G. G. Bishop

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 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.

Scattering of 3He from the NiO(001) surface

A scattering experiment employing 3He as the probe has been performed in an attempt to measure the magnetic structure of the NiO(001) surface. A nozzle beam ∼5% 3He in a carrier gas of 4He was used to produce a narrow velocity spread of the incident 3He atoms, Δv/v≊3.5%. Measurements of the angular distributions (scattering intensity versus incident angle) yielded sharp Bragg peaks, indicating that a high quality NiO(001) surface had been produced by cleaving in situ. The diffraction pattern obtained with 3He is similar to that obtained with 4He. A careful examination of the angular region where Bragg peaks from the magnetic structure might be expected revealed no features. This negative result implies that the hyperfine interaction is much weaker than the nonmagnetic portion of the atom–surface potential, and is consistent with the magnitude of the hyperfine interactions found in electron spin resonance measurements. Features attributable to bound‐state resonances in the He–surface potential were observe...

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.

Surface lattice dynamics of KBr epitaxially grown on RbCl(0 0 1)

Abstract High resolution He atom scattering experiments were performed on epitaxially grown layers of KBr on a RbCl(0 0 1) substrate for coverages of one, two, and three monolayers. The in situ growth of the KBr was monitored by following the intensity of the specularly scattered He beam versus coverage. The inelastic scattering was measured for each deposited layer to determine the surface phonon dispersion curves as a function of coverage in both the Γ M ¯ and Γ X ¯ high symmetry directions of the surface Brillouin zone. Fits to the data from the KBr/RbCl system, which is nearly perfectly lattice matched, were obtained from a shell model calculation whose parameters are found to provide not only a good match to the measured dynamics of these deposited films, but in addition, to the dispersion of bulk and clean surfaces of KBr, RbCl, KCl, and RbBr.

Multiphonon scattering of thermal energy He atoms from the LiF(001) surface

Results from the scattering of nearly monoenergetic (ΔE/E≊2%) thermal energy He atoms from the LiF(001) surface are presented and discussed with particular emphasis on multiphonon interactions. These experiments represent the next step in a series of experiments exploring the multiphonon scattering of He from alkali halides in that previous work [G. G. Bishop et al., Phys. Rev. B 47, 3966 (1993)] has now been extended to include nonspecular angles. The LiF surface was prepared as in the previous experiments by cleaving the crystal in air and cleaning it by sublimation in the ultrahigh vacuum system. The measurements consist of time‐of‐flight spectra of the scattered He atoms taken for a number of incident angles as a function of the surface temperature ranging from 290 to 720 K and for He incident wave vectors ranging from 7 to 11 A−1. Analysis of the data was carried out using a theoretical model developed by Manson [Phys. Rev. B 43, 6924 (1991)], which allows the determination of the interaction potenti...