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Previous time-of-flight experiments at 20-meV He beam energies, which provided the dispersion curve of Rayleigh surface phonons, have been extended to 90-meV energies on NaF(OOl) along (100). Despite a large multiphonon background from the Rayleigh phonons it has been possible to detect small peaks due to optical surface phonons. The results provide the first direct experimental evidence in the dispersion curves of microscopic surface optical modes.PACS numbers: 68.30.+z, 79.20.Rf Inelastic scattering of He atoms from crystal surfaces has been recently used for the first direct observation of surface phonons over the entire surface Brillouin zone. 1 The dispersion curves of Rayleigh waves have now been determined from time-of-flight (TOF) spectra in five ionic crystals, a semiconductor, and the three noble metals. 2 " 5 More recently electron energyloss spectroscopy (EELS) has also been employed and the dispersion of acoustical surface modes in a clean Ni(100) surface and optical modes with an oxygen over layer have been obtained. 6 He scattering has a resolution ten times better than EELS and no drawbacks due to surface charging. Still, it has until now been considered to be relatively insensitive to surface optical modes. Unlike neutrons, He atoms interact with crystal nuclei via the surrounding electrons at relatively large distances, from which, in the absence of net charge, they can only weakly couple to the opticalmode counterphase motion of oppositely charged ions. 7 Notwithstanding these unfavorable conditions, we have been able to obtain the first extensive evidence of microscopic optical surface vibrations in an ionic crystal using He scattering. The dispersion curves of the two optical modes, which are polarized in the scattering plane defined by the incident and outgoing wave vectors (sagittal plane), and are denoted by S A (mainly longitudinal) and S 2 (mainly transverse), have been measured in NaF(OOl) along (100) out to the zone boundary. 8 We have observed also the quasilongitudinal acoustic surface mode S 6 in the region of strong hybridization with S4. 8 Microscopic surface optical modes have been theoretically predicted by Lucas as early as 1968, 9 but no clear experimental evidence was given until now.The apparatus, the crystal-surface preparation, and the procedure used in the present experiment are the same as those thoroughly described in Ref. 2. Compared to the previous investigation of NaF(OOl) Ray-leigh waves, 2 we have used a lower surface temperature (T 5 = 180 K) and a higher beam-source temperature (r He -450 K) resulting in a larger incident momentum (^ = 13.36 A -1 ; £ , / = 93.0 meV). At this energy a multiphonon background arises on the phonon creation side and the lower-energy part of the phonon spectrum including Rayleigh waves is broadened by multiphonon processes.Most of the spectra have been recorded for incident angles 0,(0/ = 90°-0,) in the range 30 to 40° where we expect a resonance enhancement of the scattering intensity due to the in-plane inelastic resonances (m,n) v =...

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