Coherent phonons from the CDW state inη−Mo4O11

The charge-density-wave (CDW) phase transition in $\eta -{\mathrm{Mo}}_4{\mathrm{O}}_{11}$ is studied by femtosecond time-resolved reflection. We observed coherent optical phonons in the reflectivity signal in both the normal and the CDW states. Below the transition temperature $T_{c1\mathrm{}},$ six vibrational modes were clearly detected. Four of them, centered at 64, 78, 85, and $98{\mathrm{cm}}^{\mathrm{-}1},$ were enhanced below $T_{c1\mathrm{}}.$ We conclude that the generation of these coherent phonon modes are closely related to the CDW state.