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We present Raman measurements on the iron-pnictide superconductorsCeFeAsO_{1-x}F_{x} and NdFeAsO{1-x}F_{x}. Modeling the Fe-As plane in terms ofharmonic and a cubic anharmonic Fe-As interaction we calculate the temperaturedependence of the energy and lifetime of the Raman active Fe B_{1g} mode andfit to the observed energy shift. The shifts and lifetimes are in goodagreement with those measured also in other Raman studies which demonstratethat the phonon spectrum is well represented by phonon-phonon interactionswithout any significant electronic contribution. We also estimate theanharmonic expansion from Fe (56->54) isotope substitution to \Delta a=5.110^{-4}\AA and \Delta d_{Fe-As}= 2.510^{-4}\AA and the shift of harmonic zeropoint fluctuations of bond lengths <\Delta x^2><=3 10^{-5}\AA^2, giving a totalrelative average decrease of electronic hopping integrals of |\delta t|/t<= 2.010^{-4}. The results poses a serious challenge for any theory ofsuperconductivity in the pnictides that does not include electron-phononinteractions to produce a sizable Fe-isotope effect.

Anharmonic softening of Raman active phonons in Iron-Pnictides; estimating the Fe isotope effect due to anharmonic expansion