For the first time, definite evidence for the existence of two-phonon collective states in deformed nuclei has been established through the measurement of the absolute transition rate for the decay of the double-γ ${\mathit{K}}^{\mathrm{\pi }}$=$4^{+}$ vibration in ${}_{}{}^{168}{}_{}{}^{}\mathrm{Er}$.

• Received 19 September 1990

DOI:https://doi.org/10.1103/PhysRevLett.66.691