We have studied the unresolved $\gamma$-ray spectrum of three ytterbium nuclei produced in heavy-ion fusion reactions and measured the two damping widths, ${\Gamma }_{\mu }$, the spreading (compound-damping) width, and ${\Gamma }_{\mathrm{rot}}$, the rotational damping width, together with $P_{\mathrm{nar}}$, the probability that the population enters and leaves a given state via the same component of the wave function. Our results for ${\Gamma }_{\mu }$ and ${\Gamma }_{\mathrm{rot}}$ are in good agreement with theoretical expectations, as well as with those of other groups using different methods. Our results for $P_{\mathrm{nar}}$ lead to a new method for quantitative evaluation of the full order-to-chaos transition in these Yb nuclei. Finally, we provide the first evidence for the motional narrowing of ${\Gamma }_{\mathrm{rot}}$ in nuclei. The basic physics occurring in this region is level mixing, which our simulation technique reproduces surprisingly well.

• Received 14 March 2008

DOI:https://doi.org/10.1103/PhysRevC.78.034303