Femtosecond pump-probe ionization of small NaI-S-n clusters, S : H2O,NH3: A tool to probe the structure of the cluster

Femtosecond pump-probe ionization experiments on small NaI-Sn=1-7 clusters, S:NH3,H2O, are reported. The excitation of the clusters by the pump laser induces the NaI bond breaking within the cluster as well as evaporation of solvent molecules both in the excited and ionic states. Depending on the de tection scheme used, namely one-photon or resonant two-photon ionization, t he resulting Na+-S-n time-dependent signals are not similar. This reflects the major role of the ionization process in the nature of the dynamics prob ed. Furthermore, a solvent-selective behavior evidenced on the Na+-S-n sign als has been related to different cluster structures according to the solve nt nature: NaI seems to be embedded within ammonia clusters while it sticks on the surface of water clusters, when they are larger than four molecules . (C) 2000 American Institute of Physics. [S0021-9606(00)00920-X].