Nonequilibrium dynamics of transverse-optical (TO) phonons and low-frequency interlayer shear and breathing modes are mapped in femtosecond midinfrared pump-probe experiments. Time-resolved changes of TO phonon absorption reveal a TO phonon lifetime of 1.2 ps, while low-frequency excitations decay with a time constant of 22 ps. The coupling of intralayer TO and interlayer motions manifests in a quasi-instantaneous redshift of the TO phonon resonance by some 10 ${\mathrm{cm}}^{-1}$. Theoretical calculations account for the coupling scenario and underline the relevance of interphonon interactions for the nonlinear phonon response.

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• Phonons
• 2-dimensional systems
• Density functional calculations
• Infrared spectroscopy
• Pump-probe spectroscopy
• Reflectivity