  We measure the anisotropic mid-infrared response of electrons and phonons in
bulk YBa2Cu3O7 after femtosecond photoexcitation. A line shape analysis of
specific lattice modes reveals their transient occupation and coupling to the
superconducting condensate. The apex oxygen vibration is strongly excited
within 150 fs demonstrating that the lattice absorbs a major portion of the
pump energy before the quasiparticles are thermalized. Our results attest to
substantial electron-phonon scattering and introduce a powerful concept probing
electron-lattice interactions in a variety of complex materials.
