The long light filaments generated in air by powerful ultrashort laser pulses, previously attributed to self-channeling, were investigated by use of gigawatt pulses from a Ti:sapphire chirped-pulse-amplification laser system. A filament contained only a small fraction of the pulse energy and always ended at the diffraction length of the beam (~100 m), independently of the pulse energy. These features are explained by the moving-focus model, which is presented as an alternative to the self-channeling model. Computer simulations involving ionization of the air also support the moving-focus model.

Some 'Keldysh-like' theories are analysed leading to a more pragmatic definition of the tunnelling regime of ionization of atoms. Rather than using the more extreme definition of tunnelling (i.e. gamma <<1, F<

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A broad collection of experimental tunnel ionization data obtained using intense 10.6 mu m CO2 laser radiation, is presented. The variety of species studied includes three rare gas atoms (Xe, Kr and Ar), three homonuclear diatomic molecules (H2, O2 and N2), two heteropolar diatomic gases (CO and NO) and one triatomic molecule (CO2). The ionization behaviour going from the neutral particle to its associated singly charged ionic state, for all of the above species, is compared with a quasistatic tunnel ionization model. Good agreement between this theoretical model and the complete range of experimental results is found.

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Detailed measurements of and dissociation fragment kinetic energy dependences on laser intensity, using 150 fs, 800 nm pulses, are presented. The yields for both molecular and atomic ions are also given. The observed three-peak kinetic energy spectrum carries within it the signature of the different stages of the interaction. The two lower energy peaks are a product of bond softening (and above threshold) dissociation of the molecular ion from Franck - Condon populated vibrational levels. The third higher-energy peak results from enhanced ionization of the dissociating molecular ions. No light-induced vibrational trapping need be invoked to interpret the higher-energy fragments.

We discuss the mechanisms of multielectron dissociative ionization of diatomic molecules in intense laser fields. We show that during dissociation of molecular ions the nuclei pass through a critical range of internuclear distances where ionization is enhanced by several orders of magnitude for several successive charge states of the molecule. The critical range of internuclear distances depends only weakly on the laser frequency, laser intensity, and the charge state. Both numerical and analytical models are developed, and the effect of enhanced ionization on the kinetic energy and angular distributions of charged fragments is discussed.

The non-resonant interaction of an intense nanosecond CO2 laser pulse with Hg2, N2, and H2 shows that the molecules are tunnel ionized as if they were atoms with the same ionization potentials as the molecules. Moreover, dissociation of the neutral molecule appears to be much less probable even though the dissociation energies are lower than the ionization potentials.