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Autoionization Inhibited by Internal Interferences

Neukammer, J; Rinneberg, H; Jönsson, G; Cooke, W. E; Hieronymus, H; König, A; Vietzke,K; Spinger-Bolk, HPublished in:Physical Review Letters - Moving Physics Forward

DOI:10.1103/PhysRevLett.55.1979

1985

Link to publication

Citation for published version (APA):Neukammer, J., Rinneberg, H., Jönsson, G., Cooke, W. E., Hieronymus, H., König, A., Vietzke, K., & Spinger-Bolk, H. (1985). Autoionization Inhibited by Internal Interferences. Physical Review Letters - Moving PhysicsForward, 55(19), 1979-1982. https://doi.org/10.1103/PhysRevLett.55.1979

Total number of authors:8

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VOLUME 55, NUMBER 19 PHYSICAL REVIEW LETTERS 4 NOVEMBER 1985

Autoionization Inhibited by Internal Interferences

J. Neukammer, H. Rinneberg, G. Jonsson, ' %. E. Cooke, H. Hieronymus, A. Konig,and K. Vietzke

Institut fiir Atom un-d Festko'rperphysik, Freie Uni versita tBe'rlin, D 1000 B-erlin 33, West Germany

H. Spinger-BolkCoherent GmbH, Rodermark, 8'est Germany

(Received 25 July 1985)

We have measured the linewidths of [Sd3t2n2d3t2] J Qau=toionizing Rydberg states of barium withprincipal quantum numbers ranging between n 2 ——14 and n 2

= 100. Dramatic deviations of theautoionization rates from the normal n 2' scaling law have been observed. Whereas the[Sd3t260d3/2]J=O level exhibits a linewidth I' (FWHM) as low as 6 MHz, higher (n2 ——80)members of this series are broadened up to 5 GHz. Autoionization is inhibited for principal quan-tum numbers n2=60 because of configuration interaction between the [Sd3t2n2d3t2]J=o and[Sdst2n3dsi2]j=o Rydberg series. This causes the state at n2= 60 to be metastable.

PACS numbers: 32.80.Dz, 31.60.+b, 32.70.Jz

High-energy autoionizing states have generated con-siderable interest recently as possible storage states forextreme-uv lasers. ' By sharing energy between twoexcited electrons, it is possible for the state to store anenergy greater than the atomic ionization limit. How-ever, these states usually autoionize very rapidly( —10 ' sec) through the interelectron Coulomb in-teraction, so that interest has generally been directedtowards metastable autoionizing states. Alkali-atomquartet states were the first observed examples ofmetastable autoionizing states; autoionization is in-hibited in these cases because the Coulomb interactionconserves the total spin S while all the possible finalstates of an ion plus a continuum electron are doub-lets. Similarly conservation of parity and total orbitalangular momentum can also inhibit autoionization,although most of these selection rules begin to fail inheavy atoms because of spin-orbit coupling.

A variety of methods have been proposed for theuse of external fields to create metastable autoionizingstates. Strong laser fields4 have been predicted to pro-duce metastable states by canceling out the continuumcoupling. Magnetic field tuning has been proposed tonarrow the resonances of a Rydberg series convergingto an excited Landau level, above the ionization limit.Most recently, the Stark-field —ionization resonances insodium have been shown to narrow near a crossingand have been described quantitatively by WKB quan-tum defect theory. 6 (In these last two cases, theautoionization involves only one electron which usestwo degrees of freedom to store the energy. ) Starkfields have also been used to produce interference nar-rowing between barium autoionizing states. 7

In this Letter we report measurements of the[Sd3tzn2d3l2]J=a autoionizing states of barium whichshow that the interelectron Coulomb interaction can

be adjusted to inhibit autoionization, by variation ofthe principal quantum number n in a doubly excitedRydberg series. In this fashion, we have observed 3orders of magnitude change in the autoionization ratefor states whose principal quantum numbers differonly by 10%. Because of the strong spin-orbit interac-tion of the Sd electron, the [Sd3lznzd3t2] J —

Q Rydbergstates are generally jj coupled and hence are mixturesof Sdn2d So and Sdn2d Po I-S-coupled components.However, because of configuration interaction withthe [Sd&lzn3d&l2]~ 0 series, the 'S&& and Po amplitudesof the [Sd3i2n2d3/2] J QRyd—berg states change dramat-ically with increasing principal quantum number,which leads to a pure Po state at n2 ——60. Below theSd D3i2 ionization limit, only Ba+(6s) ions are possi-ble, and so the continua are of 6sel type and no openPo channel of even parity exists. Because of parity

and L conservation, the autoionization of the[Sd3iz60d3l2] J —

Q state is inhibited, which results in alinewidth of less than 6 MHz corresponding to a life-time 7 & 2S nsec. To observe these metastableautoionizing states, we have used a stepwise multipho-ton excitation technique employing two narrow-bandcw dye lasers.

As shown in Fig. 1, we used two cw dye lasersto populate [Sd3iznzs ]J —t 2 and [Sd3lzn2d ]1=0, 1, 2

autoionizing Rydberg states, using two different exci-tation schemes. In the first scheme, one ring dye laserexcited atoms from the ground state to the intermedi-ate Sd6p D& state and a second ring dye laser then ex-cited the [Sd3lzn2d3i2]z autoionizing states as shown inFig. 2(a). In the second scheme, a dc discharge popu-lated the metastable Sd6s 'D2 state and ring dye laserswere used to populate first the Sd6p 'P& intermediatestate and then the final [Sd3l2n2d3l2]J autoionizingstates, as shown in Fig. 2(b). In both schemes the

198S The American Physical Society 1979

VOLUME 55, NUMBER 19 PHYSICAL REVIEW LETTERS 4 NOVEMBER 1985

DYE LASER DYE LASER

AMPLIFIER

BQ 5dq(2BQ' 5d3/2

b)5d3/2 ns, nd

Ba' 6s«2

BQ 5dg(2

5d3(2 ns, nd

BQ' 6s1i2

QUADRUPOLE

MASS FILTER

EINZELLENS I I

ELECTRONMULTIPLIER

CAPACI TORPLATES

DISC

RAT EMETER

RECORDER

440 nm &X2& 465 nm

5d 6p D„

g1 = 41 3.359nm

s2 1$o

545nm&X2& 550nm

5d 6P P1

&„=582.789 nm

5d 6s D2

DC DISCHARGE

S2 1~

FIG. 2. Excitation schemes.

BARIUM OVEN

FIG. 1. Experimental setup.

lasers were stabilized to a bandwidth of about 1 MHzwith typical power levels of 5 and 100 mW for the twolasers, respectively. The linearly polarized laser beamscrossed an atomic beam at 90' and the ions producedby autoionization were focused into the entrance aper-ture of a quadrupole mass spectrometer by means ofan electrostatic lens. The ions were detected with asecondary-electron multiplier. The quadrupole massfilter was especially important to suppress the back-ground ions which were present when the dc dischargewas used. A parallel-plate capacitor served to compen-sate the electric field in the excitation region caused bythe electrostatic lens.

%hen we employed the excitation scheme shown inFig. 2(a), the dye lasers were operated with stilbene 1

and stilbene 3, respectively. Spectra were obtained byour tuning the frequency of the second dye laser(Coherent model CR 699-29 Autoscan) while thewavelength of the first laser was kept fixed atA.

&

——413.359 nm. The broad tuning range (5000 GHz)and narrow bandwidth of the Autoscan made it possi-ble to record narrow as well as broad autoionizing reso-nances. The total angular momenta J of the Rydbergstates were determined by our taking spectra withparallel as well as perpendicular polarizations of thelaser beams. To measure term values, a ' Te-vapor130

cell was used to determine the wavelength of thesecond laser. For measurements of linewidths theinternal frequency scale provided by the wavemeter ofthe Autoscan was used. The second excitation scheme[(Fig. 2 (b) ] via the Sd 6p 'I', intermediate levelwas found to be particularly suited to reach[513/2n213/2]J 0 Rydberg states (n =71—84) of pre-dominantly singlet character. This excitation scheme

n=50 n=55 n=60

1 GHz

I

I/

1 GHz 100 MHz

J=2" J=2

1 GHz 1 GHz

I

si'

II

FREQUENCY

FlG 3. Typical line shapes of some [513/3II213/3] J 0states. Note the expanded scale at n2 = 60.

requires dye lasers operating in the rhodamine 6G andrhodamine 110 spectral range. The first laser was sta-bilized to the 516s 'D2 516p 'I'& transition(lL&

——582.789 nm). The tuning range of the seconddye laser (Spectra Physics model 380D) was only 30GHz. Two marker cavities with free spectral ranges of150 MHz and 5 GHz provided the frequency calibra-tion.

In Fig. 3 we show the narrowing and broadening of[513/2n213/2] J I3 autoion—izing resonances for principalquantum numbers between n2 ——50 and 69. At higherenergies resonances falling within the broad contour ofthe [Sds/21415/2] J =o state are asymmetric and exhibitpronounced Fano-type line shapes (cf. Fig. 4). Wesummarize our data in Fig. 5 by plotting the normal-ized linewidths I'(n2 ) versus the effective principal

1980

VOLUME 55, NUMMBER 19 pH Ys/CAL REVIEW LETTER 4 NOVEMBER 1985

"P = 5d3, 2 ns, nd545.4nm

S 2 5828nm

t 5d3/2 78d3/2lj (]

5 GHZ

FREQUENCY

] =0 states for n2= 7 aand 79.FIG.. 4 Broad [Sds/2n2d3/2 J=Q s a

i =2, 3) isber n3 (modl . AAs usual, n; i =e to thed'determined from the bin ing

ionization limi,corresponding

Sd, and 515/2 states,e the Ba+ 6s, S 3/2,erturb-i. 5, thein u

5/2 3 5/2 J=0

resonance.3 orders o mf magnitude is ob

them al behavior by

ental poin t corresponds toTh lowest experim~

1

elinewidth o

lar er than that of the n2= sJ = 1, 2 fine-structure

largerRydberg states,~Sd3/2&2d3/2& J = 0

ed by interference etoo are narrowewe focus

components toL tter, however, wgp

3/»sent a particular y ran

11owing 0 au Oi8, 9

f interference narrotheory (MQDT

0 in1 uantum-defect e

b in-'n of autoion' '

~ ~

descri etl "nfi u"timplicitly. Recen y,terference effects imp ici

ents:

d, (l)d ) + sinOISd5/2n3d5/2Iws) =cosOIsd3/2n2 3/2

1 ted to L,S-coupledstates are re a ewhere the jj-coupled sstat as follows:es

=- (-')"'I'& ) + (-')"'I'~0),ISd3/2n2d3/2) =- —,

= (-')"'I'~ ) —(-,')"'I'~0),I Sds/2&3d5/2) =

t 1 d per unit energy,e; =, so th t the wave functiontions are norma tze

ns(

1' '1'""'1 'n'1not depend on n; at a .

tan8b 'f rerepresents the rratio of the num er

hat this is not the samarne as(

orbit. Note t as ent in eac con

rentsince the con igura i

l) isnormalization,

l 0 defined in Eq.p The anglents to

t """ 'h'nn'1e er

each other as wel asboundary coIl 1 iont ns specific toSimultaneously, oun

b imposed, which re-on the effect' e q"'nt

nctions must e'

f he two jj channels. Mnumbers o t e

Rydberg series with aan autoionizing ywithin

interaction ohas been discussep g eso ane s

the k-matrix ors been analyze

h framework of thed theoretically, byh sical situation has

nel quantum-defectf the three-channe qf Ref.

p oyr discussion close y ptheory. ur is

11.r of the autoionizing states canTh bound character oe

um of the two jj-coup led Sdnd com-be written as a sum opon

—10c$2

10 =.

I

0.50

old l e showmg Qn ds/2] J = 0 sta ""hsonance. The sthe lSds/»&ds/2]J =0 «so

ertur Sd 12d ] =o state. e pbdb hb sterisks, are perturThe circles indicate

1981

Data points at n2 ——5,onance.

VOLUME 55, NUMBER 19 PHYSICAL REVIEW LETTERS 4 NOVEMBER 1985

rizes these conditions as follows:r

tan(m. r ) R)3 3 t cos(7r7 )

R)3 R23

R23 cosOcosm. (n2 +52) =0,tanjr(n3 + 53) sino cos7r(n3 + 53

where —~v is the phase shift of the continuum wave function and is obtained by the solving of Eq. (3). A t is theamount of continuum character, and the R;, values are those defined by Seaton or Cooke and Cromer" when thequantum defects 5; are chosen so that R,, = 0. In this formulation, R,, is I/vr times the sum of the Slater integralsrepresenting the coupling between channels i and j evaluated with the wave functions normalized per unit energy,so that R;1 is dimensionless. Note that this requires that Rtp ( 3 ) Rt3 In order for Eq. (3) to have a nontrivialsolution the determinant of the corresponding matrix has to vanish. By solving Eq. (3) for the phase shift —7rrand taking its derivative with respect to the energy at resonance we obtain

I (n;)3=27r 'Rt'3 [(—,')t~'tan7r(n3 +8,) —R„]'/[tan'7r(n; +8,) +R4, ].The expression for I was derived by our neglecting the energy dependence of the slowly varying effective quan-tum number n3. In addition, the small-angle approximation of the tangent was used. In Fig. 5, the solid linerepresents Eq. (4) with the following parameters: 83 ——2.435, R23 ———0.23, and Rt3 ———0.17. These values arevery close to those obtained by reduction of the four-channel analysis of Aymar, Camus, and Himdy. ' As can beseen from Fig. 5, the experimental data are well described by Eq. (4).

Theoretically the autoionization rate I vanishes when tanm(n3' +53) equals R t3R23/R tz= —', ' Rz3. At that en-

ergy [n3 (modl) =0.485], the admixture of the continuum wave function A t is zero, which thus reduces Eq. (3)to a two-channel MQDT expression

tanvr(nz +5,)tan7r(n3 +8,) =R2s.

When this condition is not satisfied, the linewidth is proportional to the mismatch from this condition, evaluated atresonance:

I (nz ) =27r Itanm. (n2 +hz)tan+(n3'+83) Rz3 I/Rtz. (5)

The states [Sdyzn2dyz] J=Q with n2 ——26 and 60 corre-sponding to n3 (modl) =0.475 and 0.470, respective-ly, are close to the point of vanishing resonance width.

In conclusion, the [Sd3~2n2d3~2]J =o autoionizingstates of barium provide an illustrative example of howinternal interactions can be used, even in very heavyatoms, to enforce simple selection rules that can in-

hibit autoionization. As a result, n2= 80 states onlylive for a couple of Rydberg-orbit periods, while the n2=60 metastable state lives for nearly 10000 orbits.Only narrow-band cw dye lasers provide the necessarydynamic range to probe such a large variation in life-times.

This work was supported by the DeutscheForschungsgemeinschaft Sonderforschungsbereit 161.One of us (W.E.C.) acknowledges the support of theAlexander von Humboldt Foundation through aSenior U. S. Scientist Award. We thank ProfessorG. Huber and Dr. J. Eberz for the loan of the ' Tecell.

&'~Permanent address: Lund Institute of Technology, S-221000, Lund, Sweden.

~b~Permanent address: Physics Department, University ofSouthern California, Los Angeles, Cal. 90089.

S. E. Harris, R. G. Caro, R. W. Falcone, D. E. Holmgren,J. E. Rothenberg, D. J. Walker, J. C. Wang, J. R. Willison,and J. F. Young, in Atomic Physics 9, edited by R. S. vanDyck, Jr. and E. N. Fortson (World Scientific, Singapore,1984), p. 462, and references therein.

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3E. J. McGuire, Phys. Rev. A 14, 1402 (1976).4P. Lambropoulos and P, Zoller, Phys. Rev. A 24, 379

(1981); K. Rzazewski and J. H. Eberly, Phys. Rev. Lett. 47,408 (1981).

5H. Friedrich and D. Wintgen, Phys. Rev. A 31, 3964(1985).

6J.-Y. Liu, P. McNicholl, D. A. Harmin, J. Ivri, T. Berge-man, and H. J. Metcalf, Phys. Rev. Lett. SS, 189 (198S).

7E. B. Saloman, J. W. Cooper, and D. E. Kelleher, Phys.Rev. Lett. 55, 193 (1985).

sM. J. Seaton, Rep. Prog. Phys. 46, 167 (1983).9U. Pano, Phys. Rev. A 2, 353 (1970).OJ. P. Connerade, A. M. Lane, and M. A. Baig, J. Phys. B

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