High-Resolution Electron Attachment to the Water Dimer Embedded in Helium Droplets: Direct Observation of the Electronic Conduction Band Formation

For bulk liquid helium the bottom of the conduction band (V 0 ) 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 (H 2 O) 2 - ion yield corresponding to V 0 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 V 0 = 0.76 ± 0.10 eV is observed, which corresponds to a droplet size of N min = 1600 ± 900. For droplet sizes below N min , a peak at ∼0 eV appears, which is well-known from neat H 2 O clusters. Hence, we interpret N min as the smallest droplet size in which the electronic band structure is formed in liquid helium droplets.