Femtosecond multiphoton ionization of ammonia clusters

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Abstract
Herein, we report on femtosecond time‐resolved experiments in ammonia clusters. The mechanisms of their ionization and the subsequent formation of the protonated ammonia cluster ions are studied using a femtosecond pump‐probe technique at 620 nm. It is found that an intermediate corresponding to C’ states of the monomer is responsible for the ionization of ammonia clusters. Femtosecond pump–probe studies show that the lifetime of the intermediate to the formation of the protonated cluster ions (NH3)nH+ (n=1–5) is the same as that leading to the formation of the unprotonated cluster ions (NH3)m+ (m=2–5). The results provide the first direct experimental proof that formation of the protonated cluster ions takes place through an absorption–ionization–dissociation mechanism.