  We study the coherent mixing between two-particle (single exciton) and
four-particle (biexciton) states of a semiconductor nanocrystal resulting from
the coulomb coupling between states with different numbers of electron-hole
pairs. Using a simple model of the nanocrystal wave functions and an envelope
function approach, we estimate the efficiency of the multiple exciton
generation (MEG) process resulting from such coherent admixture mechanism,
including all the relevant states in a very broad energy interval. We show that
in a typical ensemble of nanocrystals with 3~nm average radius, the onset of
the MEG process appears about 1~eV above the lower edge of the biexciton
density of states and the efficiency of the process reaches 50\% for photon
energies around 5~eV. The MEG onset shifts to lower energies and the efficiency
increases as the radius grows. We point out that the energy dependence of the
MEG efficiency differs considerably between ensembles with small and large
inhomogeneity of nancrystal sizes.
