The dynamics of molecular multiphoton ionization and fragmentation of a diatomic molecule (Na2) have been studied in molecular beam experiments. Femtosecond laser pulses from an amplified colliding-pulse moddocked (CPM) ring dye laser are employed to induce and probe the molecular transitions. The final continuum states are analyzed by photoelectron spectroscopy, by ion mass spectrometry and by measuring the kinetic energy of the formed ionic fragments. Pumpprobe spectra employing 70-fs laser pulses have been measured to study the time dependence of molecular multiphoton ionization and fragmentation. The oscillatory structure of the transient spectra showing the dynamics on the femtosecond time scale can best be understood in terms of the motion of wave packets in bound molecular potentials. The transient Na2+ ionization and the transient Na+ fragmentation spectra show that contributions from direct photoionization of a singly excited electronic state and from excitation and autoionization of a bound doubly excited molecular state determine the time evolution of molecular multiphoton ionization.

Abstract.Femtosecond time resolved pump-probe experiments studying wave packet dynamics in the (2) is,+ double minimum state of Na2 are reported. The experiments were performed in a molecular beam with ion Time of Flight (TOF) detection. By Fast Fourier Transformation (FFT) of the observed time domain data the energy spacings of the coherently coupled vibrational levels in the (2) tZ~ potential are obtained with an accuracy of 0.02 cm-1, although an ultrafast laser source with its inherent spectral width was used in the experiment.The wavelengths of the pump and probe laser pulses are chosen such that in this two color experiment we can control ionisation versus ionisation induced fragmentation.In order to study the influence of the potential barrier on a vibrational wave packet motion we performed simulations based on time dependent quantum calculations.

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We report on time-resolved multiphoton ionization and fragmentation dynamics of free Hgnclusters (n ≤ 110) with femtosecond laser pulses. Already at laser intensities of 10 11 W/cm 2 we observe prompt formation of singly, doubly, and triply charged Hgn clusters. In pump-probe experiments short-time (0-5 ps) oscillatory modulations of the transient Hg + n and Hg 2+ n signals are observed which indicate an intermediate state dynamics common to all observed cluster sizes and charge states. Long-time (0-200 ps) transient ionization signals show caged dissociation and recombination chromophore dynamics.PACS. 33.80.Rv Multiphoton ionization and excitation to highly excited states -36.40.Qv Stability and fragmentation of clusters

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