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We examine the effect of electronic resonance enhancement on the line broadening for Coherent Anti-Stokes Raman Scattering (CARS) and related third-order non linear processes, in low pressure gases. These phenomena are analysed theoretically within the framework of a time-ordered diagrammatic representation of nonlinear polarizations. Diagrams facilitate the classification of the various physical processes and provide a way to readily derive all the susceptibility contributions. In CARS, the susceptibility terms which are Doppler-free and thus prevail in the Doppler limit are different from those which dominate in the collision broadening regime. This causes important changes in the spectral content. On the contrary, the same terms are seen to dominate, in both regimes, in coherent Stokes Raman scattering or in stimulated Raman scattering. Typical line contours are displayed. The generalization to all nonlinear processes is outlined

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