Photodesorption of ammonia follows a curved reaction path. Electronic excitation of, or electron attachment to, the adsorbate mol. leads to a transition from the pyramidal ground state to an excited state with planar geometry. During the short residence time in the excited state, the hydrogen atoms are accelerated towards the surface, resulting in an inversion of the mol. and a strong repulsion from the surface which eventually leads to desorption. In this paper, evidence is presented that in the inverted geometry the mol. feels the corrugation of the surface potential on rotation around its symmetry axis. This gives rise to a strong torque in cases where the 3-fold symmetry of the mol. is not matched by the lattice symmetry, resulting in energy transfer to the rotational degrees of freedom. Secondly, it is demonstrated that when both ammonia and oxygen are coadsorbed on a Ru surface, in which case the rotation of adsorbed NH3 is frustrated, a large fraction of the zero point energy is released as rotational energy.