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HIGH RESOLUTION STUDY OF RYDBERG STATES OF BARIUM
E. Eliel, W. Hogervorst
To cite this version:
E. Eliel, W. Hogervorst. HIGH RESOLUTION STUDY OF RYDBERG STATES OF BARIUM.
Journal de Physique Colloques, 1982, 43 (C2), pp.C2-443-C2-446. �10.1051/jphyscol:1982236�. �jpa-
00221848�
HIGH RESOLUTION STUDY OF RYBBERG STATES OF BARIUM
E.R. Eliel and W. Hogervorst
Natuurkundig Laboratorium, Vrije Univevs-iteit, PB 7161 1081 HV Amsterdam, Holland
Résumé. - Nous avons étudié les séries 6snl, S, P, D et F de 1 atome de baryum avec des lasers à colorant monomodes. La structure de la série F est analysée en tenant compte des interactions électrostatiques, spin-orbite et hyperfine. On montre que l'adjonction d'une interaction effective du type spin-orbite à 2 corps est néces- saire. La structure des niveaux de Rydberg est très sensible à la présence de ni- veaux perturbateurs ce que l'on démontre dans les séries S et F.
Abstract. - We have studied the 6snl S-, P~, D- and F-Rydbergseries of Barium using CW dye lasers. The structure of the F-seri'es is analyzed in terms of the electrostatic repulsion between the two valence electrons, the spin-orbit inter- action and the hyperfine interaction. However, an effective two-body spin-orbit interaction has to be included. The structure of the Rydbergstates is sensitive to the presence of perturbing configurations, as shown in the S- and F-series.
We have performed a high resolution study of the S-, P-, D- and F- Rydberg- series of the alkaline-earth element Barium. The level structure reflects the interaction between the two valence electrons outside closed shells. Configurat- ion interaction with doubly excited states, which influence level positions', Lande-factors and radiative properties ' , affect this structure as was recently shown by Beigang et al.->>° for the D-series of Strontium and by Rinneberg et al.
for the S-series of Barium.
The even-parity Rydbergstates were populated by two-step excitation from the ground state using two stabilized CW dye lasers. In our experimental set-up.which is described elsewhere", the laserbeams perpendicularly intersect a well-colli- mated atomic beam of natural Barium. The first laser is tuned to the 6s2 1S0 •+
6s6p l~?i resonance transition at 553.5 nm and locked to individual hyperfine or isotopic components. The second laser then excites the atoms to the Rydbergstate.
Excitations take place in a field free region and the Rydbergatoms are detected downstream using a field ionization detector. Absorption spectra are obtained by scanning the second laser.
The odd-parity Rydbergseries were directly excited from the metastable 3D j - and
3D2-states at 9034 and 9216 cm with our frequency-doubled CW ring dye laser.9 These states are efficiently populated in a discharge in front of the beam pro- ducing oven.
In this contribution we limit ourselves to a discussion of some results ob- tained for the 1S - and 3F-series.
The structure of the Rydbergstates is expected to be mainly determined by three interactions'^1, the electrostatic^repulsion between the 6s- and nl - electron, the spin-orbit interaction £ i.s of the 1 2 n£-electron and the hyperfine contact interaction a I.s of the 6s-electron (only for the odd isotopes
1 3 5B a and 1 3 7B a with nuclear spin I = 3/2). These interactions are diagram-
matically shown in the upper part of Fig. 1. In a hydrogenic model it is pre- dicted that electrostatic repulsion and spin-orbit interaction scale as n* , where n* is the effective main quantum number. Since the Hyperfine interaction of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982236
JOURNAL DE PHYSIQUE
the 6s-electron is constant, at higher 6 5 65
values of n* the three interactions
have to be treated simultaneously.
1 -
- - - - : ; 6 } - - - o - ;A consequence of the decreasing elec- f
trostatic repulsion and spin-orbit in- el,,t,,,
,,,, ,,,,-,, ,,,
hyportlnoteraction is that the structure of the
odd-isotope Rydbergstates willtonverge 6 5
to the hyperfine structure of the
2 n d
ground-state of the Ba -ion. This is k = n l l
illustrated in Fig. 2. In the up er
part a recording of the transition % l O ~ I . C ~ ~ Y O t w o - b o d y S D I ~ I - ~ r b , t ~ n t o r a c t m n
+ 30f is shown. in the lower Dart of the transitionT3D1 +50f. The -assign
-
merit to the two isotopes 1 3 5 , 1 3 7 ~ ~ FIG. 1 . DIAGRAMMATIC REPRESENTATION OF and the single components of the even THE INTERACTIONS.
isotopes 136'138~a are indicated. The
hyperfine and spin-orbit interaction in the odd isotopes strongly mix the ' ~ 3 , 3~2,3,4-terms resulting in a complicated hyperfine spectrum. With increasing n a clear grouping appears at the high frequency side of the 136'138~a doublet. Each group corresponds to a transition from a specific hyperfine level in 6s5d3Dl. The residual structure within a group reflects the excited state sp$itting,decreasing notably with increasing n and showing the convergence to the Ba - ground state.
In Fig. 2 this grouping does not yet appear at the low-frequency side of the spectrum.
The analysis of the 6sns1s -series is particularly simple due to thg absence of spin-orbit-interactions. In Fig. 3 the
measured position of the odd i s o t ~ ~ e ~ ~ ~ B a with respect to the 138~a-isotope is
plotted, showing an increasing separation as a function of n, whereas the pure iso- tope shift is expected to become constant.
The observed effect is due to the hyper- fine induced mixing of the two levels with total angular momentum F = 312
longing to the
-
and 3 ~ 1 - term bg-,
expressed by the +nZn -zero matrixelement
- i 1 ~ 0 , F = 3 / 2 1 a I . s 1 3 ~ 1 , F = 3 / 2 > . With increasing n thecsinglet-triplet separat- ion decreases, resulting in a stronger repulsion between the two F= 312- levels.
A calculation along these lines, using known singlet-triplet splittings, results in the smooth curve in Fig. 3. The difference between experimental and cal- culated values in the interval n = 20-30 is constant and equals the isotope shift.
The irregularity at n = 18 is ascribed to the perturber 5d7d1p0, as earlier observed by Rinneberg et al.
.
We observe theonset of a marked irregularity in the interval n = 35-
40, which is still under study.The perturber is 5d7d3p1, which interacts strongly with the 3~1-state and thus via the hyperfine interaction, also with the
+v lsO-state.
The analysis of the F-series is more FIG. 2. RECORDINGS OF THE TRANSITION complicated. An attempt to fit the
3 ~ 1 + nf
.
experimental spectra with the three aforeg: LASER GHOST. mentioned interactions with corresponding interaction constants (Slater exchange
.
good agreement it proved to be necessary to includen an effective two-body spin-orbit interaction,diagram- matically shown in the lower part of Fig. 1, with interaction constant a. This second order contribut-
30. ion effectively accounts for the configuration inter- action with all neighbouring Rydbergstates of the same series. With the three interaction constants G3,
25 - Snf and a, shown as a function of n* in Fig. 4 in a log-log-plot, the experimental spectra were generally
20. reproduced within 5-10 MHz. As can be seen from this figure, where for convenience the functions n*-3 and n*-4 (n* = n
-
0.17) are also plotted, only for high values of n* the limiting situation of the hydrogenicIsotope shlft (GHZ C is
Another interesting way of representing the resultsof FIG. 3 . POSITION OF 1 3 7 ~ ~ the F-series is given in Fig. 5. Here part of the WITH RESPECT TO 13sBa IN hyperfine level structure is shown as a function of n.
THE Gsns lSo-SERIES. FULL This figure is constructed in such a way that the F =
CURVE CALCULATED FROM lS0- 112-levels fall on a vertical line. This hyperfine 3~ l - ~ ~ ~ ~ ~ ~quantumnumber occurs only in the 3~p-state, l ~ ~ . thus this hyperfine level is unaffected by hyperfine interactions with 3~3,qand ' ~ 3 . The F = 512, 712 and 912 levels- which start in the lower left section of Fig. 5 cross repeatedly in the region between n = 22 and 35 due to repulsions by nearby F = 512, 712 and 912-1evels.The group of levels F=5/2-1112, which bend off at the right hand side of Fig.5 belong to 3 ~ 4 , the four levels at the top at the right of F = 112 belong to 3 ~ 3 while the remaining levels are 3 ~ 2 . Not shown is the quartet of levels with
IF^-
character. A marked irregularity appears around n = 20, again showing thepresence of a perturber, probably of the type 5d8p3p2.
Acknowledgement. - This work was financially damenteel Onderzoek der Materie.
-+ I n n *
FIG. 4. THE INTERACTION CONSTANTS G ~ ,
Enf
AND a AS A FUNCTION OF n* FOR THEsupported by the Stichting voor Fun-
F-SERIES
.
FIG. 5. THE HYPERFINE STRUCTURE OFF-STATES AS A FUNCTION OF n.
JOURNAL DE PHYSIQUE
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