The dispersion curves of the three phonon modes of xenon, krypton, and argon monolayers on the Pt(111) surface

The dispersion curves of all three phonon modes, the shear horizontal (SH), longitudinal (LA), and perpendicular (S) vibrations of the two-dimensional monolayer inert gas solids of xenon, krypton, and argon on Pt(111) have be en measured by inelastic helium atom scattering (HAS). Previous HAS measure ments of xenon on other substrates revealed, in addition to the S mode, onl y a single dispersive mode which deviated strongly from the expected behavi or of the LA branch, to which it had been assigned. The new measurements no w identify this mode as the SH mode and all three modes are in excellent ag reement with calculations based on the inert gas pair potentials determined from gas and bulk solid data. Possible mechanisms for the excitation of th e SH branch, which should couple only very weakly to HAS on the basis of sy mmetry considerations, are discussed. (C) 2000 American Institute of Physic s. [S0021-9606(00)70104-8].