The time dependent quantum dynamics of the large amplitude motion of the NH stretching chromophore in NHD2 is investigated during and after coherent multiphoton excitation by calculation of the wave packet evolution using global analytical potential energy and electric dipole hypersurfaces of ammonia derived from ab initio calculations. Intramolecular vibrational redistribution between the NH stretching and bending motion and coupling to the radiation field induces a diffusion of probability density into the NH chromophore space, which includes the inversion coordinate. However, inversion remains essentially dominated by a tunneling process, even at average energies well above the inversion barrier. (C) 2003 American Institute of Physics.