ULTRAFAST REACTION DYNAMICS OF ELECTRONICALLY EXCITED (A) OVER-TILDE STATE OF AMMONIA CLUSTERS

Femtosecond pump-probe techniques combined with a reflectron time-of-f light mass spectrometer are utilized to study the ultrafast reaction d ynamics of the electronically excited A state of ammonia clusters. All of the detected protonated cluster ions, (NH3)nH+ n = 2 - 6, are obse rved to display two distinct features with respect to preselected pump -probe time delays; a fast decay, followed by a persistent ion signal leveling off to a finite nonzero value. The fast decay is attributed t o a predissociation process; while an intracluster reaction, which lea ds to formation of long-lived intermediates (NH3)nNH4, is responsible for the nonzero falling off regime. The results provide conclusive exp erimental evidence that both an absorption-ionization-dissociation mec hanism and an absorption-dissociation-ionization mechanism are operati ve in the A state of ammonia clusters.