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The hyperfine structure constants and the electronic gj factor of the state 4f 11 6s 2 4111/2 belonging to the holmium ground multiplet have been measured using the atomic beam magnetic resonance technique combined with a state-selective laser-induced detection of the resonant atoms. By the same method the gj factor of the level 4f t~ 6s2419/2 was determined, while the hyperfine structures of this metastable state and of two high-lying even parity states have been investigated by high resolution laser spectroscopy. The results for the experimental A and B factors of all four members of the ground multiplet allow a least-squares evaluation of the three magnetic dipole and the three electric quadrupole effective radial parameters for the configuration 4f la 6s 2 of holmium, yielding an accurate value for the spectroscopic nuclear electric quadrupole moment: Qhfs(~6SHo)=2.716(9)b (uncorrected for quadrupole shielding). From a comparison to the quadrupole moments measured in mesic holmium atoms the shielding factor could be estimated. Experimental MethodWhile the holmium energy spectrum comprises several low-lying fine structure states arising from the The atomic beam was evaporated crucible-free from a cylindrical holmium target by electron impact from a 100kV electron gun. This universal evaporation method [-7, 8], originally designed for the highly refractive 4d-and 5d-elements, was used because it allows an enhancement of the population of the metastable states by the decay of electron bombardment excited high-lying fine structure levels.Since the melting point of holmium is only 1,740 K the electron gun was run at a low emission current of 1.5 mA to produce the atomic beam at a vapour pressure of about 0.1 mm Hg. The atoms then pass conventionally equipped flop-in type ABMR magnetic field regions and are focussed on the detector slit if they have undergone an rf transition in the Cfield. The precisely known 39K hfs transitions of a simultaneously running potassium beam which emerges from a resistance heated oven and is registrated by a hot-wire detector are used for a regulation system [9] continuously controlling the magnetic field strength in the C-region of the apparatus. Behind the detector slit the rf-resonant holmium atoms are crossed at right angle by the beam 0340-2193/82/0307/0193/$01.60

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