Analysis of two-phonon resonances in and Raman spectroscopy of ammonium chloride

Using a model calculation of the unperturbed two‐phonon density of states for the frequency region encompassing the 2ν4 overtone of NH4Cl, together with the quantum field Green function approach of Ruvalds and Zawadowski, both the shape of the perturbed two‐phonon band in the Raman spectra and the detalled spectral evolution of the polariton spectra in this frequency regime as a function of wave vector q have been calculated. The calculation, which does not require a two‐phonon bound state, is in good agreement with experiment.