Abstract
For bulk liquid helium the bottom of the conduction band (V0) is above the vacuum level. In this case the surface of the liquid represents an electronic surface barrier for an electron to be injected into the liquid. Here we study the electronic conduction band for doped helium droplets of different sizes. Utilizing an electron monochromator, the onset of the (H2O)2– ion yield corresponding to V0 is determined for helium droplets doped with the water dimer. While for larger droplets the onset approaches the well-known bulk value of about 1 eV, the barrier does not continuously decrease with smaller droplet size. A minimum value of V0 = 0.76 ± 0.10 eV is observed, which corresponds to a droplet size of Nmin = 1600 ± 900. For droplet sizes below Nmin, a peak at ∼0 eV appears, which is well-known from neat H2O clusters. Hence, we interpret Nmin as the smallest droplet size in which the electronic band structure is formed in liquid helium droplets.
Highlights
Introduction toElementary processes induced by ionization; Topics in Physical Chemistry Series, Baumgaertel, H., Franck, E
The onset of the (H2O)2− ion yield corresponding to V0 is determined for helium droplets doped with the water dimer
We investigate the injection of a free electron into doped helium droplets, i.e., the reverse process of that studied in ref 8
Summary
In striking contrast, the ion yield at 10.5 K shows a pronounced peak at about 0 eV, i.e., at this electron energy the electron is attached to the water dimer without entering the conduction band The observation of this 0 eV contribution can be explained in two ways: (i) the electron attaches to an almost bare water dimer or (ii) there exists a minimum size Nmin for helium droplets, where the conduction band is present. The existence of V0,min is confirmed by the measurement at the expansion temperature of 11 K shown in Figure 2b (recorded at the water pressure of 3.1 × 10−7 mbar) In this case, the peak close to 1.1 eV completely vanished and only the 0 eV resonance is present. Letter formation in pure helium droplets a minimum droplet size of 4500 ± 700 (with the lower and upper limits representing the threshold of dynamic and energetic stabilities).[5,21] excess electrons may remain delocalized in the conduction band of pure helium droplets in the size regime between about 1600 and 4500 and are highly mobile within such droplets before escaping into the vacuum
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