  We report the inelastic light scattering studies on SmFeAsO0.65 and
SmFeAsO0.77H0.12 with iron isotopes namely 54Fe and 57Fe. In both of these
systems under investigation we observed a significant shift in the frequency of
the phonon modes associated with the displacement of Fe atoms around ~ 200
cm-1. The observed shift in the Fe mode (B1g) for SmFeAsO0.65 is ~ 1.4 % and
lower in case of SmFeAsO0.77H0.12, which is ~ 0.65 %, attributed to the lower
percentage of isotopic substitution in case of SmFeAsO0.77H0.12. Our study
reveals the significant iron isotope effect in these systems hinting towards
the crucial role of electron-phonon coupling in the pairing mechanism of iron
based superconductors.
