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Excitation and decay processes of helium clusters are investigated with fluorescence methods. The results differ remarkably from that obtained for the heavier rare gas clusters. They are discussed in view of the unusual structural and electronic properties of helium.

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It is shown that Ni3+ and Co3+ in sapphire single crystals may be reduced to Ni2+ and Co2+ by hydrogen or deuterium gas at high temperatures. The ions may be reoxidized by heating the crystals in oxygen. From studies of the ir spectra the hydrogen is shown to form hydrogen-bonded groups of the type O–H···O situated near the site of the reduced heavy-metal ion. The assignment of the visible and the uv spectra for the reduced species Ni2+ and Co2+ in the trigonal crystal field of sapphire is suggested, which also allows some corrections of the trivalent spectra. Crystal-field parameters for the divalent species obtained by second-order perturbation theory and by trial-and-error adjustment, are as follows: Ni2+:  Δ=10 700 cm−1,B=970 cm−1,C/B=4.7,v<0Co2+:  Δ=12 300 cm−1,B=915 cm−1,C/B=4.6,v>0