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The Journal of Chemical Physics

Relaxation of the mercury 6 3P and 6 3P1 states

M. Stock, E. W. Smith, Robert E. Drullinger, Merrill M. Hessel

Published Sep 15, 1977• 34 citations
Publisher

Abstract

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.

DOI
10.1063/1.435202

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The Journal of Chemical Physics•Dec 1, 1986•1 citation
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.
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.
Measurements are reported of the fundamental mode radiative decay rate of the Hg 6 1P1^{1}P_{1}1P1​ population under conditions of low collision rate and large optical thickness of the 184.9-nm resonance line. The measured data extend over a wide range of experimental conditions. The measurements were made using two different experimental methods, both employing low-pressure mercury--noble-gas discharges. In the first method the decay rate of the 6 1P1^{1}P_{1}1P1​ population was measured in the afterglow of the discharge using a dye-laser absorption technique. In the second one the decay was measured of the laser-induced fluorescence at the 184.9-nm line created by pulsed optical pumping. Radiative decay rates obtained with the two independent experimental methods for nominally the same conditions agreed within the experimental error of 5%. For zero perturber gas density the radiative decay rate as a function of the Hg density shows a pronounced minimum. This minimum disappears with the addition of an argon density of more than about 3\ifmmode\times\else\texttimes\fi{}1022{10}^{22}1022 ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$, which corresponds with an extra collisional broadening of \ensuremath{\sim}15 MHz. The experimental decay rates differ largely from rates calculated with a complete frequency redistribution theory. Comparison with rates calculated from the partial redistribution theory described in paper I (preceding paper) shows agreement within 25%. The behavior of the experimental rates at the lowest mercury densities could be interpreted as due to a deviation from the natural abundance in the 6 1P1^{1}P_{1}1P1​ excited-state population.
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.
Isotopically selective resonance ionization of mercury atoms has been accomplished by three-step excitation via the 6 3P1 and 8 1S0 excited states. Three collinear beams, respectively 254, 286, and 532 nm, from two dye lasers and a pulsed neodymium:yttrium aluminum garnet (ND:YAG) pump laser, were passed through a closed mercury vapor cell containing electrodes to which the ions were drawn by an electric field. The ion current and the 254-nm fluorescence radiation were measured as a function of laser wavelengths. Hyperfine structures for the 254- and 286-nm transitions were observed.
Decay rates of the 6 2P3/2 metastable and 6 2P1/2 ground state atomic thallium densities produced in UV photodissociation (at λ193, 222, 249, 308, and 353 nm) of TlI, TlBr, and TlCl molecular vapors have been measured for vapor densities from 1013 to 1017 cm−3. Dominant relaxation processes have been identified as (i) for the metastable state, collisional deexcitation by thallium halide molecules at high halide densities (≥1015 cm−3) and by free halogens and trace impurities at lower halide densities and (ii) for the ground state, diffusion in the ambient (argon/thallium halide) gas at high halide densities and volume reassociation under certain conditions at low halide densities. Cross sections and diffusion coefficients are evaluated.