Time-resolved velocity map ion imaging study of NH3 photodissociation

Following excitation of the A state nu(')(2)=4 mode in ammonia, we show how the time scale to dissociation of the N-H bond depends on the internal energy imparted to the NH2 photofragment. Using a combination of femtosecond pump/probe spectroscopy and velocity map ion imaging techniques, the time and energy resolved H-atom elimination can be directly related to the nonadiabatic nature of the photodissociation for high kinetic energy H atoms with evidence for adiabatic dynamics to dissociation giving the lowest energy H atoms. Extrapolation of the time scales for dissociation versus internal energy of the NH2 photofragment implies that dissociation to the vibrationless ground state of NH2 occurs in < 50 fs, in very good agreement with frequency resolved measurements. The anisotropy of the H fragments with the highest kinetic energies seems to also suggest that the NH2 partner fragment comes off with very low rotational excitation.