  We demonstrate that the absorption of femtosecond x-ray pulses can excite
quasi-spherical high-wavevector coherent acoustic phonon wavepackets using an
all x-ray pump and probe scattering experiment. The time- and momentum-resolved
diffuse scattering signal is consistent with strain pulses induced by the rapid
electron cascade dynamics following photoionization at uncorrelated excitation
centers. We quantify key parameters of this process, including the localization
size of the strain wavepacket and the energy absorption efficiency, which are
determined by the photoelectron and Auger electron cascade dynamics, as well as
the electron-phonon interaction. In particular, we obtain the localization size
of the observed strain wave packet to be 1.5 and 2.5 nm for bulk SrTiO$_3$ and
KTaO$_3$ single crystals, even though there are no nanoscale structures or
light-intensity patterns that would ordinarily be required to generate acoustic
waves of wavelengths much shorter than the penetration depth. Whereas in GaAs
and GaP we do not observe a signal above background. The results provide
crucial information on x-ray matter interactions, which sheds light on the
mechanism of x-ray energy deposition, and the study of high wavevector acoustic
phonons and thermal transport at the nanoscale.
