In conclusion, we quantified the anharmonicity of optical phonons that are impulsively generated via photoexciation of free carriers in polycrystalline thin films of (PEA) 2 PbI 4 . We find that the two primary lattice modes that are associated with the motion of the lead-iodide network exhibit distinct dynamics. The lower energy mode, labelled M1, exhibits temperature independent dephasing dynamics, indicating a low probability of phonon scattering. The higher energy modes, however, exhibit stark temperature dependence, which we analysed considering downconversion of optical phonons into two acoustic modes. We also extracted the anharomonicity constant, which is a measure of phonon-phonon scattering, and based on the comparison with other material systems, we conclude that the anharmonicity of the Ruddlesden-Popper metal halides is as large as in strongly ionic solids.