  Using femtosecond time-resolved x-ray diffraction we investigate the
structural dynamics of the coherently excited A1g phonon mode in the
Fe-pnictide parent compound BaFe2As2. The fluence dependent intensity
oscillations of two specific Bragg reflections with distinctly different
sensitivity to the pnictogen height in the compound allow us to quantify the
coherent modifications of the Fe-As tetrahedra, indicating a transient increase
of the Fe magnetic moments. By a comparison with time-resolved photoemission
data we derive the electron-phonon deformation potential for this particular
mode. The value of Delta mu/Delta z = -(1.0 - 1.5) eV/A is comparable with
theoretical predictions and demonstrates the importance of this degree of
freedom for the electron-phonon coupling in the Fe pnictides.
