Coherent atomic spectroscopy

Nuclear Instruments and Methods North-Holland, Amsterdam

COHERENT

ATOMIC

in Physics

Research

B31 (1988) 93-101

93

SPECTROSCOPY

W.R.S. GARTON Black&t Laboratory,

Imperial

College, London S W7, UK

The Argonne Spectroscopy Laboratory, initiated and advanced over several decades by F.S. Tomkins and M. Fred, has been a major international facility. A range of collaborative work in atomic spectroscopy is selected to illustrate advances in experimental physics which have been made possible by combination of the talents of Tomkins and Fred with the unique facilities of the Argonne Laboratory.

One of the greatest laboratories for optical spectroscopy of this century has been that begun some forty years ago at Argonne by Frank Tomkins and Mark Fred, who are shown together in fig. 1 at the celebrated Paschen Circle, early commissioned by them. In accepting the privilege of participating in a session held to signal the achievements of that laboratory and to salute

Fig. 1. F.S. Ton&ins

0168-583X/88/$03.50 (North-Holland

Physics

and M. Fred at the Argonne

0 Elsevier Science Publishers B.V. Publishing

Division)

its originators, my intention in choice of title was to convey a sense of the coherence in activities and webs of relationships which have centered on the Argonne Spectroscopy Laboratory. It is no exaggeration that many important achievements of recent decades have depended uniquely on the combination of the skills of one of the great experimental spectroscopists of the

Paschen

Circle. II. SYMPOSIUM

FRANK

TOMKINS

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W. R.S. Garton / Coherent atomic spectroscopy

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Fig. 2. Layout of flash-pyrolysis apparatus for absorption spectroscopy.

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Fig. 3. Ra I absorption spectrum showing principal series and some autoionization resonances.

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W.R.S. Garton / Coherent atomic spectroscopy

times - Frank Tomkins - and the facilities of the Ton&ins-Fred Laboratory. What follows may serve to illustrate that certain significant cont~butions would not otherwise have been made - even yet. The first collaborative work between Argonne and the Imperial College Spectroscopy Laboratory, 25 years ago, was an attempt to secure the absorption spectrum of radium, viz., Rydberg Series and autoionization resonances. For this purpose the experimental scheme shown in fig. 2, and originated by Nelson and Kuebler [l], was adopted. Preliminary experiments with BaCl in place of RaCl showed that this approach was not likely to succeed: the Ba I series appeared truncated by the microfields of ions and electrons in the reaction chamber. An interesting observation was that, for some 100 ps following the pyrolizing flash the barium produced by dissociation of BaCl was almost fully ionized. Tomkins later secured a portion of the RaI absorption spectrum (fig. 3) by using a furnace and reducing a radium salt by adding calcium metal. As can be seen, important parts of the Ra spectrum are obscured by Ca

Fig. 4. F.S. Tomkins

installing

induction-furnace

95

I features. However, the flash pyrolysis scheme has subsequently been used successfully in other collaborations; e.g., in observation of Rydberg series and determination of the astrophysically important photoionization cross-section of atomic sulfur [2,3]. During the period which concerns us here, our understanding of atomic structure and quantum mechanics, has made strong advances through observation and inte~retation of atomic absorption spectra which arise by multiple or/and inner-shell excitation. For obvious reasons such experiments have involved the use of high-temperature furnaces and vacuum ultraviolet spectroscopy, with the attendant problems of using windowless systems and/or overcoming the presence of gaseous impu~ties. We owe to Tomkins the design, construction and operation of excellent induction-heated furnace assemblies for work of this sort [4]. Fig. 4 shows one of these in the process of installation by the designer. Exemplifying spectra of SC I and Y I are shown in fig. 5. These were obtained [5] in collaboration with W.C. Martin and V. Kaufmann, who made available the

assembly

in a Mach-Zehnder

interferometer.

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b

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-

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-

Fig. 5. Series and autoionization

resonances

in {a) SC I, (b) Y I.

W.R.S. Garton / Coherent atomic spectroscopy

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Fig. 7. Diamagnetic

structures

in Ba I (upper)

St I (lower), showning quasi-Landau solenoid (Garton et al. [13]).

resonances.

l- and n-mixing,

obtained

with

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W.R.S. Garton / Coherent atomic spectroscopy

10 m normal-incidence vacuum spectrograph at the NBS. While it was possible to do some partial analysis, including the first recognition of a number of Rydberg series, with consequent determinations of ionization potentials, much remains for further study in these spectra. For example, both spectra contain many unassigned autoionization resonances. Future experiments of the MCD-MOR type and by laser-fluorescence may help to unravel problems of uncertain J-values and to relate particular continua to the autoionization processes. Such ambitions are included in current collaborative programs between Imperial College and the Physikalisches Institut, Bonn. In other collaborative research, at Harvard College Observatory [6] the apparatus of fig. 4 was used for determination of f-values in the alkaline-earth principal series, and in SC I. An illustration of the excellence of the spectra obtained is displayed in fig. 6. Work started at Argonne in the late 60s on the effects, ascribable to atomic diamagnetism, on long Rydberg series and annexed continua, produced surprises which, in turn, have led to many further experimental and theoretical developments. Only brief

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Fig. 9. Two-step excitation scheme for Zeeman spectroscopy of even-even transitions

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Resonance positions as a function of H. Curves are obtained by numerical integration of Eq. (1). The two data points defined to agree with the theory are marked by the arrow. Circles, Srr resonance peak positions; crosses, Bar resonance minimum positions; triangles. Bar peak positions. The lowest datum point corresponds to the 6~31s ‘S, level.

Fig. 8. Dipole magnet of Rutherford Laboratory under test.

Fig. 10. Plots of quasi-Landau resonance positions versus energy. Circles are experimental, curves are theoretical (Starace, ref. [S]). II. SYMPOSIUM

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TOMKINS

W. R.S. Carton / Coherent arornic spectroscopy

25s

I

Fig. 11. Combined Paschen-Back and diamagneticpatterns in In I.

semi-historical remarks will be made here to some of the leading aspects of the work. The first results concerned magnetic structures in the principal series of Ba I and Sr I, and were obtained at the Argonne Paschen Circle with a large laboratory electromagnet, so that 7r and (unseparated) (I spectra were obtained. The fact that the more fascinating spectra were in u polarization encouraged acquisition of a superconducting solenoid, capable of a field of some 5 T over about 100 cm. Commercial production of large superconducting magnets had not commenced at that time, and so the solenoid was manufactured at Argonne National Laboratory. It was now possible to obtain separated D (+ ) and u (- ) spectra, as illustrated in fig. 7, which displays very clearly what have been called quasi-Landau resonances [7,8]. Following the work with this solenoid, a large dipole magnet, capable of 4.7 T was built at the Rutherford Laboratory in England for use at Argonne, At the date concerned this magnet was an ambitious undertaking, and a picture of it is shown in fig. 8. It was now possible to acquire good rr spectra at higher fields. A very successful use of this new system was reported by Fonck et al. [9,10] in which Ba or Sr contained in a diode ionization chamber was stepwise-excited by a tuneable laser, and the resulting ions or electrons collected along the field-lines. The excitation scheme for these experiments is shown in fig. 9, and some typical results in fig. 10. Later work with both magnets on the spectrum of In I has a bearing on a misconception some 50 years old.

In a celebrated classic work Van Vleck [ll] recognized two different types of Quadratic Zeeman effect, one arising from the diamagnetic term in the Hamiltonian, and the other representing partial Paschen-Back Effect, Van Vleck - probably having in mind simple systems, e.g., H-like or light-alkali - concluded that the two effects would be mutually exclusive. This proves not to be true in heavier atoms, in which the spin-orbit interaction decays more slowly along a Rydberg series. As an example, in In I the fine-structure interval in the D-doublets remains of the same order as the Lorentz interva1 and the di~a~etic shift at about n = 20 in a field of 4.5 T. What seems to be the first formulation in quantal terms of the combined partial-P-B and diamagnetic problem resulted from this work (Garton et al. ref. [12]). The simultaneous occurrence of partial PaschenBack effect and diamagnetic shift is illustrated in fig. 11. Thus, the latter shift is clear at about n = 20, and the asymmetry of the u patterns is due to the spin-orbit interaction. While the association of Frank Ton&ins with the technology of laser-spectroscopy began as mentioned above [PJO], his important subsequent and continuing contributions to the production of coherent light are described in the paper of R. Mahan.

References [l] L.S. Nelson and N.A. Kuebler, J. Chem. Phys. 37 (1962) 47.

W.R.S.

Garton / Coherent atomic spectroscopy

[2] G. Tondello, Astrophys. J. 172 (1972) 771. [3] Y.N. Joshi, M. Mazzoni, A. Nencioi, W.H. Parkinson and A. Cantu, J. Phys. B20 (1987) 1203. [4] F.S. Tomkins and B. Ercoli, Appl. Opt. 6 (1967) 1299. [5] W.R.S. Garton, E.M. Reeves, F.S. Ton&ins and B. ErcoIi, Proc. Roy. Sot. (London) A333 (1973) 1,17. [6] W.H. Parkinson, E.M. Reeves and F.S. Ton&ins, J. Phys. B9 (1976) 157; Proc. Roy. Sot. (London) A351 (1976) 569. [7] A.R. Edmonds, J. Phys. (Paris) 31 (1970) C-4 71. [8] A.F. Staracc, J. Phys. B6 (1973) 585. [9] R.J. Fonck, F.L. Roessler, D.H. Tracy, K.T. Lu, F.S.

101

Tomkins, and W.R.S. Garton Phys. Rev. Lett. 39 (1977) 1513. [lo] R.J. Fonck, D.H. Tracy, D.C. Wright and F.S. Tomkins, Phys. Rev. Lett. 40 (1978) 1366. [ll] J.H. Van VIeck, Theory of Electric and Magnetic Susceptibilities, (OUP, Oxford, 1932). [12] W.R.S. Garton, H. Crosswhite, H.M. Crosswhite and F.S. Tomkins, Proc. Roy. Sot. (London) A400 (1985) 55. [13] W.R.S. Garton, F.S. Tomkins and H.M. Crosswhite, Proc. Roy. Sot. (London) A373 (1980) 189.

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