Employing two non-destructive methods, Fourier-transform infrared spectroscopy (FTIRS) and helium atom scattering (HAS), the adsorbate CO 2/NaCl(001) was investigated in the frequency range from < 10 cm −1 to 5000 cm −1. The 'internal' vibrations ν 2, ν 3, ν 3 + 2ν 2 and ν 3 + ν 1 were studied using FTIRS. The ν 2 and ν 3 split due to a correlation field between two molecules in each adsorbate unit cell. In addition the degeneracy of the ν 2 bending vibrations is lifted on the surface, yielding a quartet absorption. Outside the experimental resolution the combination vibrations ν 3 + 2ν 2 and ν 3 + ν 1 are not split, as expected from their low absorption cross section. The temperature dependence of the ν 3 doublet is explained in terms of small changes of the adsorption geometry, possibly due to the excitation of adsorbate phonons ('external' modes, i.e. vibrations of the molecules in the adsorption potential). In HAS experiments these phonons were studied. Below 80 cm −1 a total of eight different modes was observed, some of which could be followed across the whole Brillouin zone. Above 80 cm −1 several overtone/multiphonon excitations were also observed.