  The evolution of charge carriers in photoexcited room temperature ZnO
nanoparticles in solution is investigated using ultrafast ultraviolet
photoluminescence spectroscopy, ultrafast Zn K-edge absorption spectroscopy and
ab-initio molecular dynamics (MD) simulations. The photoluminescence is excited
at 4.66 eV, well above the band edge, and shows that electron cooling in the
conduction band and exciton formation occur in <500 fs, in excellent agreement
with theoretical predictions. The X-ray absorption measurements, obtained upon
excitation close to the band edge at 3.49 eV, are sensitive to the migration
and trapping of holes. They reveal that the 2 ps transient largely reproduces
the previously reported transient obtained at 100 ps time delay in synchrotron
studies. In addition, the X-ray absorption signal is found to rise in ~1.4 ps,
which we attribute to the diffusion of holes through the lattice prior to their
trapping at singly-charged oxygen vacancies. Indeed, the MD simulations show
that impulsive trapping of holes induces an ultrafast expansion of the cage of
Zn atoms in <200 fs, followed by an oscillatory response at a frequency of ~100
cm-1, which corresponds to a phonon mode of the system involving the Zn
sub-lattice.
