Results from quasi-classical and quantum-mech. calcns. for the UV-laser induced photodesorption of NH₃ and ND₃ from Cu (111) are compared. A 2-dimensional, 2-electronic state model was used to describe the dynamics of this desorption system, in which the out-of-plane bending mode was identified as playing an important role. Quantum-mech. calcns. employ the wave-packet method to solve the time-dependent Schroedinger equation. The coupling between the 2 relevant electronic states is treated by assuming a residence time in the excited state, and by a posteriori incoherently averaging the observables over this residence time, assuming an electronic lifetime for the excited state. Quasi-classical calcns. employ a surface hopping model in which the residence time in the excited state is stochastically sampled. Isotope effects, translational and vibrational (n2) state distributions are compared in the context of recent expts.