  The mechanism of electron pairing in high-temperature superconductors is
still the subject of intense debate. Here, we provide direct evidence of the
role of structural dynamics, with selective atomic motions (buckling of
copper-oxygen planes), in the anisotropic electronlattice coupling. The
transient structures were determined using time-resolved electron diffraction,
following carrier excitation with polarized femtosecond heating pulses, and
examined for different dopings and temperatures. The deformation amplitude
reaches 0.5 % of the c-axis value of 30 A when the light polarization is in the
direction of the copper-oxygen bond, but its decay slows down at 45 degrees.
These findings suggest a selective dynamical lattice involvement with the
anisotropic electron-phonon coupling being on a time scale (1 to 3.5 ps
depending on direction) of the same order of magnitude as that of the spin
exchange of electron pairing in the high-temperature superconducting phase.
