PHONON-LIBRON DYNAMICS OF ACETYLENE ADSORBED ON NACL(001)

The phonon-dispersion curves of the high-temperature (3 root 2 x root 2)R45 degrees phase of acetylene on NaCl(001) have been measured using high-resolution time-of-flight inelastic helium-atom scattering and c ompared with semiempirical calculations. The observed two Einstein vib rational modes at 8 and 14 meV are assigned to a collective perpendicu lar motion of the molecule centers of mass in the monolayer and to a c ollective librational motion of the molecular axes, respectively. With the same semiempirical potential used in the dynamical calculations, the 0 K adsorbate monolayer structure is predicted to be (root 2 x roo t 2)R45 degrees which is at variance with the (7 root 2 x root 2)R45 d egrees structure determined at T < 80 K by helium diffraction. The res ults are compared with structure assignments based on infrared spectro scopy and ab initio calculations.