  Coherent grazing-incidence small-angle X-ray scattering is used to
investigate the average kinetics and the fluctuation dynamics during
self-organized nanopatterning of silicon by Ar$^+$ bombardment at 65$^{\circ}$
polar angle. At early times, the surface behavior can be understood within the
framework of linear theory. The transition away from the linear theory behavior
is observed in the dynamics through the intensity correlation function. It
quickly evolves to exhibit stretched exponential decay on short length scales
and compressed exponential decay on length scales corresponding the dominant
structural length scale - the ripple wavelength. The correlation times also
peak strongly at the ripple length scale. This behavior has notable
similarities but also significant differences with the phenomenon of de Gennes
narrowing. Overall, this dynamics behavior is found to be consistent with
simulations of a nonlinear growth model.
