Coherent acoustic phonons in PbS quantum dots are observed using femto
second optical techniques. This is the first observation of coherent a
coustic phonons in a semiconductor quantum dot; the phonons are genera
ted through the deformation-potential coupling to the quantum-dot exci
ton. The acoustic modes are weakly damped, and we also find extremely
weak coupling (S similar to 0.01) to the optical. modes. These conclus
ions have important consequences for the vibronic nature of the excito
n transition in the quantum dot and its dephasing.