  According to a recent experiment, the instantaneous electronic Kerr effect in
air exhibits a strong intensity dependence, the nonlinear refractive index
switching sign and crossing over from a self-focusing to a de-focusing
nonlinearity. A subsequent theoretical work has demonstrated that this has
paradigm-changing consequences for the understanding of filamentation in air,
so it is important to subject the idea of higher-order nonlinearities to
stringent tests. Here we use numerical modeling to propose an experiment
capable of discriminating between the standard and the new intensity-dependent
Kerr-effect models.
