Mode Conversion And Long-Lived Vibrational Modes In Lead Monolayers On Silicon (111) After Femtosecond Laser Excitation: A Molecular Dynamics Simulation

The dynamics of vibrations localized in a monolayer of Pb on Si(111) is studied using density functional theory and molecular dynamics methods. Recently, a pump-probe experiment has given direct access to the relaxation dynamics of the vibrational excitations in Pb, revealing two distinct time scales which are separately related to the directions of the Pb vibrational motions [Ultramicroscopy 127, 2 (2013)]. We analyze the experimental findings theoretically using atomistic modeling. After excitation of two-dimensional optical phonons, equilibration between parallel and perpendicular modes is observed before dissipation into the Si substrate. Even after several nanoseconds, equilibration with the substrate is not achieved due to energy trapped in acoustic Pb modes.