J. Duan

Surface dynamics of NaCl(001) by inelastic He atom scattering

The relatively simple structure of alkali halide crystals has made these materials the paradigm for the understanding of the dynamics of ionic insulators. We have employed a time‐of‐flight (TOF) technique to determine the energy loss/gain due to single phonon creation/annihilation events in the scattering of He atoms from the NaCl(001) surface. From these data the surface dispersion curves over the ΓM region of the surface Brillouin zone have been constructed. The results compare favorably with both the slab dynamics and the Green’s function calculations reported for this crystal, and with previous measurements in the 〈100〉 direction. Of particular interest, we report the first evidence for the crossing mode in the 〈100〉 direction, which had been predicted for NaCl. Additionally, we have employed the TOF technique to examine the Debye‐Waller attenuation of the elastic (specular) intensity and the relative height of an inelastic multiphonon ‘‘foot’’ at the base of the specular beam as a function of the c...

Epitaxial growth of KBr onto NaCl(001) by high-resolution He atom scattering

Abstract The vapor deposition and subsequent growth of KBr onto the cleaved (001) surface of single crystal NaCl were studied by measuring the scattering behavior of a nearly monoenergetic (Δ E / E ⋍ 2%) He atom beam. Oscillations in the intensity of the specular beam due to layer-by-layer growth were observed for the first several layers. The structure of the surface for each deposited layer was determined by He diffraction via angular distribution measurements. The surface lattice spacings for NaCl and KBr have a mismatch of about 17% (3.99Afor NaCl and 4.66Afor KBr) and, hence, 6 KBrs can fit very nearly onto 7 NaCls (∼ 28A). The measured diffraction patterns showed peaks corresponding to a surface corrugation with this length. For the first adlayer, peaks only for NaCl and this “superstructure” were observed. As additional layers were deposited, the NaCl peaks disappeared and the superstructure peaks at first increased and then diminished as the Bragg peaks corresponding to the KBr lattice spacing grew in. By 6 monolayers only the diffraction peaks due to the KBr were observed. Additionally, time-of-flight energy analysis of the inelastically scattered He atoms was employed to determine the surface phonon dispersion curves of the adlayer. After only 4 monolayers, the Rayleigh and crossing modes were found to be almost identical to those measured from a cleaved KBr(001) surface with the exception of a non-zero value for the Rayleigh mode at the zone center. However, possible evidence for strain was seen in the S 2 mode, a surface optical mode lying in the gap between the acoustic and optical bulk bands. Similar behavior in the measured diffraction has been found in current complementary experiments for NaCl deposited onto KBr(001).

Surface lattice dynamics of NiO (001) by inelastic He atom scattering

High‐resolution inelastic He atom scattering experiments, employing a time‐of‐flight (TOF) technique, have been carried out on NiO (001) in the 〈100〉 and 〈110〉 high symmetry directions. The targets were prepared by cleaving single‐crystal NiO in air and then baking in vacuum. Nearly all of the experiments reported here were carried out at a target temperature of 575 K, which is above the Neel point (TN = 523 K), and with He atom wave vectors in the range of 7–9 A−1 (25–45 meV). Because of these high temperatures, the peaks in the TOF spectra were found to be somewhat broader and the multiphonon background somewhat higher than in reported spectra from this instrument for alklai halide surfaces. Nonetheless, sufficient data were obtained to allow the determination of the Rayleigh wave almost to the Brillouin zone boundary. The results for this mode are found to be in good agreement with recently published, room‐temperature electron energy loss spectroscopy experiments. In contrast, there is a substantial di...

Surface lattice dynamics of the Rbl (001) crystal surface via inelastic he atom scattering

Abstract High-resolution He atom scattering experiments (HAS) are reported, which were carried out on a cleaved Rbl (001) surface (T s = 120K). The nearly monoenergetic ( Δv v ≈ 1%) He beam, produced from a high pressure nozzle source, was chopped into pulses for the analysis of the inelastic scattering by a time-of-flight (TOF) technique. The incident He atom energies employed in these studies ranged from 20 to 24 meV, corresponding to wavevectors of 6.2 to 6.8 A −1 . Data from single phonon annihilation and creation events were obtained in both the and surface high symmetry directions and used to construct the surface phonon dispersion curves. The experimental results are compared with previous theoretical calculations for this surface. In general, the observed Rayleigh wave is found to be in very good agreement. However, a crossing mode is found which was not predicted initially, but which has now been seen in the (001) surfaces of several other alkali halides. Perhaps most interesting is that the data support the existence of the predicted high-lying surface optical branch, S 2 , pushed above the optical bulk bands because of the surface relaxation. In addition, another newly discovered branch, extending about a third of the way from the M point to the \ gG point near the top of the acoustic band is probably also due to the surface relaxation. It is possibly associated with second layer vibrations.

Homoepitaxial growth investigated by high‐resolution He atom scattering: NaCl onto NaCl(001)

The homoepitaxial growth of NaCl vapor deposited onto the cleaved (001) surface of NaCl was studied with a nearly monoenergetic (ΔE/E≂2%) He atom beam. In comparison with reflection high‐energy electron diffraction, the He atom is unique for epitaxial studies in that it is uncharged, nondestructive, and sensitive only to the outer surface layer. The oscillations in the scattered intensities of the specular and Bragg peaks due to the layer‐by‐layer growth were examined by measuring the angular distributions as a function of coverage, surface temperature (150–400 K) and incident He atom wave vector (6–8 A−1). The periodic variation in the relative heights of the specular and Bragg peaks with Δkz=‖kfz−kiz‖ showed that under these conditions the growth is two‐dimensional with a terrace height of 2.8 A or half the bulk face‐centered‐cubic lattice spacing. In addition, scattering cross sections for adsorbed NaCl at low coverages were determined with respect to these same parameters. The cross sections for subst...

Surface and bulk phonon‐assisted resonances with surface bound states in the inelastic scattering of He atoms on NaCl (001)

High‐resolution He atom scattering experiments have been carried out on the NaCl (001) surface in both the 〈100〉 and 〈110〉 surface high symmetry directions. Time‐of‐flight (TOF) arrival spectra of the inelastically scattered He atoms were analyzed to obtain the surface phonon dispersion curves. Several of the TOF peaks corresponding to Rayleigh mode phonons were found to have relatively large intensities and there appeared to be a number of well‐resolved peaks in the time‐of‐flight which did not correspond to surface‐localized modes when the data were compared with previous calculations for NaCl. These data appear to be associated with surface and bulk phonon‐assisted resonances with surface bound states. Analysis of these resonances yields results consistent with the four bound states of He on NaCl (001) which had been previously reported.

Temperature dependence of He atom scattering from NaCl(001)

Abstract A nearly monoenergetic He atom beam was scattered from the NaCl(001) surface in the direction as a function of crystal temperature. The scattering in the specular direction was examined, employing a time-of-flight technique to separate the scattered elastic and inelastic He atom signals. The crystal temperature was varied from 120K to 670K, covering the range over which the scattering goes from mainly elastic to mostly multiphonon inelastic scattering. The elastic specular signal decreased with increasing crystal temperature in accordance with the Debye-Waller effect and eventually could no longer be detected. However, with increasing temperature the inelastic signal grew relative to the elastic signal and formed a well-defined “foot” directly around the elastic peak. A theory based on small energy exchanges in the collisions and the Debye model was developed which shows the main features of the measurements; namely, the behavior of the inelastic intensity and the FWHM of the foot as a function of target temperature.

Measurement of the surface dynamics of RbCl(001) via high resolution He atom scattering

High resolution He atom scattering experiments have been carried out on the cleaved RbCl(001) surface in the 〈100〉 direction. For most of the experiments reported here the incident He atom wave vector ki was ∼8 A−1 (corresponding to incident energy Ei≊33 meV) and the target temperature was ∼190 K. Both the Rayleigh wave and a surface optical mode lying in the gap between the bulk acoustic and optical bands (the S2 mode) were observed. The measured surface dispersion branches are compared with model calculations of surface dispersion for both a relaxed and an unrelaxed RbCl(001) surface. The calculated Rayleigh mode dispersion is very nearly the same in both models and agrees well with the experimental results. The experimental optical mode data appear to fit the calculations only in part for each model. A comparison of the RbCl results with previously measured KBr surface dispersion curves shows that their dynamics are largely the same, but with significant differences for part of the S2 mode close to the...

Surface dynamics of CsF by He atom scattering

Abstract The dynamical behavior of the CsF(001) crystal surface was explored experimentally by He atom scattering. The time-of-flight method was used to energy analyze the inelastically scattered He atoms to obtain dispersion relations for the entire surface Brillouin zone for both the 〈100〉 and 〈110〉 high symmetry directions. Intense signals from the Rayleigh modes were found in both directions. Some evidence of a longitudinal acoustic mode was observed in the 〈110〉 direction. A few weak points that could be associated with an optical mode were also found in both directions. Finally, the results of CsF are compared with those of its “mirror” compound NaI.

He atom scattering study of the surface dynamics of an epitaxial one monolayer KBr film on RbCl(001) : a shell model analysis

Abstract High resolution inelastic He atom scattering experiments have been carried out on an epitaxially grown one monolayer film of KBr on RbCl(001) in the temperature range of 180–220K for the ΓM direction. This adsorbate/substrate combination represents a crystal interface with a misfit less than 0.2%. Measured time-of-flight spectra yield the surface dispersion of the film with branches corresponding to a Rayleigh wave, a longitudinal resonance and a surface optical mode that lies in the gap between the KBr bulk optical and acoustic bands. The Rayleigh mode of the film is very similar to the Rayleigh modes for the cleaved (001) surfaces of both KBr and RbCl; however, the optical mode is softened by about ten percent. Because of the small misfit in lattice constants, the unit cell can be approximated by that of the substrate, RbCl. Thus, this system lends itself to a theoretical treatment by the shell model approach, similar to that which has guided theoretical work on the surface dynamics of bulk ionic crystals. Further, shell model parameters can be taken from the literature for the interactions within the overlayer and within the substrate, and adapted for the interaction between the deposited ions and the substrate ions at the interface. A suitable and consistent procedure for the latter has proven to be the most challenging. The surface dispersion and relaxation are calculated by the slab dynamics technique in the usual way. A comparison between experiment and theory is good on the whole considering that the model is without adjustable parameters. In particular, the calculation predicts the softening for the optical gap mode. The calculation also predicts an outward relaxation of the KBr monolayer which is found by the measurements. However, the extent of the relaxation is not predicted to be as great as was observed.