Ultrafast fragmentation of small alkali atom-ammonia clusters

Ultrafast dynamics of small, free alkali atom-ammonia clusters Na(NH3)n is studied using amplified 150 fs Ti: sapphire laser radiation (red: 770–820 nm). Pump-probe experiments using the red photon to excite and the second harmonics (blue: 385–410 nm) to ionize the clusters show that the first electronically excited state of NaNH3(Ã 2E) is stable for times τ > 1 ns. Larger cluster ions can only be observed for short delay times, indicating life times of the excited states of typically τ ∼ 30 ps for Na(NH3)2. In addition, a significant Na+ ion signal rises on the same time scale, indicating fragmentation of the clusters and demonstrating a genuine ‘pump-control’ experiment.