Observation of ultrafast NH3 (Ã) state relaxation dynamics using a combination of time-resolved photoelectron spectroscopy and photoproduct detection

Abstract

The ultrafast excited state relaxation of ammonia is investigated by resonantly exciting specific vibrational modes of the electronically excited NH₃ (Ã) state using three complementary femtosecond (fs) pump–probe techniques: time-resolved photoelectron, ion-yield and photofragment translational spectroscopy. Ammonia can be seen as a prototypical system for studying non-adiabatic dynamics and therefore offers a benchmark species for demonstrating the advantages of combining the aforementioned techniques to probe excited state dynamics, whilst simultaneously illuminating new aspects of ammonia's photochemistry. Time-resolved photoelectron spectroscopy (TRPES) provides direct spectroscopic evidence of σ* mediated relaxation of the NH₃ (Ã) state which manifests itself as coupling of the umbrella (ν₂) and symmetric N–H stretch (ν₁) modes in the photoelectron spectra. Time-resolved ion yield (TRIY) and time-resolved photofragment translation spectroscopy (TRPTS) grant a measure of the dissociation dynamics through analysis of the H and NH₂ photodissociation co-fragments. Initial vibrational level dependent TRIY measurements reveal photoproduct formation times of between 190 and 230 fs. Measurement of H-atom photoproduct kinetic energies enables investigation into the competition between adiabatic and non-adiabatic dissociation channels at the NH₃ (Ã)/NH₃ () conical intersection and has shown that upon non-adiabatic dissociation into NH₂ () + H, the NH₂ () fragment is predominantly generated with significant fractions of internal vibrational energy.