The Raman spectra of thin films of C-60 were recorded as a function of
potassium coverage under ultrahigh vacuum conditions. Both clean and
air-exposed C-60 films were studied. The growth of the K3C60 and K6C60
phases are observed but no spectroscopic evidence of a K4C60 phase is
found. The spectra indicate that potassium and C-60 co-exist as disti
nct C-60, K3C60 and K6C60 phases as opposed to solid solutions with va
riable dopant concentrations. Facile potassium diffusion into vacuum-s
ublimed C-60 films is observed even at 50 K resulting in the formation
of K3C60 and K6C60. The electrical resistivity of films were recorded
in situ at 373 K as a function of K exposure. Raman spectra recorded
periodically at various levels of K exposure and at multiple locations
on the film indicate that the doped C-60 films are granular with aver
age grain sizes of the order of one to several micrometers. The occasi
onal simultaneous detection by Raman of all three phases indicates tha
t a small quantity of undoped C-60 remains even at levels of potassium
exposure sufficiently high to form K6C60. This implies that while pot
assium infiltration into C-60 crystallites is rapid, intercrystallite
potassium exchange is slow.