Two-phonon contributions to meV-resolved, inelastic x-ray scattering spectra of MgB_2 at 300 K are identified, in good agreement, in both intensity and energy, with a harmonic calculation using the force constant matrix from ab initio calculations. This contribution impacts the determination of the linewidth of the E_(2g) phonon mode that is so important for the high T_c of this material. To the best of our knowledge, this is the first observation of peaks in measurements of phonon dispersion (q > 0) due to two-phonon scattering in a non-rare-gas solid.
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