Ultrafast fragmentation and ionisation dynamics of ammonia clusters

The formation of protonated and unprotonated ammonia cluster ions is studied by femtosecond two colour two photon pump-probe techniques applied to (NH3)(n) and (ND3)(n) clusters with n up to 8. The fourth harmonic (similar to 200 nm, 6.2 eV, 160 fs) of a Ti: Sapphire laser pulse is used to excite the clusters in a state corresponding to the <(Angstrom)over tilde> state of NH3 while the third harmonic (267 nm, 4.65 eV) is used for the subsequent ionisation step. Employing a combination of the optical Bloch equations for the excitation process and rate equations for the cluster dynamics we calibrate the zero time delay and carefully analyse the time dependence of the pump-probe signal. Several distinct intermediate steps in the time evolution can be distinguished, having characteristic time constants ranging from 40 fs to over 100 ps. They are discussed in a consistent scheme for the excitation, ionisation and protonation dynamics, accounting also for characteristic differences observed between deuterated and undeuterated species. A particularly remarkable time dependence of the homogeneous (NH3)(2)(+) cluster ion signal is interpreted as a fingerprint of internally protonated neutral precursors of the type NH3NH2NH4.