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Physical Review

Measured Lifetime of the ( 3p54s ) P13 State of Argon

John L. Morack, Clifford E. Fairchild

Published Nov 5, 1967• 18 citations
Publisher

Abstract

A direct measurement of the natural lifetime for the ($3{p}^{5}4s$) $^{3}P_{1}$ state of argon has been made using a modified delayed-coincidence method. Argon atoms are repetitively excited by pulses of electrons. For each pulse, the decaying light signal is sampled for a time $\ensuremath{\Delta}t$ at some delay time ${t}_{1}$ after the excitation is terminated. The exponential decay is then reconstructed as the delay time is changed. Collision effects are not important in the pressure region (5.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$-2.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ Torr) of the experiment and the effects due to cascading are eliminated by keeping the energy of the bombarding electrons low enough so that excitation of the $3{p}^{5}4p$ levels is negligible. The measured lifetime is dependent upon the argon pressure, and this effect is attributed to resonance trapping; a value of 21\ifmmode\pm\else\textpm\fi{}2 nsec is obtained for the natural lifetime by using the existing theories of resonance trapping to extrapolate the data to zero pressure.

DOI
10.1103/physrev.163.125

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A direct measurement of the natural lifetime for the (3p54s3{p}^{5}4s3p54s) 3P1^{3}P_{1}3P1​ state of argon has been made using a modified delayed-coincidence method. Argon atoms are repetitively excited by pulses of electrons. For each pulse, the decaying light signal is sampled for a time \ensuremathΔt\ensuremath{\Delta}t\ensuremathΔt at some delay time t1{t}_{1}t1​ after the excitation is terminated. The exponential decay is then reconstructed as the delay time is changed. Collision effects are not important in the pressure region (5.0\ifmmode\times\else\texttimes\fi{}10\ensuremath−6{10}^{\ensuremath{-}6}10\ensuremath−6-2.0\ifmmode\times\else\texttimes\fi{}10\ensuremath−4{10}^{\ensuremath{-}4}10\ensuremath−4 Torr) of the experiment and the effects due to cascading are eliminated by keeping the energy of the bombarding electrons low enough so that excitation of the 3p54p3{p}^{5}4p3p54p levels is negligible. The measured lifetime is dependent upon the argon pressure, and this effect is attributed to resonance trapping; a value of 21\ifmmode\pm\else\textpm\fi{}2 nsec is obtained for the natural lifetime by using the existing theories of resonance trapping to extrapolate the data to zero pressure.
A delayed-coincidence technique has been used to measure the lifetime of the 5d6p1F35d6p^{1}F_{3}5d6p1F3​ state of atomic barium. An intermediate, 6s5d1D26s5d^{1}D_{2}6s5d1D2​ metastable state of barium atoms in an atomic beam was populated by electron bombardment, and a pulsed cw laser was employed to excite atoms from the metastable state to the 5d6p1F35d6p^{1}F_{3}5d6p1F3​ state. A time-to-amplitude convertor was used to measure the time distribution of the fluorescent light. The value obtained for the lifetime was (44.5\ifmmode\pm\else\textpm\fi{}1.8) nsec.
The photon-photon delayed coincidence technique is used to measure the lifetime; the value (6.21+or-0.10) ns is found. The measurements are obtained under conditions where pressure-dependent effects, as well as other known systematic errors, are negligible. A procedure for data analysis which has not previously been used with the present technique is employed. In this procedure, the apparatus response function is obtained directly and the measured time spectrum is deconvoluted using this response function to yield the lifetime.
We report the first observation of the atomic cascade $9^{1}P_{1}\ensuremath{-}7^{3}S_{1}\ensuremath{-}6^{3}P_{1}$ in mercury. The exponential distribution of time delays between detection of the $9^{1}P_{1}\ensuremath{-}7^{3}S_{1}$ (5676-\AA{}) photon and the $7^{3}S_{1}\ensuremath{-}6^{3}P_{1}$ (4358-\AA{}) photon was measured, and a value of 8.2(0.2) nsec was obtained for the mean lifetime of the 73S17^{3}S_{1}73S1​ level.
The lifetime of the 7s6d7s6d7s6d 1D2{^{1}D}_{2}1D2​ state of atomic radium is determined to be $385(45)\text{ }\ensuremath{\mu}\text{s}$ using cold 226Ra^{226}\text{R}\text{a}226Ra atoms prepared in a magneto-optical trap. The 1D2{^{1}D}_{2}1D2​ state is populated from the decay of the 1P1{^{1}P}_{1}1P1​ state which is excited by a pulse of 483 nm light. The decay of the 1D2{^{1}D}_{2}1D2​ state is observed by detecting delayed fluorescence at 714 nm from the last step in the decay sequence ${^{1}P}_{1}\text{\ensuremath{-}}{^{1}D}_{2}\text{\ensuremath{-}}{^{3}P}_{1}\text{\ensuremath{-}}{^{1}S}_{0}$. The measured lifetime is compared to a number of theoretical calculations. An improved value of the 7s7p7s7p7s7p 1P1{^{1}P}_{1}1P1​ level of $20\text{ }715.598(6)\text{ }{\text{cm}}^{\ensuremath{-}1}$ is obtained.
The radiative lifetime of the 21P12^{1}P_{1}21P1​ and 31P13^{1}P_{1}31P1​ state of atomic helium has been measured by the level-crossing technique. In the experiment, a beam of 21S02^{1}S_{0}21S0​ metastable atoms was first produced and then excited to the 21P12^{1}P_{1}21P1​ or 31P13^{1}P_{1}31P1​ state with resonance radiation. The lifetimes were measured by observing the zero-field level-crossing signals in the detected 584- or 537-\AA{} fluorescent radiation when these 1P1^{1}P_{1}1P1​ states decayed to the 11S01^{1}S_{0}11S0​ ground state. The results are $\ensuremath{\tau} (2^{1}P_{1})=(5.57\ifmmode\pm\else\textpm\fi{}0.15)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec and $\ensuremath{\tau} (3^{1}P_{1})=(1.73\ifmmode\pm\else\textpm\fi{}0.11)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ sec. These values are in excellent agreement with theory.
The effect of imprisonment of radiation in sodium vapor on the measured lifetime of the 32P3 ^{2}P32P states has been investigated in an experiment in which the atoms in the vapor were excited with 10-nsec pulses of resonance radiation and the elapsed times between the exciting and the fluorescent pulses were recorded using a time-to-amplitude converter and kicksorter. At vapor densities below 1010{10}^{10}1010 atoms/cm3{\mathrm{cm}}^{3}cm3, where there is no multiple scattering, the measured lifetime reaches a constant value of (1.63\ifmmode\pm\else\textpm\fi{}0.04)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ sec, equal to the natural lifetime of the 32P3 ^{2}P32P states. In the density range 109{10}^{9}109-1013{10}^{13}1013 atoms/cm3{\mathrm{cm}}^{3}cm3, the variation of the effective lifetime with the vapor density is in good agreement with the predictions of Milne's theory of radiation trapping; agreement with Holstein's theory is observed only in the range 1012{10}^{12}1012-1013{10}^{13}1013 atoms/cm3{\mathrm{cm}}^{3}cm3. The results of a subsidiary Hanle experiment show that a multiply scattered photon of sodium resonance fluorescence loses its original polarization and that, at high sodium vapor pressure, coherence is not preserved.