Motional narrowing and ergodic bands in excited superdeformed states of {sup 194}Hg

The E{sub {gamma}}-E{sub {gamma}} coincidence spectra from the electromagnetic decay of excited superdeformed states in {sup 194}Hg reveal surprisingly narrow ridges, parallel to the diagonal. A total of 100-150 excited bands are found to contribute to these ridges, which account for nearly all the unresolved E2 decay strength. Comparison with theory suggests that these excited bands have many components in their wave functions, yet they display remarkable rotational coherence. This phenomenon can be explained in terms of the combination of shell effects and motional narrowing.