Resonant and Non Resonant Susceptibility

1. n0_3sc

265
The imaginary part of the $$\chi ^3$$ susceptibility can be split into the resonant and non resonant contributions.
The resonant is the contribution from satisfying a resonant vibrational state/mode, but what contributes to the non resonant term?

2. olgranpappy

1,273
everything else, apparently.