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The Journal of Chemical Physics

Formation of supersonic beams of metastable mercury, Hg(6 3 P ). Kinetics of photoexcitation and intramultiplet quenching in a nozzle

J. A. Haberman, Bruce E. Wilcomb, F. J. Van Itallie, Richard B. Bernstein

Published Jun 1, 1975• 9 citations
Publisher

Abstract

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.

DOI
10.1063/1.430352

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Journal of the Optical Society of America•Jan 1, 1979•4 citations
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.
Inclusion of collisional destruction mechanisms as sink terms in the theory of imprisonment of resonance radiation appears to have been neglected in the case of the 2537-Å mercury line. In this paper, the Holstein photon transport equation is employed to determine the effect of destructive or self-quenching collisions between resonance state and ground state atoms, processes in which the resonance state is converted to another excited state with the release of kinetic energy. These mechanisms seem to explain experimental work in which the decay constant of the 2537-Å mercury line reaches a minimum and then increases with density. Comparison of theory with experimental data yields a cross section between 3.0 and 10.0 × 10−18 for the conversion of Hg(3P1) to metastable Hg(3P0) by two-body collisions with Hg(1S0). The sharp increase at high density of the decay constant of the 2537-Å mercury line has not been predicted by previous theories but is a general consequence of the inclusion of collisional destruction mechanisms.
Les atomes métastables Hg(6 3P0) sont créés dans un mélange mercure-azote, éclairé par une impulsion lumineuse à la longueur d'onde de résonance 253,7 nm et suivant le processus : Hg(6 3P1) + N2(X 1Σ g+, v = 0) → Hg(6 3P0) + N2(X 1Σ+g, v = 1). Dans la post-excitation la loi de décroissance de la concentration des atomes métastables dépend de la valeur du flux lumineux à l'excitation. Les phénomènes observés sont partiellement expliqués par (i) la présence de molécules d'azote dans le niveau vibrationnel v = 1 en concentration supérieure à sa valeur à l'équilibre thermique (ii) les collisions inélastiques : Hg(6 3P0) + Hg(6 3P0) → Hg(6 1S0) + Hg*, où Hg* désigne un atome de mercure dans un état très excité. Les sections efficaces d'excitation des niveaux 8 1S0, 8 3S 1, 6 1D2, 6 3D1, 6 3D2 et 6 3D3 ont été mesurées et valent respectivement 15, 9,12, 4,10 et 6 Å2.
The relaxation of 6 3P0 mercury atoms excited by a laser pulse was measured for densities ranging from 2×1017 to 8×1018 cm−3. The decay of the 253.7 nm resonance line was also measured for densities ranging from 5×1016 to 1018 cm −3. The decay coefficients were analyzed to yield rate coefficients for three body molecular formation, the 3P1→3P0 collision rate and a collisional quenching rate for the 3P0 state. Small loss rates due to diffusion and radiation were also observed and found to be consistent with theoretical estimates of these quantities.
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.
The total cross section for excitation of ground state mercury atoms to the 63P1 state by 50 eV electrons was experimentally determined to be 0.434 ± 0.04 × 10−16 cm2. The apparatus used a pair of single-photon detectors to observe correlated optical photons in the 7 1S0 → 6 3P1 → 6 1S0 and 7 3S1 → 6 3P1 → 6 1S0 cascades. The product of the number of singles counts in the two detectors divided by the number of time-correlated cascade events was used along with angular distribution and correlation factors to determine N, the total number of atoms excited to the 6 3P1 state in the interaction region. The correct number N was arrived at in this way without making a separate determination of the overall photon detection efficiency. No standard lamp was needed for calibration nor was reference made to another standard cross section. The apparatus was self-calibrating. The absolute excitation cross section was determined from N and other parameters of the bombardment apparatus.