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, are reported. The modes, at frequencies of (h) over bar omega = 8 meV and (h) over bar omega = 10 meV, are nearly dispersionless, similar to the previously known low-frequency mode at (h) over bar omega = 7 meV, which was also observed in the present measurements. From the intensity 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 also polarized parallel to the Pt(lll) surface and, further, that they are associated with the bridge bonded CO molecules. (C) 1998 American Institute of Physics. [S0021-9606(98)70145-X].