ULTRAFAST FRAGMENTATION AND IONIZATION DYNAMICS OF AMMONIA CLUSTERS

The formation of protonated and unprotonated ammonia cluster ions is s tudied by femtosecond two colour two photon pump-probe techniques appl ied to (NH3)(n) and (ND3)(n) clusters with n up to 8. The fourth harmo nic (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 <(Angst rom)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 equ ations for the cluster dynamics we calibrate the zero time delay and c arefully analyse the time dependence of the pump-probe signal. Several distinct intermediate steps in the time evolution can be distinguishe d, having characteristic time constants ranging from 40 fs to over 100 ps. They are discussed in a consistent scheme for the excitation, ion isation and protonation dynamics, accounting also for characteristic d ifferences observed between deuterated and undeuterated species. A par ticularly remarkable time dependence of the homogeneous (NH3)(2)(+) cl uster ion signal is interpreted as a fingerprint of internally protona ted neutral precursors of the type NH3NH2NH4.