be of the order of magnitude of 10^-7 sec [1]. If they escape collision during this period they return to their normal state in the ordinary wave-emitting radiation (resonance). They may, however, while activated, collide with nitrogen molecules, normal mercury atoms or normal thallium atoms. We shall assume, for the present, that the amount of nitrogen and thallium sufficiently exceeds the amount of mercury, so that we may disregard totally the collisions between activated and normal mercury atoms. \n \nWe assume furthermore that all collisions between activated mercury atoms and thallium atoms result in the activation of the latter, producing the fluorescence radiation of thallium with a constant energy distribution between its different lines.

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Radio-Frequency Resonance of the Metastable State (2P322 3S122)2 of Neon Produced and Aligned by Electron Impact

T. Hadeishi, Orilla A. McHarris, William A. Nierenberg

The metastable state ${(2^{2}P_{\frac{3}{2}} 3^{2}S_{\frac{1}{2}})}_{2}$ of neon was produced and aligned by electron impact. The diodestructure electron gun was operated at a high electron-current density under space-charge neutralization conditions. Radio-frequency resonance of the aligned metastable state ${(2^{2}P_{\frac{3}{2}} 3^{2}S_{\frac{1}{2}})}_{2}$ was observed by monitoring the change of absorption of the $\ensuremath{\lambda}6143$-resonance radiation passing through the gap between the cathode and anode. The neon metastable-state resonance was compared with the mercury metastable-state $6^{3}P_{2}$ radio-frequency resonance obtained by using the same gun for both, and observed by monitoring the absorption of the $\ensuremath{\lambda}5461$-resonance radiation passing through the same excitation region. The observed $\frac{{({g}_{J})}_{\mathrm{Ne}}}{{({g}_{J})}_{\mathrm{Hg}}}$ was 1.008\ifmmode\pm\else\textpm\fi{}0.024. This method could be applied to other atoms having a metastable state with $J>0$.

Physical Review•May 17, 1965•9 citations

The Excitation of Sodium by Ionized Mercury Vapor

Harold W. Webb, S. C. Wang

Excitation of sodium by excited and by ionized mercury vapor.---The excitation of the sodium spectrum by ionized mercury distilling from an arc was studied in a tube in which conditions were under control. An auxiliary electrode in the stream of mercury vapor was used to increase the number of excited atoms at the point of observation until the excitation due to them predominated over that due to the ions. The spectrum produced by excited mercury atoms was thus compared with that due to mercury ions. The spectrum due to the excited atoms showed the marked "resonance" effect shown earlier by Beutler and Josephy. The spectrum due to mercury ions also showed excitation corresponding to the excitation energy of the metastable mercury atom. This may be accounted for as due to metastable atoms formed as the result of recombination of mercury ions. An alternative explanation is that the excitation of a sodium atom by a mercury ion is by a three body impact involving a mercury ion, a sodium atom and an electron. This was suggested by the fact that when sodium is introduced into the ionized mercury there is a strong enhancement of the mercury lines originating at the two $3S$ levels, which seems to be best explained as due to three body impacts. It seems probable that both this and the above process are of the same nature. It was noted further that the spectrum excited by ionized mercury resembles the chemiluminescence spectrum resulting from mixing sodium and mercuric chloride vapors. The spark line of mercury 3984, not previously found in the luminous mercury stream, was found when sodium was present.

Physical Review•Mar 1, 1929•5 citations
Metastable state produced in mercury vapor at low pressures by electron impacts of 4.9 volts.---The persistence of the "radiation" produced by 4.9 volt impacts in mercury vapor was measured in a four electrode tube of the type usually employed in resonance potential measurements, consisting of an equipotential cathode of simple design, an inner grid GGG for controlling the velocity of the impacting electrons, an outer photo-electric grid HHH and a photo-electric plate PPP. A.c. and d.c. voltages were applied to the two grids in such a way that the electrons had sufficient velocity to excite the radiation only in alternate half-cycles and that the photo-electric current to the electrometer alternated in direction simultaneously. As a result of the persistence of the radiation there resulted a decrease in the current with increasing frequency, reaching a minimum at 1800 cycles for a distance between grids of 17 mm and at 3800 cycles for a distance of 8.5 mm. These results did not vary much with pressure,.003 to.032 mm. They show a lapse of time between the excitation and the arrival of the first radiation at the plate PPP of about 1/3600 and 1/7600 sec. respectively. A mathematical discussion shows that the diffusion of the radiation by repeated emission and re-absorption (the "imprisonment" of radiation theory) cannot account for these results. A calculation, based on the assumption that the excited atoms remain in a metastable state and carry the energy of excitation to the photo-electric surfaces and there give it up, gives results in very close agreement with the observations. The conclusion is that a metastable state is formed by the atoms excited by the 4.9 volt electron impacts and that these are the effective ones in producing the photo-electric response in the tubes. In these experiments the 2p22{p}_{2}2p2​ state is the only one excited by the impacts in appreciable amount. How these results can be reconciled with the Bohr theory, according to which the 2p22{p}_{2}2p2​ state is not metastable, is not clear.
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.
The mean life of the mercury line $\ensuremath{\lambda}2537$ was measured by the alternating voltage method previously described by Webb. The radiation was excited in a quartz tube by impact with electrons from a hot cathode. The photoelectric system was contained in a separate tube also of quartz. The photoelectric surface was zinc evaporated on a nickel plate. Measurements of the apparent life were made with diminishing mercury vapor pressures until further reduction of pressure produced no change in the measured rate of decay of the radiation. At these low pressures (below 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ mm, for the geometry used) the absorption and reemission of the radiation was negligible and the true life of a single excitation process was measured. This was found to be 1.08\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ sec. with an estimated precision of one percent.Absorption coefficient of $\ensuremath{\lambda}1849$. An absorption cell with a one mm thick absorbing layer of mercury vapor was mounted so as to move in and out between the excitation and photoelectric tubes used in the above measurements. The absorption of $\ensuremath{\lambda}1849$ was measured at pressures such that the absorption of $\ensuremath{\lambda}2537$ was negligible. From this the atomic absorption coefficient for the main component of the line was found to be 6.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}11}$ from which the mean life of the 21P12^{1}P_{1}21P1​ state was calculated to be 2.9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec. with an estimated precision of 25 percent.
Let us consider a mixture of mercury vapor, thallium vapor and nitrogen in a closed quartz tube under well defined conditions of pressure and temperature, illuminated by λ 2537A of constant intensity. \n \nDue to the absorption of the illuminating radiation a constant number N of mercury atoms will be activated in unit time. Those activated atoms persist in the 2p2-state for a finite time t which is known to be of the order of magnitude of 10^-7 sec [1]. If they escape collision during this period they return to their normal state in the ordinary wave-emitting radiation (resonance). They may, however, while activated, collide with nitrogen molecules, normal mercury atoms or normal thallium atoms. We shall assume, for the present, that the amount of nitrogen and thallium sufficiently exceeds the amount of mercury, so that we may disregard totally the collisions between activated and normal mercury atoms. \n \nWe assume furthermore that all collisions between activated mercury atoms and thallium atoms result in the activation of the latter, producing the fluorescence radiation of thallium with a constant energy distribution between its different lines.
The absorption of the resonance radiation (2536A) of mercury was studied by dividing the radiation from a resonance lamp into two parts, each part going into a separate photo-electric cell. An absorption cell containing mercury vapor whose pressure could be controlled by cooling a side tube was placed in the path of one beam. The ratio of the amount of light passing into each beam could be varied by a mechanical device. From the setting of the mechanical device, giving a balance between the two photo-electric currents, the absorption of the mercury vapor could be inferred. The absorption per atom was calculated and found to decrease rapidly as the total amount of absorbing vapor was increased, thus showing that the resonance radiation is not homogeneous. The maximum atomic absorption coefficient was 11\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$, which indicates that an atom can stop the radiation which falls upon an area one hundred times larger than its cross sectional area.
The metastable state (22P3232S12)2{(2^{2}P_{\frac{3}{2}} 3^{2}S_{\frac{1}{2}})}_{2}(22P23​​32S21​​)2​ of neon was produced and aligned by electron impact. The diodestructure electron gun was operated at a high electron-current density under space-charge neutralization conditions. Radio-frequency resonance of the aligned metastable state (22P3232S12)2{(2^{2}P_{\frac{3}{2}} 3^{2}S_{\frac{1}{2}})}_{2}(22P23​​32S21​​)2​ was observed by monitoring the change of absorption of the $\ensuremath{\lambda}6143$-resonance radiation passing through the gap between the cathode and anode. The neon metastable-state resonance was compared with the mercury metastable-state 63P26^{3}P_{2}63P2​ radio-frequency resonance obtained by using the same gun for both, and observed by monitoring the absorption of the $\ensuremath{\lambda}5461$-resonance radiation passing through the same excitation region. The observed (gJ)Ne(gJ)Hg\frac{{({g}_{J})}_{\mathrm{Ne}}}{{({g}_{J})}_{\mathrm{Hg}}}(gJ​)Hg​(gJ​)Ne​​ was 1.008\ifmmode\pm\else\textpm\fi{}0.024. This method could be applied to other atoms having a metastable state with J>0J>0J>0.
Excitation of sodium by excited and by ionized mercury vapor.---The excitation of the sodium spectrum by ionized mercury distilling from an arc was studied in a tube in which conditions were under control. An auxiliary electrode in the stream of mercury vapor was used to increase the number of excited atoms at the point of observation until the excitation due to them predominated over that due to the ions. The spectrum produced by excited mercury atoms was thus compared with that due to mercury ions. The spectrum due to the excited atoms showed the marked "resonance" effect shown earlier by Beutler and Josephy. The spectrum due to mercury ions also showed excitation corresponding to the excitation energy of the metastable mercury atom. This may be accounted for as due to metastable atoms formed as the result of recombination of mercury ions. An alternative explanation is that the excitation of a sodium atom by a mercury ion is by a three body impact involving a mercury ion, a sodium atom and an electron. This was suggested by the fact that when sodium is introduced into the ionized mercury there is a strong enhancement of the mercury lines originating at the two 3S3S3S levels, which seems to be best explained as due to three body impacts. It seems probable that both this and the above process are of the same nature. It was noted further that the spectrum excited by ionized mercury resembles the chemiluminescence spectrum resulting from mixing sodium and mercuric chloride vapors. The spark line of mercury 3984, not previously found in the luminous mercury stream, was found when sodium was present.