The lifetime of the 1s2s3S1 level of the He-like O6+ ion has been measured using the Electron Beam Ion Trap in the magnetic trapping mode. A value of 956-4+5 μs is found, which corresponds to a radiative transition rate of 1046-5+4 s-1 for the magnetic dipole transition to the 1s21S0 ground state. This value is in excellent agreement with recent theoretical predictions and distinguishes among different treatments of negative energy states and correlation in multiconfiguration Dirac-Fock calculations

Beiersdorfer, P.

Lopez-Urrutia, J. R. Crespo

Savin, Daniel Wolf

Widmann, K.

Columbia University Academic Commons

PHYSICAL REVIEW A JULY 1998VOLUME 58, NUMBER 1Precision measurement of the lifetime of the 1s2s 3S1 metastable level in heliumlike O61J. R. Crespo Lo´pez-Urrutia,1 P. Beiersdorfer,1 D. W. Savin,2 and K. Widmann11Department of Physics and Space Technology, Lawrence Livermore National Laboratory, Livermore, California 945502Department of Physics and Columbia Astrophysics Laboratory, Columbia University, New York, New York 10025~Received 29 December 1997!The lifetime of the 1s2s 3S1 level of the He-like O61 ion has been measured using the Electron Beam IonTrap in the magnetic trapping mode. A value of 9562415 ms is found, which corresponds to a radiative transitionrate of 10462514 s21 for the magnetic dipole transition to the 1s2 1S0 ground state. This value is in excellentagreement with recent theoretical predictions and distinguishes among different treatments of negative energystates and correlation in multiconfiguration Dirac-Fock calculations. @S1050-2947~98!05307-4#PACS number~s!: 32.70.Cs, 52.70.La, 95.30.KyI. INTRODUCTIONLine emission from metastable levels that decay only viaelectric dipole-forbidden transitions, such as the 1s2s 3S1level in heliumlike O61, plays an important role in plasmadiagnostics @1#. The metastable level will be collisionallydepopulated when the electron density approaches a criticalvalue, transforming the line emission into a diagnostic ofelectron density. Accurate knowledge of the radiative rate is,therefore, a requisite for inferring reliable electron densitiesfrom such lines. It is also important for testing calculationsof atomic wave functions, as small inaccuracies may lead tolarge changes in the rates of forbidden transitions. Thus theserates are sensitive indicators of the accuracy of atomic cal-culations.The 1s2s 3S1 level is the lowest-lying excited level in theheliumlike system. It is forbidden to decay to the 1s2 1S0ground level by an electric dipole transition. At low densitiesthe level decays via a magnetic dipole transition to the1s2 1S0 ground level. At sufficiently high collision rates, i.e.,sufficiently high electron densities, the level is collisionallydepopulated by transferring its population to the nearby1s2p 3P1 level. The intensity ratio of the 1s2s 3S1!1s2 1S0 forbidden transition to the 1s2p 3P1!1s2 1S0intercombination line is, thus, sensitive to the electron den-sity, as shown by several modeling calculations @2–4#. Thedensity at which the electron collision rate rivals the pre-dicted radiative decay rate of the 1s2s 3S1 level is in therange 1010–1012 cm23. This density range is typically foundin active solar and stellar plasmas. High-resolution measure-ments of the O61 K-shell line emission from the Sun haveindeed shown this density dependence @5#.Early estimates of the 3S1 radiative decay rate were pro-vided by Gabriel and Jordan @6# using the relationAr54.43109l5s21. ~1!Their estimate yielded a value of 1.198 ms based on thetransition wavelength l522.0977 Å. This value was re-vised in many subsequent ab initio calculations @7–11#.Some of those predictions ~e.g., Ref. @9#! are only by-products of calculations testing methods to be applied toPRA 581050-2947/98/58~1!/238~4!/$15.00more complex atoms, and did not intend to reach the preci-sion that could have been achieved when specifically appliedto two-electron ions. The newer calculations provided valuesbetween 943 and 955 ms employing a variety of differenttheoretical approaches. One of these approaches, the multi-configuration Dirac-Fock method, was recently shown by In-delicato @12# to provide significantly different values depend-ing on whether and how correlations and the negative energycontinuum are included in the calculations. The largest scat-ter among the different approaches was found in low-Z ions,i.e., for the smallest radiative rates. Similar differences werefound by Johnson, Plante, and Sapirstein @11#. For low-Zions such as oxygen, these authors calculated the radiativedecay rate using the Hartree potential, as it produced moreaccurate results than using the Coulomb potential because ofa significantly smaller need for negative energy corrections.In order to experimentally test and distinguish among thecalculations, a measurement with an accuracy better than afew percent is needed. However, only recently have methodsbeen developed that allowed measurements of the long-lived1s2s 3S1 level in heliumlike ions. Radiative lifetimes in themillisecond regime have become accessible to experimentalscrutiny in storage rings @13#; measurements of radiative life-times in the microsecond regime were demonstrated at theElectron Beam Ion Trap ~EBIT! facility at Livermore@14,15#. The latter facility used an electron beam to confinethe ions and excite the transition of interest, and adjustmentsfor collisional effects had to be made.Recently, a new mode of operation, dubbed the magnetictrapping mode, has been developed @16#. This mode dis-penses with the need to account for electron collisions whenmaking radiative lifetime measurements on the EBIT facil-ity. The magnetic mode allows the measurement of the decayof much longer-lived metastable levels than before and withhigher accuracy. The method relies on the observation thations produced in EBIT remained trapped for many secondsafter the electron beam was turned off @16–18#. In the ab-sence of the electron beam, the fluorescent decay of meta-stable levels can be observed without collisional perturba-tions. This has been illustrated in a measurement of the1s2s 3S1 level in heliumlike N51, which has been measuredwith a 3% accuracy @16#. In the following we use the capa-bility provided by the magnetic trapping mode method to238 © 1998 The American Physical SocietyPRA 58 239PRECISION MEASUREMENT OF THE LIFETIME OF . . .measure the radiative lifetime of the 1s2s 3S1 level in heli-umlike O61 within a 0.5% accuracy.II. EXPERIMENTAL ARRANGEMENTThe measurement was carried out at the Livermore EBITfacility. Oxygen was introduced into the trap by injecting amolecular beam of CO2 into the trap. We used a continuousinjection of CO2 at a gas injector pressure of 1027 torr, giv-ing a neutral CO2 density of roughly 106 cm23, or 10211torr in the trap. The beam energy was 1.2 keV, and the cur-rent around 25 mA. The beam dissociated and ionized theCO2 molecules. Although the beam energy was above thethreshold to strip oxygen completely, it was necessary tochoose such a high energy given the nonequilibrium, tran-sient ionization conditions of the measurement to producesufficient amounts of heliumlike oxygen ions.Similar to our earlier measurement of N51 and Ne81@14,16#, we measured the total intensity of the Ka emissionwith a lithium drifted silicon Si~Li! detector mounted side-onin EBIT. The advantage of the low resolution Si~Li! detectorlies in a high counting efficiency, which is essential forachieving good statistics.The measured Ka emission comprised three 2!1 transi-tions. These were the transitions 1s2p 1P1!1s2 1S0,1s2p 3P1!1s2 1S0, and 1s2s 3S1!1s2 1S0, labeled w , y ,and z in standard notation, as illustrated schematically in Fig.1. To observe the fluorescent decay of the electron popula-tion in the 1s2s 3S1 metastable level, we turned off the elec-tron beam for periods of 10 ms by switching off the powersupply used for the anode extraction potential of the electrongun. The beam was then turned on again for another 10 ms.Upon turning off the beam, direct electron-impact excita-tion immediately ceased. Transitions w and y have transitionrates that are many orders of magnitude larger than z . Theiremission thus ceased almost instantaneously, as is indicatedby the steep drop in the recorded Ka emission shown in Fig.2. This drop is slightly rounded by the finite time resolutionof the detector electronics. By contrast, the decay of the1s2s 3S1 level produced emission that was seen to last wellinto the excitation-free period.The time scale was calibrated to the ppm level with thehelp of a calibrated signal generator. Since the rate of dataaccumulation was on the order of a few counts per cycle,pulse pile-up and dead time effects can be totally neglected.FIG. 1. Schematic level diagram of the n51 and n52 levels ofheliumlike O61. The prominent decay paths are indicated.III. RESULTSWe fitted the temporal evolution of the 3S1 fluorescentdecay to a single exponential with statistical weighting and aconstant background, as shown in Fig. 3. The resulting life-time is t5955.9 ms. The statistical uncertainty is 3.5 ms.Fits of double exponentials have been attempted but failed todescribe the data.Systematic errors might arise from ion loss from the trapduring the magnetic trapping mode. Previous experiments,however, have shown that ions remain trapped on the orderof seconds @16–18#. The dominant loss mechanism is chargeexchange reactions with neutral background gases. Since weFIG. 2. Ka emission from heliumlike O61. The emission dropsimmediately after the electron beam is turned off, revealing thefluorescent decay signal of the 1s2s 3S1 level.FIG. 3. Fit of a single exponential to the x-ray emission mea-sured during the magnetic trapping mode ~top!. Residuals of the fitare given below.240 PRA 58J. R. CRESPO LO´ PEZ-URRUTIA et al.continuously injected CO2 into the trap, charge exchange ofO61 ions with CO2 was a possible loss mechanism in thepresent measurement. Moreover, charge capture of O71 ionswas a possible feeding mechanism of the heliumlike 3S1level.Given the estimated neutral density of 106 cm23 in thetrap, an ion temperature of less than 1 keV @19#, and chargeexchange cross sections on the order of less than 10214 cm2@20#, an upper bound of less than 0.1 s21 can be placed onthe rate of charge exchange recombination losses. This isnegligible compared to the measured radiative decay rate ofabout 103 s21. Similarly, population of the metastable levelby charge exchange is also negligible.As an upper limit on ion losses, we assume an ion storagetime of 1 s, which is less than the shortest storage time mea-sured in Fourier-tranform ion-cyclotron measurements@17,18#. Incorporating such a systematic error into our errorlimits, we find a lifetime of 9562415 ms for the heliumlike 3S1level and a radiative transition rate of 10462514 s21.IV. COMPARISON WITH THEORYThe measured radiative lifetime can be compared to vari-ous theoretical results. Using nonrelativistic wave functions,Drake @7# obtained a value of 957.9 ms. This value is withinthe uncertainty limits of our measurement. The relativisticcalculation by Johnson and Lin @8# giving 950.6 ms is inslightly less good agreement. A very recent calculation em-ploying exact relativistic wave functions by Johnson, Plante,and Sapirstein @11# yielded a value of 955.1 ms, which is inbest agreement with our measured value.By contrast, two other predictions based on relativisticwave functions yielded results that do not agree with ourmeasurement. These are the value of 945.2 ms computed byJohnson and Lin @9# and the value of 943 ms computed byLin, Johnson, and Dalgarno @10#. There is also poor agree-ment with the rates calculated with the single-configurationDirac-Fock method by Indelicato @12#, as illustrated in Fig.4. However, the agreement clearly improves as correlationand negative energy states are included in the calculations.Figure 4 also shows the predictions of Johnson, Plante,and Sapirstein @11# using exact relativistic wave functions.The present measurement is only the second measurementwith a sub-1% uncertainty among low-Z ions and is clearlyable to distinguish between the calculations of Johnson,Plante, and Sapirstein and the best multiconfiguration Dirac-Fock calculations from Ref. @12#. Our present measurementand that of C41 by Schmidt et al. @13# are in excellent agree-ment with the predictions of Johnson, Plante, and Sapirstein,but differ significantly from multiconfiguration Dirac-Fockcalculations.ACKNOWLEDGMENTSThis work was performed under the auspices of the U.S.Department of Energy by LLNL under Contract No. W-7405-ENG-48, and was supported in part by NASA’s X-RayAstronomy Research and Analysis Program under Work Or-der W-19127.FIG. 4. Comparison of the measured magnetic dipole transitionrate in O61 with results from single-configuration Dirac-Fock cal-culations ~dashed line!, multiconfiguration Dirac-Fock calculationswithout ~dotted line! and with taking negative energy states intoaccount ~dot-dashed line! from Ref. @12#. Also shown are the valuescalculated in Ref. @11# ~solid line! using exact relativistic wavefunctions. All results are expressed relative to the single-configuration Dirac-Fock values. Previously measured values arefrom the following references: C41, Ref. @13#; N51, Refs. @13,16#;Ne81, Ref. @14#; Mg101, Ref. @15#.@1# C. de Michelis and M. Mattioli, Nucl. Fusion 21, 677 ~1981!.@2# R. Mewe and J. Schrijver, Astron. Astrophys. 65, 99 ~1978!.@3# R. Mewe and J. Schrijver, Astron. Astrophys., Suppl. Ser. 33,311 ~1978!.@4# A. K. Pradhan and J. M. Shull, Astrophys. J. 249, 821 ~1981!.@5# G. A. Doschek, U. Feldman, P. B. Landecker, and D. L. Mc-Kenzie, Astrophys. J. 249, 372 ~1981!.@6# A. H. Gabriel and C. Jordan, Phys. Lett. 32A, 166 ~1970!.@7# G. W. F. Drake, Phys. Rev. A 3, 908 ~1971!.@8# W. R. Johnson and C. D. Lin, Phys. Rev. A 9, 1486 ~1974!.@9# W. R. Johnson and C. D. Lin, Phys. Rev. A 14, 565 ~1976!.@10# C. D. Lin, W. R. Johnson, and A. Dalgarno, Phys. Rev. A 15,154 ~1977!.@11# W. R. Johnson, D. R. Plante, and J. Sapirstein, in Advances ofAtomic, Molecular, and Optical Physics Vol. 35, edited by B.Bederson and H. Walther ~Academic Press, San Diego, 1995!,p. 255.@12# P. Indelicato, Phys. Rev. Lett. 77, 3323 ~1996!.@13# H. T. Schmidt, P. Forck, M. Grieser, D. Habs, J. Kenntner, G.Miersch, R. Repnow, U. Schramm, T. Schu¨ssler, D. Schwalm,and A. Wolf, Phys. Rev. Lett. 72, 1616 ~1994!.@14# B. J. Wargelin, P. Beiersdorfer, and S. M. Kahn, Phys. Rev.Lett. 71, 2196 ~1993!.@15# G. S. Stefanelli, P. Beiersdorfer, V. Decaux, and K. Widmann,Phys. Rev. A 52, 3651 ~1995!.@16# P. Beiersdorfer, L. Schweikhard, J. Crespo Lo´pez-Urrutia, andPRA 58 241PRECISION MEASUREMENT OF THE LIFETIME OF . . .K. Widmann, Rev. Sci. Instrum. 67, 3818 ~1996!.@17# P. Beiersdorfer, B. Beck, R. E. Marrs, S. R. Elliott, and L.Schweikhard, Rapid Commun. Mass Spectrom. 8, 141 ~1994!.@18# P. Beiersdorfer, B. Beck, St. Becker, and L. Schweikhard, Int.J. Mass Spectrom. Ion Processes 157/158, 149 ~1996!.@19# P. Beiersdorfer, V. Decaux, S. Elliott, K. Widmann, and K.Wong, Rev. Sci. Instrum. 66, 303 ~1995!.@20# R. E. Olson, in Electronic and Atomic Collisions, edited by N.Oda and K. Takayanagi ~North-Holland, Amsterdam, 1980!, p.391.

Precision measurement of the lifetime of the 1s2s3S1 metastable level in heliumlike O6+

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Precision measurement of the lifetime of the 1s2s3S1 metastable level in heliumlike O6+

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