Large liquid helium clusters (He L , n+10) produced in a supersonic jet are doped with alkali atoms (Li, Na, K) and characterized by means of laser induced fluorescence. Each cluster contains, on average, less than one dopant atom. Both excitation and emission spectra have been recorded. The observed excitation spectra are analyzed, calculating the transitions within an approach based on the hypothesis that the chromophores are trapped in a dimple on the cluster's surface as predicted by the theoretical calculations of Ancilotto et al. [9]. The results of the model calculations are in good qualitative agreement with the experimental findings. In spite of the very weak binding energy (a few cm\), some of the excited atoms remain bound to the surface, provided the excitation occurs at frequencies not too far from the alkali's gas phase absorptions. These bound-bound excitations produce very broad, red shifted emission spectra. At other, blue shifted frequencies, the excited atoms desorb from the cluster's surface, giving rise to unshifted, free atom, emission spectra. The heavier alkali metals (Na, K) show, compared to the calculations, an additional broadening which is attributed to surface excitations on the helium droplet.
Thulium atoms are trapped in a magneto-optical trap using a strong transition at 410 nm with a small branching ratio. We trap up to 7 × 10 4 atoms at a temperature of 0.8(2) mK after deceleration in a 40 cm long Zeeman slower. Optical leaks from the cooling cycle influence the lifetime of atoms in the MOT which varies between 0.3 -1.5 s in our experiments. The lower limit for the leaking rate from the upper cooling level is measured to be 22(6) s −1 . The repumping laser transferring the atomic population out of the F=3 hyperfine ground-state sublevel gives a 30% increase for the lifetime and the number of atoms in the trap.
We have studied both theoretically and experimentally the optical pumping of Cs atoms trapped in ͑bodycentered-cubic and hexagonal-close-packed͒ crystalline 4 He matrices. The theoretical approach is based on rate equations for which time-dependent and asymptotic solutions are obtained in the case of depopulation and repopulation pumping. Comparison with experiments show that repopulation pumping, i.e., a process in which spin polarization in the excited state is not destroyed, is the dominant pumping mechanism in both crystalline phases.
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