Femtosecond time-resolved photoelectron spectra of ammonia molecules and clusters

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Abstract
The first femtosecond pump-probe photoelectron spectroscopy experiment for neutral clusters is reported. The à 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.