Real time observation of hydrogen transfer: Femtosecond time-resolved photoelectron spectroscopy in the excited ammonia dimer

The energy flow in ammonia dimers excited to the electronic à state is analyzed by combining the femtosecond pump–probe technique and the photoelectron–photoion coincidence detection. We use ∼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 (≳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 à 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−.