Ultrafast laser pump and control in small sodium-ammonia clusters

Ultrafast dynamics of Na(NH3)(n) (n = 1,2) is studied using amplified 150 is Ti:Sapphire laser radiation. Femtosecond pump probe experiments using the red photon (820 nm) to excite and the second harmonics (410 nm) to ionise the clusters show that the first electronically exited state of NaNH3((A) over tilde(2)E) is stable for at least time tau similar to 1 ns. In contrast, larger clusters ions can only be observed for short pump probe delay times of typically tau similar to 32 +/- 3 ps for Na(NH3)(2) while a significant Na+ ion signal is seen to rise on the same time scale, indicating fragmentation of the clusters. If the fatter process is investigated for Na(ND3)(2) we find tau similar to 1500 +/- 500 ps. The observations are discussed in terms of two photon excitation by the red photon into a high lying excited state of the clusters, followed by a delayed internal rearrangement corresponding to the NH3 (or ND3) tunnelling motion. The process demonstrates a genuine 'pump-control' mechanism in ultrafast gas phase chemical dynamics.