Ultrafast excitation process in NaNH3: a combined theoretical and experimental analysis Original Research Article

The conditions for spectroscopic studies of fast decaying excited states with femtosecond laser pulses are examined both experimentally and by quantum dynamical calculations. The sodium-ammonia complex (NaNH3) acts as a model system which can be treated in quantum chemical calculations and is of chemical relevance for metal-molecular interaction in polar liquids. The state selective dynamics is studied in a two colour pump-probe experiment. The fundamental wavelength of a Ti:sapphire generated femtosecond laser pulse excites the NaNH3 complex and the second harmonic is used for ionization. The calculations performed on ab initio potential energy and transition dipole moment surfaces show that for laser intensities with a Rabi cycle less than π the pump-probe process is a pure two-photon ionization process. Under these conditions the observed ion signal reflects directly the population of the excited state and the observed wavelength dependence resembles the vibrational spectrum of the NaNH3 complex.