Ultrafast reaction dynamics of electronically excited A state of ammonia clusters

Femtosecond pump–probe techniques combined with a reflectron time-of-flight mass spectrometer are utilized to study the ultrafast reaction dynamics of the electronically excited à state of ammonia clusters. All of the detected protonated cluster ions, (NH3)nH+ n=2−6, are observed 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 to a predissociation process; while an intracluster reaction, which leads to formation of long-lived intermediates (NH3)nNH4, is responsible for the nonzero falling off regime. The results provide conclusive experimental evidence that both an absorption–ionization–dissociation mechanism and an absorption–dissociation–ionization mechanism are operative in the à state of ammonia clusters.