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Two-color two-photon femtosecond ionization experiments have been performed on NaI. The wave packet evolution of the A excited state has been followed by detecting photoions and photoelectrons. The results indicate that the Na+ ions are formed when the wave packet is located at the outer turning point of the excited state. Surprisingly, the NaI+ ions are also observed to be in phase with the Na+ signal. Photoelectron spectra show that high kinetic energy electrons are produced when ionizing around the outer turning point, in agreement with the NaI+ formation. The absence of signal corresponding to ionization from the covalent part of the excited state potential can only be understood if the absolute ionization cross section is much smaller in the covalent region of the A state (where the molecule can be considered as a van der Waals complex) than in the ionic Na+···I- part of the A state potential (where the interatomic distance is such that the ionization process may be considered as a photodetachment of the electron from I- anion). Simulations taking into account that ionization occurs only when the wave packet is in the ionic region of the A state are in good agreement with experimental data.

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This work presents a quantitative comparison between experiment and molecular dynamics simulations for the excitation spectra of large van der Waals clusters. The emission and excitation spectra of mixed Ba(Ar)n clusters have been obtained for average cluster sizes ranging between 300 and 4000. The simulation is performed by using classical dynamics and pairwise additive potentials for two cases corresponding to the barium atom at the surface or inside the argon cluster. A very good agreement with the experiment is found when the barium atom is at the surface.