How "hot precursors" modify island nucleation: A rate-equations model

We propose a novel island nucleation and growth model explicitly including transient (ballistic) mobility of the monomers deposited at rate $F$, assumed to be in a hot precursor state before thermalizing. In limiting regimes, corresponding to fast (diffusive) and slow (ballistic) thermalization, the island density $N$ obeys scaling $N \propto F^\alpha$. In between is found a rich, complex behavior, with various distinctive scaling regimes, characterized by effective exponents $\alpha_{\rm eff}$ and activation energies that we compute exactly. Application to $N(F,T)$ of recent organic-molecule deposition experiments yields an excellent fit.