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Hyperfine-Structure Studies of Highly Excited D and F Levels in Alkali Atoms Using acw Tunable Dye Laser

Svanberg, Sune; Tsekeris, P; Happer, W

Published in:Physical Review Letters - Moving Physics Forward

DOI:10.1103/PhysRevLett.30.817

1973

Link to publication

Citation for published version (APA):Svanberg, S., Tsekeris, P., & Happer, W. (1973). Hyperfine-Structure Studies of Highly Excited D and F Levelsin Alkali Atoms Using a cw Tunable Dye Laser. Physical Review Letters - Moving Physics Forward, 30(18), 817-820. https://doi.org/10.1103/PhysRevLett.30.817

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VOLUME 30, NUMBER 18 PHYSICAL REVIEW LETTERS 30 APRir. 19?3

*Work supported by the National Aeronautics and

Space Administration Contract No, NGR 44-006-156and the Atmospheric Sciences Section, National Sci-ence Foundation Grant No. GA 27169.

S. S. Huang, Astrophys. J. 108, 854 (1948).A, Burgess and M. J. Seaton, Mon. Notic. Astron.

Soc. 120, 121 (1960).D. W. Norcross, J. Phys. B: Proc. Phys. Soc. ,

London 4, 652 (1971).V. Jacobs, Phys„Bev. A 4, 989 (1971).J. S. Howard, J. P. Riola, B. D. Bundel, and R. F.

Stebbings, Phys. Rev. Lett. 29, 821 (1972).B. D. Rundel, F. B. Dunning, J. S. Howard, J. P.

Biola, and B. F, Stebbings, Bev. Sci, Instrum. 44, 60(197'}.

E. D. Stokes, F. B. Dunning, R. F, Stebbings, G. K.Walters, and R, D. Rundel, Opt. Commun, 5, 267(1972).

F, B, Dunning and E. D. Stokes, Opt. Commun. 6,160 (1972}.

F. B. Dunning, E. D. Stokes, and B. F, Stebbings,Opt. Commun. 6, 68 (1972}.

F, B. Dunning, F. K. Tittel, and B. F. Stebbings, tobe published."B, D. Rundel, K. L. Aitken, and M. F, A. Harrison,

J. Phys. B: Proc. Phys, Soc., London 2, 954 (1969).

Hyperfine-Structure Studies of Highly Excited D and I' Levelsin Alkali Atoms Using a cw Tunable Dye Laser*

S. Svanbe rg, t P. Tseke r is, and ~. Happe rColumbia RaCiation Laboratory, Department of Physics, Columbia University, Nen~ York, Nem York 10027

(R,eceived 26 March 1973)

Highly excited S, D, and E levels in cesium and rubidium have been populated utilizinga two-step optical excitation method. In the first step an rf lamp is used to transferatoms into the first P states, and the subsequent excitation is performed with the tunableradiation of a cw dye laser. We give results for the hyperfine structure of several Dand E states in Cs 3~ and Rb8', obtained in level-crossing and optical double resonanceexperiments.

Until recently, accurate hyperfine structurestudies of excited state. s in alkali atoms have beenconfined to the sequences of 'P levels, which arereadily available for observation through directexcitation with resonance radiation. Detailedstudies of P levels have revealed several effectswhich cannot be explained by a simple, one-elec-tron picture of the alkali atoms. These includemagnetic core polarization, electric-quadrupoleshielding, and possible deviations from expectedLande g~ factors. ' Clearly, a correspondingknowledge of the properties of non-P states is ofgreat interest for the understanding of the vari-ous atomic interactions.

It is well known that the D-state fine-structureintervals are often anomalously small and eveninverted, and recent experimental work has shownthat similar, large anomalies occur in the D-statehyperfine structure. Data on D-state hyperfinestructure are still quite limited, and F-state dataare nonexistent. The scarcity of data can betraced to inadequacies of existing experimentalmethods. Archambault et al. ' have made roughestimates of the hyperfine structure of the O'D»,state in Na" and the O'D», state in Cs'", which

were produced by electron bombardment. Kiththe recently introduced cascade decoupling' andcascade rf spectroscopy methods' the Columbiagroup has investigated the hyperfine structure ofseveral $ and D levels in the alkali atoms. ' Inthese experiments the states are produced by cas-cading from a P level, excited by a conventionalrf lamp. However, for reasons of intensity, thismethod is limited to comparatively low-lying $and D levels as the absorption oscillator strengthsin the $-P transition sequence decrease very rap-idly with increasing P-state energy, and conven-tional lamps are not very efficient sources forthe higher resonance lines. To get access tomore highly excited non-P levels we have utilizeda two-step excitation method, ' taking advantageof recent advances in the technology of tunablela,sers. In the first step the strong D, and D,lines from a powerful rf lamp are used to trans-fer atoms from the ground $ state to the first ex-cited P states. In the second step, the intensetunable radiation from a cw dye laser, operatingwith rhodamine 60, is used to excite atoms froma P level to a highly excited S or D level. Ab-sorption of light by atoms in the O'P», level of

Vox.UMz 30, NUMszR 18 30 APRj:r. 1973

Rb has earlier been demonstrated by Bradley etal. ,

' utilizing high-power pulsed dye lasers. Fur-ther, a two-step excitation of the O'D term inlithium has been used in a fine-structure investi-gation by Smith and Eck.' In this case the lightfor the excitation was provided in a discharge inthe studied alkali vapor. However, as in theelectron excitation experiments of Archambaultet ul. ,

' it was hard to achieve a very well definedexcitation situation. The excitation method de-scribed in this Letter is particularly useful sinceit can be used in conjunction with conventionaloptical double resonance (ODR) and level cross-ing (LC) methods to investigate the hyperfinestructures and fine structures of a large numberof excited states. As an example, we discusssome of our recent measurements for a numberof excited D and F states in rubidium and cesium.

In performing experiments on the highly excitedalkali states it is very valuable to have reliableestimates on absorption oscillator strengths, nat-ural lifetimes, and branching ratios. We havemade extensive computer calculations of thesequantities utilizing the Coulomb approximationmethod of Bates and Damgaard. '

Qur experimental arrangement is based on theapparatus described in Refs. 4 and 5. A power-ful rf lamp was constructed for producing thelight for the first excitation step. The beam ofD, and D, light entered the resonance cell in thedirection of the external magnetic field, which isalso the detection direction. The laser beam,which has a diameter of about 3 mm, passesthrough the resonance cell at right angles to thefield direction. We use a Spectra Physics model370 tunable dye laser, which is pumped by the5145-A line of a Spectra Physics model 165 argonion laser. In multimode operation the dye lasergives up to 300 mW output for an input power of2 W. With rhodamine 6G as a dye, the usablewavelength region extends from about 5550 toabout 6300 A. A bandwidth of about 0.3 A is ob-tained. Fluorescent light released in the decayof the highly excited state is detected by an RCAC31000F photomultiplier. The rf field requiredin the QDR measurements is produced in a cavity,tuned to about 147 MHz. Lock-in detection isused in the experiments and the signals are storedin a PDP-8/S computer, which also controls thelinear magnetic field sweep.

In the two-step excitation process it is possibleto take advantage of radiation trapping of the D,and D, resonance lines. In the multiple scatter-ing process, an incoming photon can excite sev-

eral ground-state atoms into a 'P state so thatmore P-state atoms will be available for laserexcitation. In this way we have achieved adequatepopulations for QDR and LC spectroscopy of the

9, 10, 11'S», and the 8, 9, 10'D levels in Cs andthe 8, 9'$ and 6, 7 D levels in Rb. The interac-tion region between the atoms and the two lightbeams is about 0.5 cm' and contains at the usualoperation temperature (100'C for Cs, 120'C forRb) about 10"atoms. The number of detectedphotons per second in a solid angle of about 0.15sr is 10'-10', depending on which particularstate is studied and what detection line is used.

We have used the LC method to measure thehyperfine structure of the 8 and O'D, &, states inCs'". Because of the multiple scattering of theD lines the different sublevels of the 6'P», statehave approximately equal populations. W'ith thetunable laser, cr transitions to the 'D», state areinduced and the decay down to the 6'P&(, level isobserved. To avoid direct leakage of the detec-tion line from the rf lamp, a, Schott RG10 colorglass filter is used in the exciting beam to blockall lines except the infrared ones. An interfer-ence filter with a half-width of 50 A is used toisolate the detection line from the stray lightfrom the strong laser beam. A rotating linearpolarizer in the detected beam is used for lock-in detection of the LC signals. In Fig, 1 exam-ples of the measured LC curves are shown. In

20

M&GNETIC FIELD (gauss)

FIG. 1. . Level-crossing signals for the 8 and 9 D3y&states in Cs'33. The sampling time for each of thecurves is about j. h.

818

VOLUME 30, +UMBER 18 PHYSICAL RKVIKW LKTTKRS 30 APRIL 1973

&- cA

(y) Z2 DLLI

03 D3/22

CP'V

p 0

5 P5

7 Ds/22

2I/2

2 S0 0

~0 00

2

52 p PI/2

2S I/2

I i «& I

IO 15

MAGNETIC FIELD (gauss)

I I I I I I I I I

5I

20

FIG. 2. Level crossing signals for the 6 and 7 D3/2states in Hb . Total sampling time for each curve,1.5 h.

[a(9'D,/, Cs"')~

= 2.35(7) MHZ.

The quadrupole interaction constant b is in bothcases small, as the quadrupole moment of Cs'"is only —3 mb. In order to determine the signsof the coupling constants additional measure-ments have to be performed. 4

Corresponding LC measurements were alsomade in the 6 and 7'D, ~, states of Rb". In Fig. 2

experimental curves are shown. From the posi-tions of the resolved crossings we obtain

~a(6'D„, Rb")~=7.72(20) MHz,

~b(6'D, , Rb")~

= 0.6(4) MHz, b/a & 0,

~a(7'D„, Rb")~=4.55(15) MHz,

ib(7 D, i2 Rb' ) i=0.36(18) MHz, b/a&0.

We have also performed QDR measurements inthe Paschen-Back region of the 8 and 9'D», statesin Cs'". The same geometry was used and therf was 100% square-wave modulated for lock-indetection. Eight signals of equal intensity are ex-pected around the center of gravity, correspond-ing to g~ =0.8. The structure obtained is in bothstates unresolved and agrees closely with the the-

both 'D„, states one resolved and two nonresolvedcrossing signals are obtained. From the posi-tions of the crossings we obtain for the dipole in-teraction constant a

la(8'D31, Cs"')l=3.98(12) MHz

oretical structure calculated from the LC results.As the transition between a 'D», state and a

P~ /2 state is forbidden and it is not possible touse the laser transition for the detection due tothe inevitable stray light, a different techniquewas used for measurements in the highly excited'D„, states in Cs. These states decay with abranching ratio of about 15/o into the second ex-cited 'P„, level, and the wavelength (4555 A forcesium) for the transition from this state to theground state can be well isolated from both excit-ing lines. Schott BG3 and BG18 color glass fil-ters were useful in further suppressing the back-ground light. The LC method is not very usefulin this case as the coherence at the crossingpoints is strongly degraded in the passage throughthe second state. Thus we used the QDR tech-nique. The 8, 9, and 10'D», levels in Cs'" allturn out to have a very small hyperfine structure.In the Paschen-Back region an unresolved signalstructure, consisting of eight closely spaced sig-nals, is obtained. The width of a single compo-nent is calculated from the theoretically estimat-ed natural lifetimes. In extrapolations to zerorf power we find the signal structure half-widths4.5(5), 2.7(5), and 2.3(5) G. From these resultsonly rough values for the dipole coupling con-stants can be obtained:

~a(8'D, &,Cs'")

~=0.9(4) MHz,

~a(9'D», Cs'")~=0.5(2) MHz,

~a(10'D, , Cs'")~

= 0.4(2) MHz.

In Cs the 'E levels from O'E to 7'E can be pop-ulated by cascading from the 'D levels excited inthese experiments. The 8'D»2 and 8'D», levelshave a 2% branching ratio into the 5'E», and theO'E», levels, respectively. The decay of the Fstates to the 5'D», and 5'D», levels can be ob-served at 8079 and 8016 A. %e have observedunresolved QDR signals in these E states asshown in Fig. 3. The 5'E doublet is inverted witha fine-structure splitting at only about 4410 MHz.Thus even for zero hyperfine structure, the QDRsignals at an rf frequency of 147 MHz would bespread over 0.6 and 0.9 G in the O'E», levels, re-spectively. The estimated individual signal fullhalf-width is 2.7 and 2.8 G, whereas the mea-sured half-width is about 4.0 G in the O'E7/2 aswell as in the O'E», states. From these values,rough upper limits for the hyperfine structure

VOLUME 30, NUMBER 18 PHYSICAL REVIEW LETTERS 30 APRIL 1973

F5/2R

for determining the signs of the coupling con-stants in the states studied in this work are inprogress.

The authors would like to thank Dr. R. Guptafor valuable advice and assistance.

0-

GC

CO

Ct

0 CO

o oooI

l I 5 l20 'I la5

C-G. ('F5/a)

l 30 'l gauss

C.G.( 0 y/p )

27/2

oor

80 85 &' 900 ~ I

~95 l00 gauss

C.G.( Dg/p) C 6( Fy/p)

FIG. 3. Optical double resonance curves with signalsfrom the 5 E'Vg2 and 5 &5i2 levels in Cs'~ . The centerof gravity {C.G.) for the E states and the position ofthe D resonance also occurring in these experimentsare indicated. Sampling time for each of the curves isabout 1 h.

constants are estimated:

la(5'F„, cs'") l&1.0 MHz

la(5'I'„, cs'")1«v MHz.

Measurements of the hyperfine structure ofother highly excited alkali states and experiments

*Work supported in part by the Joint Services Elec-tronics Program (U.S. Army, U.S. Navy, U.S. AirForce) under Contract No. DAAB07-69-C-0383, in partby the Air Force Office of Scientific Research underGrant No. AFOSB 72-2180, and in part by the SwedishNatural Science Research Council.

)Permanent address: Department of Physics, Chal-mers University of Technology, Fack, S-40220 Gote-borg, Sweden.

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R. Gupta, S. Chang, and W. Happer, Phys. Rev. A 6,529 (1972); L. K. Lam, R. Gupta, and W. Happer, Bull.Amer. Phys. Soc. 18, 121 (1972); C. Tai, R. Gupta,and W. Happer, Bull. Amer. Phys. Soc. 18, 121 (1973).

S. Svanberg, P. Tsekeris, and W. Happer, Bull ~

Amer. Phys. Soc. 18, 611 (1973).'D. J. Bradley, G. M. Gale, and R. D. Smith, J. Phys.

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Determination of the He -He Repulsive Potential up to 0.14 eV by Inversionof High-Resolution Total —Cross-Section Measurements

R. Feltgen, H. Pauly, F. Torello, * and H. VehmeyerMax-Planch Institut fu'r Stromun-gsforschung, Gottingen, Germany

(Received l6 February 1973)

High-resolution measurements of the He -He total scattering cross section are pre-sented for reduced collision energies between 0.2 and 200 meV. Clear evidence for theRamsauer-Townsend effect is observed and thirteen backward glory extrema are re-solved. From these extrema we derive the energy dependence of the s phase shift, Ap-plying the semiclassical inversion method proposed by Miller we then compute the repul-sive potential up to 0.14 eV.

Knowledge of the interaction between two He'atoms in the ground state has been greatly in-creased in recent years. Relative differential

scattering cross-section measurements at fixedenergies' ' give sufficient information on the in-teraction potential only at energies which are not

820