  We discuss the mechanism of coherent phonon generation by Stimulated Ionic
Raman Scattering, a process different from conventional excitation with near
visible optical pulses. Ionic Raman scattering is driven by anharmonic coupling
between a directly excited infrared-active phonon mode and other Raman modes.
We experimentally study the response of YBa2Cu3O6+x to the resonant excitation
of apical oxygen motions at 20 THz by mid-infrared pulses, which has been shown
in the past to enhance the interlayer superconducting coupling. We find
coherent oscillations of four totally symmetric (Ag) Raman modes and make a
critical assessment of the role of these oscillatory motions in the enhancement
of superconductivity.
