Surface-phonon dispersion spectrum

Fig. 51a, b. Rh(lll) (1x1) H. (a) Surface phonon spectrum for the bare Rh(lll) surface. The dashed lines represent the limits of the projection of the vertically and longitudinally polarised bulk phonons on the surface [95Wit]. (b) The surface phonon dispersion in presence of the (1x1) H overlayer as measured by HATOF [95Witl]. The inset reports the SBZ. 

The first successful measurement of surface phonons by means of inelastic He scattering was performed in Gottingen in 1980. By using a highly monochromatic He beam (Av/t 1%) Brusdeylins et al. were able to measure the dispersion of the Rayleigh wave of the LiF(001) crystal surfae. In earlier attempts the inelastic events could not be resolved satisfactorily due to the low beam monochromaticity. In Fig. 10a we show a typical TOF spectrum. 

Fig. 10. (a) He time-of-flight spectrum taken from a LiF(001) surface along the < 100) azimuth at an incident angle Si = 64.2°. The primary beam energy was 19.2 meV. (After Ref 25.). (b) Measured Rayleigh phonon dispersion curve of LiFfOOl) < 100), including a scan curve (dashed) for the kinematical conditions in (a). (After Ref. 25.)... 

The surface phonon spectrum of bare Mo(llO) consists of the RW, of the L mode and of a further resonance at 240 cm" (see Fig. 23). The dispersion curves were measured by HREELS (open symbols) [97Krd, 98Krdl, 98Kr52] as well as by HATOF (filled circles) [92Hul2]. 

Fig. 17. Cu(lOO) c(2x2) CO. Surface phonon spectrum as measured by HATOF. The solid and the dashed curves are the dispersion of the RW for the clean surface and its backfolded branch. Losses corresponding to the excitation of the RW are plotted as open circles. Filled and open squares denote the frustrated translation mode of the CO molecules in the c(2x2) overlayer and at defects [95E11].

To date, the phonon confinement effects have not been explicitly detected for CVD diamond films and results remained unsatisfactory in the case of DND. To improve the agreement between the predictions of the model and experimental Raman spectra of DND, effects such as crystal size distribution, lattice defects, and the energy dispersion of the phonon modes were taken into consideration and incorporated into the PCM. This work has shown that phonon wave vectors from small vibration domains lead to a broad shoulder peak at 1250 cm", that is often observed in the Raman spectrum of DND. Although the agreement between experimentally obtained and calculated Raman spectra has been significantly improved, some limitations remain, as was pointed ont in Ref. 98. The limitations imposed by the small ND size on the applicability of the PCM arise from the assumption that nanocrystals of 3-20 nm in size, showing extensive surface reconstruction and lattice defects, are assumed to have the phonon density of states of bulk diamond. 

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