EXPERIMENTAL FEMTOSECOND PHOTOIONIZATION OF NAI

Two-color two-photon femtosecond ionization experiments have been perf ormed on NaI. The wave packet evolution of the A excited state has bee n followed by detecting photoions and photoelectrons. The results indi cate that the Naf ions are formed when the wave packet is located at t he outer turning point of the excited state. Surprisingly, the NaI+ io ns are also observed to be in phase with the Na+ signal. Photoelectron spectra show that high kinetic energy electrons are produced when ion izing around the outer turning point, in agreement with the NaI+ forma tion. The absence of signal corresponding to ionization from the coval ent part of the excited state potential can only be understood if the absolute ionization cross section is much smaller in the covalent regi on of the A state (where the molecule can be considered as a van der W aals complex) than in-the ionic Na+... I- part of the A state potentia l (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 agreemen t with experimental data.