A new time-dependent quantum mechanical scheme is proposed to study the uv photodesorption dynamics of ammonia from Cu(111). The desorption is considered here as the result of substrate-mediated electronic excitation and subsequent de-excitation. The transition from a short-lived excited electronic state to the ground state is described in a coherent manner. The dynamics of the system is represented by nuclear wave packets on two quasicoupled potential energy surfaces with two degrees of freedom (the desorption and N-H-3 inversion modes). The desorbed molecules are found to have significant vibrational excitation and their translational energy distributions are highly structured due to the dominance of a predesorption mechanism. The desorption yield and the isotope effect are found to depend sensitively on the excited state lifetime. The results are compared with previous wave packet models and with experiments.