The adsorption of carbon monoxide on the MgO(100) surface, prepared in
situ by cleaving a single crystal in ultra high vacuum, has been stud
ied at temperatures between 32 and 56 K using polarization Fourier tra
nsform infrared spectroscopy. Above 45 K a single IR absorption at 215
0.5 cm(-1) is detected with p-polarization only which indicates a dyna
mic CO dipole perpendicular to the surface. Below 45 K the spectra of
the monolayer are completely different. Using p-polarization the main
IR absorption is detected at 2151.2 cm(-1). Two additional features ar
e observed at 2137.2 and at 2132.5 cm(-1), only the latter is also obs
erved with s-polarization. The spectrum indicates an ordered monolayer
with two energetically different adsorption sites per unit cell. A c(
4 X 2) symmetry is inferred in concert with helium atom scattering [Ge
rlach et al., Surf. Sci. 331-333 (1995) 1490]; from LEED [Audibert et
al., Surf. Sci. 273(1992) L467; Panella et al., J. Phys. Paris 4 (1994
) 905] a (4 X 2) symmetry was derived. The p-polarized absorption at 2
151.2 cm(-1) is assigned to a dynamic CO dipole perpendicular to the s
urface, the doublet is attributed to the in-phase and the out-of-phase
vibrational exciton of two tilted CO molecules at energetically equiv
alent sites. Their projections on the surface are oriented antiparalle
l, the dipole of the in-phase exciton at 2137.2 cm(-1) is perpendicula
r to the surface and has no azimuthal component, the dipole of the out
-of-phase exciton at 2132.5 cm(-1) is parallel to the surface. A weak
prevalence of one domain is inferred from the spectra.