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.

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.

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