M.T. Cvitas and A. Siber, Institute of Physics, Zagreb, Croatia
We show that the multiphonon backround observed in the time-of-flight
(TOF) spectra of He-atoms inelastically scattered from surfaces can in
certain regimes show pronounced structures. These structures look like
the one-phonon creation/destruction events and can be mistakenly interpreted
as such. On the example of He->Xe(111) scattering, we interpret these structures,
clearly observed in the experimental TOF spectra, as the double convolutions
of the Z-polarized surface phonon density of states. The density of states,
being (almost) singular at the edge of the surface Brillouin zone, when
convoluted with itself indeed looks like a one-phonon emission/absorption
TOF peak. Therefore, we call such structures two-phonon deceptons. The
precise calculations combining the slab lattice dynamical approach to the
calculation of surface phonon dispersions and polarization vectors and
the quantum formalism of multiphonon scattering are shown to reproduce
the experimental data in all of their aspects.