A new mechanism of nonlinear absorption of intense femtosecond laser radiation in air in the intensity range I = 10(11)-10(12) W/cm(2) when the ionization is not important yet is experimentally observed and investigated. This absorption is much greater than for nanosecond pulses. A model of the nonlinear absorption based on the rotational excitation of molecules by linearly polarized ultrashort pulses through the interaction of an induced dipole moment with an electric field is developed. The observed nonlinear absorption of intense femtosecond laser radiation can play an important role in the process of propagation of such radiation in the atmosphere.

The vibrational-translational relaxation time of the ν3 state of ozone was deduced from the phase shift of the photoacoustic detector signal relative to the amplitude-modulated radiation of the CO2 laser used for excitation of O3. A special photoacoustic cell with a third electrode is used to eliminate an instrumentation phase shift caused by inertia of the microphone membrane. A three-level kinetic model of O3 is presented and used to fit the experimental and calculated phase shifts to determine the vibrational relaxation rate coefficients for pure O3 and binary mixtures of O3 with O2, N2, and noble gases He, Ne, Ar, Kr, and Xe. These results are presented and compared with experimental data obtained for O3, O3–O2, and O3–N2 by fluorescence and double resonance techniques. Experimental data for ν3 state relaxation in binary mixtures with all noble atoms have been obtained for the first time. These new results are compared with the simplest model of interaction. Thus we obtain a very good agreement for the decrease of constants with the increase of the colliding partner mass.

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