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Physical Review A

Radiative transport at the 184.9-nm Hg resonance line. II. Extensive experiments

H. A. Post, Peter van de Weijer, R. M. M. Cremers

Published Mar 1, 1986• 33 citations
Publisher

Abstract

Measurements are reported of the fundamental mode radiative decay rate of the Hg 6 $^{1}P_{1}$ 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 $^{1}P_{1}$ 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{}${10}^{22}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$, 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 $^{1}P_{1}$ excited-state population.

DOI
10.1103/physreva.33.2017

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Collisional redistribution in Hg-Kr: Polarization spectrum of the redistributed light

I. M. Bell, C. J. K. Quayle, Keith R. Burnett +1 more

We have measured the alignment of Hg(6 $^{3}$${\mathit{P}}_{1}$) atoms following off-resonant excitation of the Hg 6 $^{1}\mathrm{S}_{0}$--6 $^{3}\mathrm{P}_{1}$ transition in the presence of Kr. We present a polarization spectrum for both red and blue wings out to detunings beyond 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This represents a considerable extension to the existing results, which has been made possible by the use of a pump-and-probe excitation scheme. In the experiment a pulsed laser, detuned from the resonance, creates an alignment in the upper state. This is probed by tuning a delayed second laser to a higher transition and monitoring laser-induced fluorescence from the two-step process. The alignment of the asymptotic Hg(6 $^{3}$${\mathit{P}}_{1}$) atoms is determined by comparison of the signals obtained for different relative polarizations of the lasers. We discuss the advantages of this method and the relation of the raw data to the polarizations presented. The results are discussed within the context of a simple quasiclassical model of the process.

Physical Review A•Apr 1, 1993•8 citations
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 m\ensuremath−3{\mathrm{m}}^{\mathrm{\ensuremath{-}}3}m\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.
Measurements are reported of the radiative decay rate of the Hg 6 1{\mathrm{}}^{1}1P1{P}_{1}P1​ population under conditions of strong radiation trapping at the 184.9-nm resonance line. The experiments were made in the afterglow of a low-pressure Hg discharge with a dye-laser absorption method. The radiative decay rate as a function of the Hg pressure shows a pronounced minimum. The value at minimum is lower than the limiting value at higher Hg densities by a factor of 2. Furthermore, the experimental rates are up to a factor of 8 lower than those calculated from theory with the usual assumption of complete redistribution. Indeed, complete redistribution is not expected in the present case, where the optical thickness is large and elastic collision rates are smaller than the single atom spontaneous emission rate. We derived an analytical expression for the fundamental-mode radiative decay rate in a homogeneous medium from a theory based on partial redistribution in frequency. A condition for the existence of a fundamental mode is discussed and is shown to be largely satisfied in our experiments. The expression for the decay rate was obtained for a resonance line with a hyperfine structure, and was elaborated for a cylindrical geometry. The resulting calculated decay rates agree with the experimental ones within 25%.
The decay rate of the Hg 61P1 level was measured as a function of cold spot temperature (Hg density) and buffer gas pressure in cylindrical, sealed fused silica cells. The decay rates were obtained using a time-resolved laser-induced 185 nm fluorescence experiment with multi-step excitation. Cold spot temperatures from 25 to 100 °C were studied. The Hg densities for this temperature range and with no buffer gas yield the lowest possible decay rates due to radiation trapping with partial frequency redistribution. Decay rates with argon buffer gas pressures of 3 and 30 Torr were also studied. The results are in agreement with published data from a discharge afterglow experiment. Monte Carlo simulations of radiation transport in the cells, including the effects of hyperfine and isotope structure, the effects of foreign gas broadening, and partial frequency redistribution are compared to the experimental data. Reasonably good agreement is obtained, however there is evidence of quenching of Hg 61P1 atoms in collisions with ground state Hg and Ar atoms. An analytic formula for the fundamental mode trapped decay rate of the 61P1 level, which is applicable over a substantial region of parameter space, was devised from the Monte Carlo results.
The cross section for the transition $6^{3}P_{0}\ensuremath{\rightarrow}6^{3}P_{1}$ in mercury, induced in collisions with N2{\mathrm{N}}_{2}N2​ molecules, has been determined by the method of delayed coincidences. The Hg 3P1^{3}P_{1}3P1​ state was excited by pulses of 2537-\AA{} Hg resonance radiation and the decay of the long-lived 2537-\AA{} afterglow was studied in relation to N2{\mathrm{N}}_{2}N2​ pressure. The resulting cross section ${Q}_{12}(^{3}P_{0}\ensuremath{\rightarrow}^{3}P_{1})=4.18\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ A˚2{\mathrm{\AA{}}}^{2}A˚2 was found to be good agreement with the prediction of the principle of detailed balancing when compared with the cross section ${Q}_{21}(^{3}P_{1}\ensuremath{\rightarrow}^{3}P_{0})$. It was also found that, within the sensitivity of the method, collisions with N2{\mathrm{N}}_{2}N2​ molecules do not quench 3P1^{3}P_{1}3P1​ or 3P0^{3}P_{0}3P0​ Hg atoms to the ground state.
The predictions of the incoherent scattering theory of the transport of resonance radiation developed by Holstein and by Biberman are shown to agree with laboratory measurements of the decay constants for the intensity of the mercury 2537 A resonance radiation following a period of optical excitation. Also, the relative importance of the diffusion of resonance atoms and the escape of resonance radiation as mechanisms for the destruction of mercury atoms in the 3P1^{3}P_{1}3P1​ or resonance state are determined from measurements of decay constants as a function of mercury density and cell radius. The experimental results show that diffusion of the resonance atoms is negligible and that the predictions of imprisonment theory are confirmed to within 15%. The experiment sets an upper limit to the diffusion coefficient at unit gas density for atoms in the 3P1^{3}P_{1}3P1​ state of 5\ifmmode\times\else\texttimes\fi{}1017{10}^{17}1017 ${\mathrm{cm}}^{\ensuremath{-}1}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ at 340\ifmmode^\circ\else\textdegree\fi{}K, which is consistent with a value of 4\ifmmode\times\else\texttimes\fi{}1016{10}^{16}1016 ${\mathrm{cm}}^{\ensuremath{-}1}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ predicted using the frequency of excitation transfer collisions calculated by Holstein. The success of the theories based on completely incoherent scattering of resonance radiation points to the desirability of including this feature in the treatment of those astrophysical problems in which the spectral line shape is determined by Doppler broadening or by collision broadening.
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.
We have measured the alignment of Hg(6 3^{3}3P1{\mathit{P}}_{1}P1​) atoms following off-resonant excitation of the Hg 6 1S0^{1}\mathrm{S}_{0}1S0​--6 3P1^{3}\mathrm{P}_{1}3P1​ transition in the presence of Kr. We present a polarization spectrum for both red and blue wings out to detunings beyond 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This represents a considerable extension to the existing results, which has been made possible by the use of a pump-and-probe excitation scheme. In the experiment a pulsed laser, detuned from the resonance, creates an alignment in the upper state. This is probed by tuning a delayed second laser to a higher transition and monitoring laser-induced fluorescence from the two-step process. The alignment of the asymptotic Hg(6 3^{3}3P1{\mathit{P}}_{1}P1​) atoms is determined by comparison of the signals obtained for different relative polarizations of the lasers. We discuss the advantages of this method and the relation of the raw data to the polarizations presented. The results are discussed within the context of a simple quasiclassical model of the process.