Normal Incidence Standing X-ray Wavefield Research Articles

Electrostatic ordering of the lanthanum endoatom inLa@C82adsorbed on metal surfaces

An investigation of the dynamic behavior of the endohedral La atom in $\mathrm{La}@{\mathrm{C}}_{82}$ adsorbed on Ag(111) or Cu(111) using the normal-incidence x-ray standing wavefield (NIXSW) technique is presented. For the surface-normal (111) reflection, NIXSW demonstrates different La absorption profiles between the $\mathrm{La}@{\mathrm{C}}_{82}$ monolayer and multilayer. Analysis of the profiles reveals significant La ordering in the monolayer; the degree of ordering increases as the film is cooled. First-principles calculations provide a description of the endohedral metal atom-cage bonding and the fullerene-surface interaction. Combined, these effects give rise to preferential binding sites for the endoatom resulting from electrostatic interactions within the adsorbed fullerene.

The structure of the [formula omitted] surface phase: a new normal-incidence X-ray standing wave study

The local geometry of the adsorbed sulphur atoms in the Cu(111)( 7 × 7 )R19°-S surface phase has been investigated with normal-incidence X-ray standing wavefield (NIXSW) absorption using the (111), (1̄11) and (200) Bragg reflections. The results are compared with the predictions for previously proposed models based on earlier investigations using NIXSW [but only the (111) reflection], surface extended X-ray absorption fine structure, quantitative low energy electron diffraction, surface X-ray diffraction (SXRD) and scanning tunneling microscopy. The best agreement is found for the model deduced from SXRD based on Cu 4S adclusters, but for the specific overlayer substrate registry in which these clusters are centred atop an outermost substrate layer Cu atom. All of the other models show significant inconsistencies with these new data.

A structural study of methanethiolate adsorbed on Cu(100)

The interaction of methanethiol, CH3SH, with Cu(100) has been studied by S 1s photoemission, S K-edge near-edge x-ray absorption fine structure and normal-incidence standing x-ray wavefield absorption at both (200) and (111) reflections. The results indicate that a single methanethiolate species, CH3S-, is formed and this is bonded to the unreconstructed surface via the S atoms which adopt the fourfold symmetric hollow site. This conclusion confirms the results of a recent near-edge and surface extended x-ray absorption fine structure study, but contrasts with the established adsorbate-induced reconstruction produced by this species on Cu(111).

SEXAFS and NIXSW investigation of the [formula omitted] surface

P K-edge surface extended X-ray absorption fine structure (SEXAFS) of the Rh(111)-( 7 × 7 ) R19.1°- P surface is reported. The results indicate that a recently proposed model for this surface is valid in which a mixed overlayer of composition Rh 5P 3 is formed on the Rh(111) surface. However, some modifications to atomic positions are necessary to acheive consistency with the RhP bond lengths determined by SEXAFS; this leads to a reappraisal of our normal incidence X-ray standing wavefield (NIXSW) data from the same surface.

Structural studies of the [formula omitted] surfaces of chlorine and iodine on Rh (111)

The surface structure of Rh(111)–( 3 × 3 ) R30°– Cl has been investigated by surface extended X-ray absorption fine structure (EXAFS). A nearest neighbour Cl–Rh bond length of 2.38±0.04 Å was found, although the adsorption site could not be unambiguously determined. The surface structure of Rh(111)–( 3 × 3 ) R30°– I was also studied by the normal incidence X-ray standing wavefield (NIXSW) technique. Iodine was found to adsorb predominantly in the ‘fcc’ hollow site, with at most 10% of atoms in the ‘hcp’ site.

Computation of adsorbate vibrational frequencies: methyl, methylidyne and methoxy adsorbed on the Al(111) surface

A general strategy for computing adsorbate vibrational frequencies using cluster calculations is illustrated for methyl, methylidyne and methoxy adsorbed on an Al(111) surface. The method uses energy second derivatives rather than the numerical fitting procedures often used in cluster calculations and avoids having to make guesses at the form of the normal modes for the vibrating polyatomic adsorbate. Using energy derivatives also facilitates performing a convergence check on the adsorbate frequency values. The methyl and methylidyne computed frequencies are in good agreement with vibrational frequencies for the CH3I dissociation product observed by high resolution electron energy loss spectra (HREELS). The computed methoxy adsorbate frequencies are also consistent with HREELS experiments, but the optimized methoxy structure, while agreeing with chemical intuition, differs from the methoxy adsorption at the hcp threefold site recently determined by normal incidence standing X-ray wavefield absorption (NISXW) experiments.

Normal incidence X-ray standing wavefield (NIXSW) study of the Rh(111)-(√7 × √7)R19.1°-P surface

A positional analysis of P in the Rh(111)-(√7 × √7)R19.1°-P surface has been carried out with the NIXSW technique using the (111) and (11 1 ) reflections. The results favour a model based on a mixed overlayer with the P atoms coplanar with respect to the (111) reflection. This is in accord with a tensor LEED analysis [W. Liu, K.C. Wang, K.A.R. Mitchell, Surf. Sci. 372 (1997) 312.] of the same surface.

The structure of (√3 × √3)R30°-sulphur on Rh(111) surface studied by X-ray standing wavefield absorption and surface EXAFS

The surface structure of (√3 × √3)R30°-S-Rh(111) has been investigated by normal incidence X-ray standing wavefield (NIXSW) absorption and surface-extended X-ray absorption fine structure (SEXAFS). NIXSW measurements show that the most likely site of adsorption is at the threefold “fcc” hollow, with some lateral disorder. The location of the S atoms at the “fcc” hollow site is consistent with S adsorption on the neighbouring fcc (111) transition metal surfaces. The coherent position relative to the Rh(111) lattice plane was measured to be 1.73±0.05 A ̊ . SEXAFS analysis revealed an S-Rh nearest neighbour bond distance of 2.25±0.04 A ̊ . The combined results of the NIXSW and SEXAFS measurements suggest that there is a net outward surface relaxation of the topmost Rh(111) layers of 0.1±0.05 A ̊ .

Structural study of Rh(100)-c(2 × 2)-S using the normal-incidence standing X-ray wavefield method

The adsorption site for sulphur in the Rh(100)-c(2 × 2)-S system has been investigated using the normal-incidence standing X-ray wavefield adsorption (NISXW) technique. Two Bragg reflecting planes were used for the measurements, the (200) plane (parallel to the surface) and the (111) plane (inclined 54.7° to the surface). The distance between the S atoms and the surface plane was found to be 1.38 ± 0.05 A ̊ . The adsorption site was determined by triangulation of the distances between the S atoms and the (200) and (111) planes, coupled with comparison of the measured coherent fractions with calculations of the expected values. The results indicate that S adsorbs in the symmetric four-fold hollow site, as found for S adsorption on other fcc(100) surfaces.

The structure of sulphur adsorption phases on Ni(111) studied by X-ray standing wavefield absorption

The technique of normal-incidence X-ray standing wavefield (NIXSW) absorption has been applied to an investigation of several structural phases of S on Ni(111), specifically the (2 × 2), (√3 × √3)R30° and (5√3 × 2)rect. phases, with a view in particular to clarifying recent controversy over the structure of the last of these. Absolute adsorbate site determination has been effected through real-space triangulation using both (111) and (1̄11) Bragg reflections. For the (2 × 2) phase the results are consistent with a simple overlayer structure with S atoms occupying the “fcc” hollow sites (directly above third-layer Ni atoms), in agreement with earlier determinations by LEED, ion scattering and SEXAFS. The same local site is found to be occupied in the (√3 × √3)R30° phase, although the NISXW data indicate poorer local order. For the (5√3 × 2)rect. phase, a simple undistorted missing-row model is found to be inconsistent with the data, as is any ideal “coincidence lattice” structure, including a simple undistorted pseudo-(100) surface reconstruction. A modification of this latter model, in which some S atoms penetrate the pseudo-(100) layer to bond to the underlying (111) substrate does, however, contain the main ingredients needed to fit the data. By using not only (111) and (1̄11) NIXSW but also (11̄1) and (111̄) absorption profiles, recorded from a near single-domain structure formed on a miscut crystal surface, specific information on some of the structural parameters of this modified reconstruction model is obtained and compared with complementary data from other published studies.

A structural study of the Al(111)( square root 3* square root 3)R30 degrees -Rb phase at different temperatures

Normal-incidence standing x-ray wavefield (NISXW) measurements have been made of the local adsorption site of Rb on Al(111) surfaces, particularly in an ordered ( square root 3* square root 3)R30 degrees phase, as a function of the sample temperature during adsorption or subsequent annealing. The results confirm the a top-site occupation for low-temperature (around 150 K) preparation, but show that room-temperature preparation leads to a structure having Rb atoms in surface substitutional sites. The overall structural situation is therefore essentially the same as that found previously by low-energy electron diffraction LEED for the Al(111)( square root 3* square root 3)R30 degrees -K phases. However, experiments involving annealing of the low-temperature prepared surface to room temperature indicate that only a small part of the surface easily transforms to the higher-temperature form, and indeed there is evidence that even in room-temperature preparations some fraction of the adsorbed atoms may remain in atop sites. The apparent conflict of this result with that from recent photoemission core-level shift and LEED data is discussed.

Structural studies of adsorbed and coadsorbed molecular species

A short review is presented of the techniques of backscattering photoelectron diffraction and normal incidence standing X-ray wavefield absorption in the particular context of applications to the study of the local adsorption structure of molecular species on surfaces, including cases of coadsorption. Specific examples of NO and NO/O coadsorption on Ni(111), methoxy on Al(111) and PF3 on Ni(111) are used to illustrate the ability of these methods to obtain local structural information in situations of especial interest in understanding structural effects in surface reactions. Further developments of these methods to the study of vibrational anisotropies of adsorbed species (such as that associated with the wagging vibrational mode of atop adsorbed NH3 and PF3 on Ni(111)), and in the use of photoemission core level (“chemical”) shifts to study more complex problems, are described.

Structural studies of adsorbed and coadsorbed molecular species

A short review is presented of the techniques of backscattering photoelectron diffraction and normal incidence standing X-ray wavefield absorption in the particular context of applications to the study of the local adsorption structure of molecular species on surfaces, including cases of coadsorption. Specific examples of NO and NO/O coadsorption on Ni(111), methoxy on Al(111) and PF 3 on Ni(111) are used to illustrate the ability of these methods to obtain local structural information in situations of especial interest in understanding structural effects in surface reactions. Further developments of these methods to the study of vibrational anisotropies of adsorbed species (such as that associated with the wagging vibrational mode of atop adsorbed NH 3 and PF 3 on Ni(111)), and in the use of photoemission core level (“chemical”) shifts to study more complex problems, are described.

The structure of sodium adsorption phases on Al(111)

The structures formed by the adsorption of Na and Al(111) have been studied by normal incidence standing X-ray wavefield absorption using both the (111) and (1̄11) Bragg reflections in order to obtain not only the Na-Al layer spacings, but also the adsorption sites. In the case of the ordered (√3 × √3)R30°-Na phase, at a nominal coverage of 0.33 ML, and at lower coverages down to approximately 0.12 ML, Na is found to adsorb in a coverage independent site which involves a substitution of some of the top layer Al atoms; if the substrate layer spacings remain unchanged by this reconstruction, the effective radius of the adsorbed Na species is found to be 1.67 Å. The higher coverage (2 × 2)-Na phase is found to involve (at least) two distinct Na adsorption sites, but each of these has a very similar layer spacing relative to the extended (111) substrate scatterer lattice planes. A specific model of this phase, involving two reconstructed layers each of stoichiometry NaAl 2 is proposed, which accounts in a quantitative way for the present X-ray standing wave data and previous X-ray absorption data, and in a qualitative way for published high resolution soft X-ray photoelectron spectroscopy results. As Al and Na are essentially immiscible in the bulk, this two-layer structure appears to be an entirely new kind of surface phase.

Structure determination for PF3 absorption on Ni(111)

The local adsorption structure of PF3 on Ni(111) has been investigated using a combination of P K-edge near-edge and surface extended X-ray absorption fine structure (NEXAFS and SEXAFS) and normal incidence standing X-ray wavefield absorption (NISXW) at both the P and F atoms in order to determine the adsorption site, the adsorbate-substrate bond-length, the molecular orientation and the internal structure of the adsorbed species. The molecule is found to be adsorbed atop a top layer Ni atom at a P-Ni nearest-neighbour distance of 2.07+or-0.03 AA, with its C3v symmetry axis perpendicular to the surface, and with an internal structure (both bond-lengths and bond angles) unchanged from that of the free molecule. The results also indicate that there is no change in the Ni(111) substrate structure induced by the adsorption, and the PF3 has a much larger vibrational amplitude parallel to the surface than perpendicular to the surface, presumably associated with a Ni-PF3 wagging mode.

Structural investigation of Rb adsorption on Al(111) using normal incidence standing x-ray wavefield absorption triangulation

The adsorption site, and adsorbate-substrate bondlength, has been investigated for the adsorption system Al(111)/Rb, using normal incidence standing x-ray wavefield absorption (NISXW), over a range of coverages from 0.12 to 0.33 ML including the ordered (2×2) and (√3×√3)R30° phases. By measuring both (111) and (1̄11) Bragg reflections in NISXW we obtain Rb–Al layer spacings in two real space directions and can therefore fully determine the adsorption structure by triangulation. For all phases we find adsorption in the atop sites, with a coverage-independent bondlength corresponding to an effective radius for the adsorbed Rb of 1.70±0.10 Å, significantly closer to the ionic (1.48 Å) than the metallic (2.43 Å) value. The NISXW results also show that the adsorbed Rb atoms have large vibrational amplitudes parallel to the surface which may be associated with the atop site adsorption.

An unusual adsorption site for methoxy on Al(111) surfaces

The adsorption site of the methoxy (CH3O) species on Al(111) has been investigated using the technique of normal incidence standing X-ray wavefield absorption. By recording the X-ray absorption of the O atom of this species at both the (111) and (111)-normal incidence Bragg scattering conditions (at normal and 70.5 degrees incidence to the surface) the adsorption site was obtained by simple real-space triangulation. The species is adsorbed in a three-fold symmetry hollow site with an O-Al layer spacing of 0.70+or-0.10 AA, (as found for chemisorbed oxygen), but the site occupied is the HCP hollow directly above an Al atom in the second layer, and not the FCC site (above an Al atom in the third layer) which is occupied by chemisorbed oxygen. This appears to be the first example of an adsorbate with a clear preference for this HCP hollow site on an otherwise clean FCC (111) surface.

Adsorption site determination for oxygen on Al(111) using normal incidence standing X-ray wavefield absorption

The structure of the chemisorption phase of oxygen on Al(111) has been studied using normal incidence standing X-ray wavefield absorption (NISXW) at the (111) and the ( 1 11) Bragg reflection conditions. The resulting OAl layer spacings in these two directions gives a measure of both the absorbate-substrate layer spacing perpendicular the surface, and the adsorption site. At the lowest coverages of oxygen, the results indicate that the adsorbed oxygen atoms occupy “fcc” hollow sites (i.e. directly above Al atoms in the third layer of the substrate) at a layer spacing of 0.70 Å. This result agrees well with previous SEXAFS results and with later LEED work, and reinforces the view that early LEED results giving much larger layer spacings were in error. The new results also indicate that this site is occupied at the lowest coverages of the chemisorbed phase, and that there is no evidence for a previously proposed precursor underlayer. Similar NISXW measurements taken from a surface containing a significant fraction of the subsequent surface oxide phase indicate that the oxygen atoms in this structure retain the same coordination, as well as bondlength, relative to the Al substrate, and favour a model in which the majority of the overlayer oxygen atoms of the chemisorbed phase retain their overlayer sites.

Structural study of 1,2-dichloroethane on Cu(111) using X-ray absorption and standing waves

The structure of 1,2-dichloroethane (DCE) adsorbed on Cu(111) in a single monolayer form has been investigated using ClK-edge near-edge X-ray absorption and normal-incidence standing X-ray wavefield absorption at the (111) reflection. These measurements indicate a small tilt (25°) of the ClC axis from parallel to the surface, and two different equally occupied ClCu layer spacings relative to the extended lattice planes of 1.0 and 1.6 Å. These resutls are found to be best reconciled with a structural model in which the DCE bonds with the ClCCCl plane of the trans conformer perpendicular to the surface and the Cl atoms over atop adsorption sites, the CuCl bondlength being compatible with the van der Waals radius for Cl. The possible role of a frustrated rotational mode of the adsorbed molecule leading to a “flipping” between two equivalent states is considered.

Normal-incidence standing X-ray wavefield absorption and SEXAFS studies of adsorption structures on Cu and Ni surfaces

The relative insensitivity of the Bragg condition to the exact angle of incidence to the scattering plane near normal incidence means that the X-ray standing wavefield absorption method can be applied to typical (imperfect) surface-science single-crystal samples in this special geometry. The method is highly complementary to the SEXAFS experiment that can be performed with the same instrumentation and we demonstrate this with examples of structures associated with Cl, CH3S— and S on Cu(111) and Hg on Ni(100). The combined approach is particularly valuable in cases in which the adsorbed species induce substrate reconstruction, and in particular reasonably definitive structures of considerable complexity have been identified for the two S-containing surfaces.