  The dynamical properties of the high-density Ru(001)-(1$\times$1)-O phase has
been investigated by a combined high-resolution electron energy loss
spectroscopy and density functional theory study. Due to a strong static
outward relaxation of the first Ru layer a soft Rayleigh phonon mode is
expected. However, a Rayleigh mode stiffening together with a new high energy
in-plane phonon mode above the bulk bands is found which is related to a strong
adsorbate-induced intralayer force constant stiffening which counteracts an
interlayer softening. This structurally rather simple system with one surface
atom per (1$\times$1) unit cell demonstrates the limited applicability of
previously adopted models.
