Surface location of alkaline-earth-metal-atom impurities on helium nanodroplets

There has been notable uncertainty regarding the degree of solvation of alkaline-earth-metals atoms, especially Mg, in free ${}^4\mathrm{He}$ nanodroplets. We have measured the electron energy dependence of the ionization yield of picked-up atoms. There is a qualitative shape difference between the yield curves of species solvated in the middle of the droplet and species located in the surface region; this difference arises from the enhanced role played by the Penning ionization process in the latter case. The measurements demonstrate that Mg, Ca, Sr, and Ba all reside at or near the droplet surface.

Figure 1

(Color online) Ion yield curves for Ca, Sr, Ba, and Xe atoms picked up by a helium nanodroplet beam. (Lines connect data points to guide the eye.) The shape difference reflects the fact that the metal atoms are located at the droplet surface and the xenon atom is solvated inside the droplet.

Figure 2

(Color online) Ionization yield curves for picked-up atoms of Mg and Xe (as in Fig. 1), indicating the difference in their locations.

Figure 3

Cross sections for metastable excitation [17, 18] (see [11] for details) and for electron-impact ionization [19] of the He atom.