REAL-TIME DYNAMICS OF AMMONIA CLUSTERS EXCITED THROUGH THE (A)OVER-TILDE STATE - FORMATION OF THE PROTONATED CLUSTER IONS

Femtosecond pump-probe techniques combined with a reflectron time-of-f light mass spectrometer are employed to study the dynamics of ammonia clusters excited to the A state. In this paper we present data on the formation of protonated ammonia cluster ions and consider a reaction s cheme for their formation through the dissociation of (NH3)(n) ((A) ov er tilde(1)A ''(2)). We propose that the formation of the radical (NH3 )(n-2)NH4 species is due to a reaction involving the dissociation of ( NH3)(n) ((A) over tilde(1)A ''(2)) to NH2 ((A) over tilde(2)A(1)). A c ompeting process in the dissociation of (NH3)(n) ((A) over tilde(1)A ' '(2)) to NH2 ((X) over tilde(2)B(1)), yields fragments that are too tr anslationally ''hot'' to react to form the (NH3)(n-2)NH4 species. We a lso quantitatively model these reactions and report the lifetimes asso ciated with the dissociation processes of (NH3)(n), and concomitant fo rmation of (NH3)(n-2)NH4. Lifetimes of 250-900 fs are observed for the dissociation of (NH3)(3-23) excited to the (A) over tilde state.