INTERNAL AND EXTERNAL VIBRATIONAL-MODES OF CO2 NACL(001) STUDIED BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY AND HELIUM ATOM SCATTERING/

Employing two non-destructive methods, Fourier-transform infrared spec troscopy (FTIRS) and helium atom scattering (HAS), the adsorbate CO(0) 2/NaCl(001) was investigated in the frequency range from <10 cm(-1) to 5000 cm(-1). The 'internal' vibrations v(2), v(3), v(3)+2v(2) and v(3 )+v(1) were studied using FTIRS. The v(2) and v(3) are split due to a correlation field between two molecules in each adsorbate unit cell. I n addition the degeneracy of the v(2) bending vibrations is lifted on the surface, yielding a quartet absorption. Outside the experimental r esolution the combination vibrations v(3)+2v(2) and v(3)+v(1) are not split, as expected from their low absorption cross section. The temper ature dependence of the v(3) doublet is explained in terms of small ch anges of the adsorption geometry, possibly due to the excitation of ad sorbate phonons ('external' modes, i.e. vibrations of the molecules in the adsorption potential). In HAS experiments these phonons were stud ied. Below 80 cm(-1) 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.