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Abstract.The formation ofprotonated and unprotonated ammonia cluster ions is studied by femtosecond two colour two photon pump-probe techniques applied to (NH3), and (ND3), clusters with n up to 8. The fourth harmonic ( ~ 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 A 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)f cluster ion signal is interpreted as a fingerprint of internally protonated neutral precursors of the type NH3NH2NH4.

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The first femtosecond pump-probe photoelectron spectroscopy experiment for neutral clusters is reported. The A state of ammonia molecules and clusters is excited with 120 fs pulses at 200 nm and probed by ionisation at 267 or 400 nm. Ions and electrons are detected in coincidence. The high repetition rate of the fs laser allows one to reduce the coincidence rate per laser pulse to less than 0.05 per pulse so that photoelectrons and photoions can be correlated unambiguously. The energy flow in the excited neutral cluster states is followed in real time, and the energy content in the ionic clusters can be related to the fragmentation patterns.