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The new methods of laser spectroscopy have not only increased the precision of optical experiments by large factors, but also have increased their sensitivity to a level where they become feasible for the small numbers of atoms produced far off stability. The author gives a brief discussion of atomic hyperfine structure and isotope shift of optical spectra before describing some of these experiments.

Microwave transitions between the Zeeman split doublet states 22 P 1/2 and 22 P 3/2 of7Li have been measured using the optical double resonance method. The fine structure separation was determined to bev fs=10.053.184(58) MHz. The hyperfine structure was well resolved and led to considerably improved precision of the diagonal coupling constantsa 1/2=45.914(25) MHz,a 3/2=−3.055(14) MHz,b=−0.221(29) MHz. Furthermore, the contact, spin dipolar and orbital magnetic hyperfine constants are evaluated using additional results from level crossing experiments, which are more sensitive on the non-diagonal magnetic hyperfine constanta 3/2,1/2. The results are in good agreement with recent theoretical calculations. The nuclear quadrupole moment is derived fromb to beQ(7Li)=−41(6) mbarn.

A resonant ionization mass spectrometer has been developed as an analytical tool for the detection of trace elements, especially of plutonium and other radionuclides. The sample, deposited on a rhenium filament, is evaporated by electrical heating and the atoms of the element under investigation are selectively ionized by laser light delivered from three dye lasers pumped by a copper vapour laser. The resulting photoions are detected in a time-of-flight spectrometer with a channelplate detector. For plutonium a mass resolution of M/δM=1500 was obtained and an overall detection efficiency of 4×10−6 was determined for stepwise excitation and ionization via autoionizing states. With a laser light bandwidth of 3–5 GHz neighbouring isotopes could be suppressed by a factor of 20 due to isotope shifts in the excitation transitions. The isotope composition of synthetic samples was measured and good agreement was found with mass spectroscopic results. The influence of the hyperfine structure on the isotope ratios is discussed.

The possibilities of determining nuclear spin, moments and variation in charge volume by optical pumping of on line masseparated isotopes are described for the example of the isotopic chain lY1Hg-205Hg. The variation in nuclear charge volume (6 ) along this chain and in particular its abrupt change for the lightest Hg-isotopes, indicating a phase transition in nuclear structure, are discussed. 1. Introduction. - This talk is based on expe- riments (I), (2), (3) performed recently by a visiting team (*) at the ISOLDE facility at CERN with the members : J. Bonn, G. Huber, H.-J. Kluge, U. Kopf, L. Kugler, J. Rodriguez, and E. W. Otten. The investigation of the hyperfine structure (hfs) and isotopic shift (IS) of optical spectra allows the determination of the basic nuclear ground state properties : spin (I), magnetic moment (p,), electric quadrupole moment (Q,), and change of the mean squared radius of the nuclear charge distribution between different isotopes (6 ). The subject has therefore widely been explored in the past for stable and long lived isotopes (4), (5) and contributed substantially to the systematic topography of nuclear structure in the valley of stability which could be established within the last two decades. Still basical and quantitative questions are open in understanding nuclear matter. In this situation it makes from first principles is missing. In this situation it makes sense to enlarge the scope of investigations principally. In particular we should be in a position to choose and change systematically the parameters N and Z of a nucleus independently and free from the boun- dary condition of being close to the stable valley. With other words, a systematic investigation of nuclear properties along lines of constant Z or N across the full array of nuclei which we can reach in our days, would give us a much wider and more reliable view on the topography of nuclear structure (*) In the course of the experiments the team changed its home university from Heidelberg to Mainz.

High frequency transitions in the hyperfine structure of the excited 7 / sup 2/P/sub 3/2/ terms of the Cs I-spectrum were detected by means of the double resonance method. The measurements were made on a 0.2 mc sample of a fission product which contained the radioisotopes Cs/sup 135/ and Cs/sup 137/ as well as the stable isotope Cs/sup 133/ in approxi mately equal concentrations. For the nuclear quadrupole moments of the radioisotopes the values Q(Cs/sup 135/) = (+0.049 plus or minus 0.002) x 10/sup -24/ cm/sup 2/ and Q(Cs/sup 137/) = (+0.05 0 plus or minus 0.002) x 10/sup -24/ cm were obtained. (tr-auth)

In this talk I will review optical pumping (OP) experiments performed by our group to ISOLDE, CERN, in a series of Hg isotopes spanning from mass number 181 to 205 (first results in refs. [1] and [2], final in refs. [3] and [4]. I will then discuss what impact these measurements had on nuclear structure research far off stability and how the results of the latter are retroacting again on the optical work. In particular they ask for isotope shifts (IS) of light even Hg isotopes which we are prepared to measure by laser spectroscopy. Further I would like to communicate briefly first results of optical pumping experiments on “exotic” Rb and Cs isotopes. Finally I will discuss plans for laser spectroscopy on fast mass-separated beams which might favourably be applied to isotopes far off stability.

The application of lasers in fundamental nuclear research has been devoted predominantly to the investigation of nuclear structure of unstable isotopes through the measurement of isotope shift and hyperfine splitting of the atomic spectrum of the nuclei in question. From the isotope shift one deduces the change of nuclear charge distribution — or more precisely the change of the mean-squared nuclear charge radius δ -, whereas the hyperfine structure yields the spin (I), the magnetic moment (μI) and the electric quadrupole moment (Qs) of the nucleus. These four nuclear properties form the cornerstones for any quantitative theory of nuclear structure. In this respect, systematic measurements, throughout the nuclear chart of elements are particularly important. This implies experiments on extended series of isotopes which reach far out into the region of unstable and short-lived nuclei.