  Longitudinal-optical (LO) mode phonon branches of KCl and NaCl were measured
using inelastic X-ray scattering (IXS) at 300 K and calculated by the
first-principles phonon calculation with the stochastic self-consistent
harmonic approximation. Spectral shapes of the IXS measurements and calculated
spectral functions agreed well. We analyzed the calculated spectral functions
that provide higher resolutions of the spectra than the IXS measurements. Due
to strong anharmonicity, the spectral functions of these phonon branches have
several peaks and the LO modes along $\Gamma$--L paths are disconnected.
