Field Adsorption of Helium on Tungsten

Using atom-probe spectroscopy we measured the activation energy for desorption and the position of He on W for fields between 3 and 5 V/Å. A tight-binding cluster model with local fields from jellium calculations reproduces the data and gives the image plane as 0.8 Å above the topmost lattice plane. The desorption energy increases by a factor of 20 and the equilibrium position reduces from 3.3 Å in $F=0\mathrm{} \mathrm{to} \sim 2$ Å in $F>\mathrm{}3\mathrm{}$ V/Å. This is interpreted as a transition from physisorption to weak field-induced chemisorption.