THE LOW-FREQUENCY VIBRATIONAL-MODES OF C(4X2) CO ON PT(111)

Two, previously unobserved, low-frequency vibrational modes of c(4x 2) CO/Pt(111), measured using high-resolution helium atom scattering, ar e reported. The modes, at frequencies of (h) over bar omega = 8 meV an d (h) over bar omega = 10 meV, are nearly dispersionless, similar to t he previously known low-frequency mode at (h) over bar omega = 7 meV, which was also observed in the present measurements. From the intensit y dependence on wave vector and relative intensity to the (h) over bar omega approximate to 7 meV mode, we suggest that the new modes are al so polarized parallel to the Pt(lll) surface and, further, that they a re associated with the bridge bonded CO molecules. (C) 1998 American I nstitute of Physics. [S0021-9606(98)70145-X].