The energy flow in ammonia dimers excited to the electronic (A) over tilde state is analyzed by combining the femtosecond pump-probe technique and the photoelectron-photoion coincidence detection. We use similar to 140 fs laser pulses (200 nm for excitation and 267 nm for ionization). For the dimer ion the photoelectron spectra change drastically from a rather broad shape (greater than or similar to 1 eV) at small delay times between pump and probe pulse to a rather narrow peak (0.25 eV) at some picoseconds. This is explained by the dynamics of an internal H-atom transfer in the electronic (A) over tilde state to an NH4...NH2 configuration. The measured photoelectron energies are consistent with ab initio potential energy surface calculations. The observed picosecond lifetime of the hydrogen-transfer state NH4...NH2 can be understood by a conical intersection with the charge-transfer state NH4+...NH2-.