Nonlinear absorption of intense femtosecond laser radiation in air.

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.