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Superfluid helium droplets HeN at 0.4 K are doped with single alkaline earth atoms (Ca, Sr). The absorption spectra for excitation of the lowest singlet transitions display broad, blue-shifted peaks if compared to the atomic lines. The shift increases with droplet size increasing from N≈1000 up to N≈10000 but converges to only about one third of the bulk value.
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Stienkemeier et al. (1997) studied this question.
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