Abstract
We studied the fragmentation of ultrathin [1--6 ML (monolayer)] ${\mathrm{C}}_{60}$ films on $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)$ surfaces under intense pulse electron irradiation using a scanning tunneling microscope for field-emission electron irradiation below (20 eV) and above (45 eV) the fragmentation threshold energy. We assessed the fragmentation yield for various film thicknesses and electron fluxes. Fragmentation resulting in coalesced spheroid structures becomes less efficient in thinner films owing to faster energy transfer into the substrate. Our observation of flux dependence revealed that two-electron excitation causes fragmentation at sub-threshold energy when the excitation rate exceeds the excited-state decay rate $(\ensuremath{\sim}{10}^{9}{\mathrm{s}}^{\ensuremath{-}1}$ for 4-ML thickness).
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