Dephasing of optical phonons in a substitutionally disordered organic solid. The lowest-frequency Raman-active phonon of a binary solid solution between p-bromochlorobenzene and p-dichlorobenzene

The temperature dependence of the line width is investigated for the lowest-frequency Raman-active phonon (28 cm^-1) in a 1:1 molar solid solution of p-dichlorobenzene (DCB) and p-bromochlorobenzene (BCB). It is found that, at 2 K, the line width is dominated by the contribution due to substitutional disorder, yet the line shape is a Lorentzian. This result is in agreement with the prediction of a theory based on a configurationally averaged Green's function. The temperature-dependence study is successfully used to determine the mechanism of dephasing due to anharmonic phonon-phonon interactions. The anharmonic contribution of the line width in the solid solution fits the mechanism of dephasing due to a T₁ relaxation in which the optical phonon decays into two acoustic phonons of half its frequency by cubic anharmonic interactions. The same mechanism has been found to explain the temperature dependence of line widths of the corresponding phonons in the p-dichlorobenzene and p-bromochlorobenzene neat crystals.