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The multiple ionization of molecular nitrogen has been studied by use of an intense picosecond laser (0.6 ps; 3x10'^ W/cm^; wavelength 600 nm). By measurement of the energies of the various fragment atomic ions (N"^, N^"^, and N^^) with a time-of-flight mass spectrometer it has proved possible to investigate the dynamics of the multielectron dissociative ionization process on a time scale of about 30 fs.PACS numbers: 33.80. Eh, 33.80.Rv We wish to report preliminary results of an experiment on the multiphoton ionization of molecular nitrogen. However, we do not wish to emphasize the multiphoton aspect of the multiple ionization processes observed, ^ but prefer to describe the laser in terms of a classical electromagnetic field. The mechanism involved is thought to be similar to that invoked in order to explain the field ionization of Rydberg atoms, ^ but in this case the laser field is so intense that it produces a potential difference along the molecular axis comparable with the binding energy of the outer electrons in the ground state.The idea behind the experiment is quite straightforward. A diatomic molecule is subjected to a large electric field created by a focused picosecond laser. As -^e molecule is ionized beyond the single-ion stage, the Tragment atomic ions mutually repel in the Coulomb field and the resulting energetic ions are detected by a timeof-flight (TOF) mass spectrometer. If additional electrons are stripped away as the molecule dissociates, the ions are now subjected to an even larger Coulomb repulsion. They will gain more energy and this will be reflected in the TOF spectrum. Thus, the detailed dynamics of multielectron dissociative ionization (MEDI) can be investigated on a time scale oi femtoseconds (the dissociation time scale), even though the laser pulse itself is of picosecond duration.The basic components of the experiment were the following. A dye laser was pumped by a mode-locked, frequency-doubled neodymium-doped yttrium-aluminum-garnet laser, producing 2.5-ps pulses (0.6-ps pulses with a saturable absorber in the dye). These individual pulses were amplified in a dye amplifier driven by a Qswitched frequency-doubled neodymium-doped yttriumaluminum-garnet laser operating at 10 Hz.

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Optical parametric chirped pulse amplifiers offer exciting prospects for generating new extremes in power, intensity, and pulse duration. An experiment is described that was used to investigate the operation of this scheme up to energies approaching a joule, as a step toward its implementation at the petawatt level. The results demonstrate an energy gain of 10(10) with an energy extraction efficiency of 20% and close to diffraction-limited performance. Some spectral narrowing during amplification was shown to be compatible with the time-varying profile of the pump beam and consistent with the measured recompressed pulse durations of 260 and 300 fs before and after amplification, respectively.