Metastable surface phase for NaxC60

NaxC60 thin films with Na concentration 0 ⩽ x ⩽ 3 were investigated using angle-dependent photoelectron spectroscopy. For low doping we observed two distinct fulleride phases coexisting with regions of pristine C60. One of these fulleride phase is predominantly formed close to the surface and disappears after annealing and further addition of Na. At higher doping a phase with stoichiometry of x = 3 is formed.