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The Journal of Chemical Physics

The Quenching of Mercury Resonance Radiation. II. The Production of Metastable (3P0) Atoms

Basil deB. Darwent, F. G. Hurtubise

Published Nov 1, 1952• 14 citations
Publisher

Abstract

The production of metastable (3P0) mercury atoms in the quenching of mercury resonance radiation has been investigated with N2, H2, C2H4, C2H6, O2, and CO2. Metastable atoms, by virtue of their ability to induce the emission of electrons from a nickel surface, were detected with the first four gases; none was detected with O2 and CO2.
DOI
10.1063/1.1700268

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The Quenching of Mercury Resonance Radiation (2537A) by Nitrogen

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When a mercury atom in the resonance $6^{3}P_{1}$ state collides with a nitrogen molecule, it may be transferred to the metastable $6^{3}P_{0}$ 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{}${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ at the lower temperature to 1.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at the upper temperature.

Physical Review•Nov 1, 1953•32 citations
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.
The quenching and depolarization of mercury resonance radiation (2537 Å), induced in collisions of Hg(63P1) atoms with various molecules has been investigated using the method of delayed coincidences. The following total quenching cross sections have been obtained for the process Hg(63P1) → Hg(63P0, 61S0): Hg–H2, 24.6 Å2; Hg–D2, 22.7 Å2; Hg–CO, 21.7 Å2; Hg–CO2, 10.2 Å2; Hg–O2, 60.5 Å2; Hg–14N2, 0.73 Å2; Hg–15N2, 0.78 Å2; and Hg–Xe, < 0.002 Å2. A cross section of 267 Å2 has been measured for the depolarization of the resonance radiation by collisions with CO2 molecules.
A nitrogen-seeded supersonic nozzle-beam source of metastable mercury, Hg(6 3P0),isdescribed.Ground-state(6 1S0)mercuryisexcited with 2537 Å resonance radiation to the 6 3P1 state, which is then converted via intramultiplet quenching to the 6 3P0 state collision with nitrogen (the carrier gas). The mercury beam is accelerated by the N2 seeding gas to a speed of 8×104 cm sec−1. Thus, the present source provides a beam of a single metastable state, Hg(6 3P0), with 4.67 eV of electronic excitation and 0.67 eV of translational energy. The metastable beam has been characterized and the kinetics of the over-all photoexcitation and quenching process studied by a secondary electron (Auger) detector utilizing a potassium surface, and an electron-bombardment ionizer, mass-filter detector. Hg(6 3P0) beam intensities of 4×1012 atoms sr−1⋅sec−1 are obtained, based upon an (assumed) 5% efficiency for the Auger detector.
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.
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.
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.