  We demonstrate numerically that the long-wavelength nonlinear dipole moment
and ionization rate versus electric field strength $F$ for different noble
gases can be scaled onto each other, revealing universal functions that
characterize the form of the nonlinear response. We elucidate the physical
origin of the universality by using a metastable state analysis of the
light-atom interaction in combination with a scaling analysis. Our results also
provide a powerful new means of characterizing the nonlinear response in the
mid-infrared and long-wave infrared for optical filamentation studies.
