  Phonons, the quantum mechanical representation of lattice vibrations, and
their coupling to the electronic degrees of freedom are important for
understanding thermal and electric properties of materials. For the first time,
phonons have been measured using resonant inelastic x-ray scattering (RIXS)
across the Cu K-edge in cupric oxide (CuO). Analyzing these spectra using an
ultra-short core-hole lifetime approximation and exact diagonalization
techniques, we can explain the essential inelastic features. The relative
spectral intensities are related to the electron-phonon coupling strengths.
