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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.

Experiments have been performed investigating the interaction of an intense CO2 laser pulse of nanosecond duration with diatomic molecules O2, CO and NO as well as triatomic CO2. New measurements relating to the tunnel ionization behaviour of noble gas atoms have been performed. These results contain less data scatter than previous work and thus provide a useful reference with which to compare the molecular results. Comparison with theory shows that in the absence of vibrational resonance with the CO2 laser (in the weak field approximation), these 'simple' molecules are tunnel ionized as if they were structureless atoms with an ionization potential equivalent to that of the molecular ground state.

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 <

Experimental results of the ionization of heavy water vapour in the tunnelling regime, at two different CO2 laser frequencies, are presented. The ionization mechanism appears to have a similar dependence on laser intensity as that for atoms, although the ionization probability seems to be higher in the case of the molecules in our experiment. It is proposed that vibrational excitation and strong field dressing of the potential energy surfaces of the molecule help in reducing its ionization potential. Numerical fits of ion number versus peak laser intensity for the ionization potential. Numerical fits of ion number versus peak laser intensity for the ionization products D2O+ and (D2O)2+ using atomic tunnelling theory provide an estimation for the rate probability of multiphoton excitation and strong field dressing prior to ionization.

The authors report experimental results of the ionization of D2O vapour by intense 10.6 mu m pulses. The processes leading to the products of D+, D2O+ and (D2O)+2 are qualitatively described by tunnel ionization from a reduced atom picture of the molecule, which is enlarged due to dressing by the strong laser field.