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Journal of Physics B Atomic and Molecular Physics

An accurate determination of the lifetime of the 4p4D7/20state in argon II

GEORGE C. KING, K.A. Mohamed, Frank H. Read, Robert E. Imhof

Published Jun 1, 1976• 14 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/9/8/010

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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.
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 $\ensuremath{\Delta}t$ 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{}${10}^{\ensuremath{-}6}$-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 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.
The electron-photon delayed coincidence technique has been used to measure precisely the lifetime for the 4p 1 P 0 state in calcium. The lifetime of this state was obtained by measuring the time correlation spectrum between the inelastically scattered electrons and the corresponding de-excitation photons. By using the least-squares fitting program to fit Gale's equation to the data points, a value of (4.15±0.17) ns was obtained. The result was compared to some of the earlier experimental measurements.
A new high-efficiency photon-photon delayed coincidence lifetime measuring apparatus has been constructed and was used to study the 5d {dollar}\\sp4{dollar}F{dollar}\\sb{lcub}9/2{rcub}{dollar} {dollar}\\longrightarrow{dollar} 5p {dollar}\\sp4{dollar}D{dollar}\\sbsp{lcub}7/2{rcub}{lcub}\\rm o{rcub}{dollar} {dollar}\\longrightarrow{dollar} 5s {dollar}\\sp4{dollar}P{dollar}\\sb{lcub}5/2{rcub}{dollar} cascade in Kr II and the 6d {dollar}\\sp4{dollar}F{dollar}\\sb{lcub}7/2{rcub}{dollar} {dollar}\\longrightarrow{dollar} 6p {dollar}\\sp4{dollar}D{dollar}\\sbsp{lcub}5/2{rcub}{lcub}\\rm o{rcub}{dollar} {dollar}\\longrightarrow{dollar} 6s {dollar}\\sp4{dollar}P{dollar}\\sb{lcub}3/2{rcub}{dollar} cascade in Xe II. A neutral atomic beam was excited and ionized by electron impact, and the spontaneously emitted photons were collected by an ellipsoidal mirror, collimated by an aspheric lens, and detected by two Amperex XP2020Q fast linear-focussed photomultipliers provided with narrow-band interference filters. The spectrum of time delays was acquired by a time-to-amplitude converter-multichannel pulse-height analyser combination. The lifetime of the Kr II 5p {dollar}\\sp4{dollar}D{dollar}\\sbsp{lcub}7/2{rcub}{lcub}\\rm o{rcub}{dollar} state was found to be 7.004 {dollar}\\pm{dollar} 0.053 nanoseconds and is in agreement with two previous measurements. This first measurement provided a calibration of the newly constructed apparatus. It also has provided an improvement in the precision with which this lifetime is known by a factor of 3. The Xe II 6p {dollar}\\sp4{dollar}D{dollar}\\sbsp{lcub}5/2{rcub}{lcub}\\rm o{rcub}{dollar} state lifetime was found to be 9.634 {dollar}\\pm{dollar} 0.067 nanoseconds. This result agrees with several measurements of lower accuracy but does not agree with two previous measurements claiming high precision. This discrepancy has not been resolved in this work.