The photodesorption dynamics of ammonia from a copper surface is studied quantum mechanically using empirical potential energy surfaces. The desorption is facilitated by substrate-mediated electronic excitation and subsequent de-excitation of the adsorbate, which are simulated in our model as Franck-Condon pump and dump between two electronic states. The delayed de-excitation populates metastable predesorption states which lay above the desorption limit. The slow decay of these resonances via energy transfer from an internal mode to the desorption mode results in incomplete and rather slow desorption. The desorbed molecules have significant vibrational excitation and their translational energy distributions are highly structured, due to the dominance of the predesorption mechanism. The desorption yield depends sensitively on the time delay between the excitation and de-excitation. Strong isotope effects are observed, consistent with experimental findings. The anomalously large NH, yield relative to ND3 is attributed to its faster motion along the inversion coordinate on the excited state.