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Journal of Applied Physics

Radiation trapping of the Hg 185 nm resonance line

Kenneth L. Menningen, James E. Lawler

Published Sep 15, 2000• 26 citations
Publisher

Abstract

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.

DOI
10.1063/1.1289050

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Chinese Physics B•Sep 1, 2014•5 citations
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.
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.
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%.
We have studied sodium resonance radiation under conditions of severe radiation trapping in sodium-argon mixtures, and we report measured values of the effective radiative decay rates. The measured values are compared with values calculated from the Holstein theory of radiation trapping in the impact-regime, foreign-gas pressure-broadening limit. The experiment was designed to satisfy all of the validity criteria imposed by the approximations of the Holstein theory, and the experiment and theory are in good agreement. Since the effective radiative decay rate depends upon the collisional line-broadening rate, confirmation of the theory allows such measurements to be used to determine unknown line-broadening constants using only broadband lasers and low resolution monochromaters. We demonstrate this technique for the sodium D lines broadened by xenon and neon perturbers. The values for the broadening constants obtained in this manner are in reasonable agreement with values from several previous studies utilizing more traditional techniques.
The effective radiation rate for Cs (6P3/2) resonance level in the presence of helium was determined. A cell filled with Cs metal and He at pressure P(He) (0-500 Pa) was established at T = 300 K. The cell was heated in the temperature range between 330 and 370 K, which produced Cs number densities approximately in the range between 10(12) and 10(13) cm(-3). Cs atoms were excited to the 6P3/2 state using a single-mode diode laser(pump laser). The transmission of the medium at the center of the Doppler envelope of the strong h. f. component of the CsD2 line due to hyperfine pumping alone amounts to approximately 5%. The assumption that has been made is that the lower-state hyperfine levels are populated in a statistical ratio. The excited-atom density and spatial distribution were mapped by monitoring the absorption of a counterpropagating single-mode diode laser beam (probe laser) tuned to the 6P3/2 --> 8S1/2 transition which could be translated parallel to the pump beam. In the presence of radiation trapping, the spontaneous radiation rate is multiplied by the transmission factor T6P3/2 -->1/2, which describes the average probability that photons emitted within the fluorescence detection region can pass through the optically thick vapor without being absorbed. The T6P3/2 --> 6S1/2 is related to the frequency dependent absorption cross section and the density and spatial distribution of atoms at the level of the transition. Position dependent 6P3/2 state densities were combined with the collisional broadening rate of 6P3/2 <-- 6S1/2 line due to the perturbation of both helium and cesium to yield T6P3/2 --> 6S1/2. The effective radiation rates of the Cs D2 line as a function of the He pressure P(He) were obtained. The helium caused line broadening and therefore increased the effective radiation rate. The fluorescence intensity I852 of the 6P3/2 --> 6S1/2 emission was measured simultaneously. The measured fluorescence ratios determined the ratios of the effective radiation rates at different He density. These ratios are in agreement with theoretical evaluation.
Experiments on one- and two-photon laser excitation of 232{}^{232}232Th+{}^{+}+ ions in a radio-frequency ion trap are reported. As the first excitation step, the strongest resonance line at 402 nm from the (6d27s)J=3/2(6{d}^{2}7s)J=3/2(6d27s)J=3/2 ground state to the (6d7s7p)J=5/2(6d7s7p)J=5/2(6d7s7p)J=5/2 state at 24874 cm${}^{\ensuremath{-}1}$ is driven by radiation from an extended-cavity diode laser. Spontaneous decay of the intermediate state populates a number of low-lying metastable states, thus limiting the excited state population and fluorescence signal obtainable with continuous laser excitation. We study the collisional quenching efficiency of helium, argon, and nitrogen buffer gases, and the effect of repumping laser excitation from the three lowest-lying metastable levels. The experimental results are compared with a four-level rate equation model, which allows us to deduce quenching rates for these buffer gases. Using laser radiation at 399 nm for the second step, we demonstrate two-photon excitation to the state at 49 960 cm${}^{\ensuremath{-}1}$, among the highest-lying classified levels of Th+{}^{+}+. This is of interest as a test case for the search for higher-lying levels in the range above 55 000 cm${}^{\ensuremath{-}1}$ which can resonantly enhance the excitation of the 229{}^{229}229Th+{}^{+}+ nuclear resonance through an inverse two-photon electronic bridge process.
The influences of different buffer gas, neon and helium, on Hg-199(+) clock transition are compared in trapped Hg-199(+) linear trap. By the technique of time domain's Ramsey separated oscillatory fields, the buffer gas pressure frequency shifts of Hg-199(+) clock transition are measured to be (df/dP(Ne)) (1/f) = 1.8 x 10(-8) Torr(-1) for neon and (df/dP(He)) (1/f) = 9.1 x 10 8 Torr(-1) for helium. Meanwhile, the line-width of Hg-199(+) clock transition spectrum with the buffer gas neon is narrower than that with helium at the same pressure. These experimental results show that neon is a more suitable buffer gas than helium in Hg-199(+) ions microwave frequency standards because of the Hg-199(+) clock transition is less sensitive to neon variations and the better cooling effect of neon. The optimum operating pressure for neon is found to be about 1.0 x 10(-5) Torr in our linear ion trap system.