Nonlinear response of a molecular gas caused by orientation effects in the field of an intense femtosecond laser pulse

The orientation anisotropy of a molecular gas induced by a high-power femtosecond pulse is studied by the method of direct numerical integration of the nonstationary Schrödinger equation. The contribution of the anisotropy to the nonlinear polarisation of the gas is calculated. It is shown that the orientation nonlinearity caused by the efficient alignment of molecules along the laser field is accompanied by a considerable repopulation of the rotational levels of a molecule and cannot be described by the perturbation theory. It is found that the inertia of this nonlinearity depends on the laser-field strength.