Electron Stimulated Desorption Ion Angular Distribution Pattern Research Articles

Adsorption state of NO on Mo(1 0 0) surface studied by ESDIAD

The adsorption states of nitric oxide and oxygen on a Mo(1 0 0) surface have been studied by electron stimulated desorption ion angular distribution (ESDIAD) and low energy electron diffraction (LEED). On a NO-exposed Mo(1 0 0) surface, only O + ions are detected by ESD and the ESDIAD pattern has a bright center spot perpendicular to the surface and four off-normal spots in the [0 0 1], [0 1 0], [ 00 1 ¯ ] and [ 0 1 ¯ 0 ] directions with a very weak intensity. The ESDIAD patterns of the NO- and O 2-exposed surfaces have the same shape but different intensities. The center spot is hazy and square at low exposure of NO below 0.5 L and becomes circular with increasing exposure. For low exposure, oxygen atoms adsorb at fourfold hollow and on-top sites, and for higher exposure, they mainly adsorb at on-top sites. Some oxygen atoms adsorb at threefold hollow sites coordinated to two first-layer molybdenum atoms and one second-layer molybdenum atom. On the O 2-exposed surface, the intensities of the center and off-normal spots decrease monotonically with increasing annealing temperature. Oxygen atoms diffuse into the bulk in the temperature range of 300–800 K. On the NO-exposed surface, these intensities decrease by a two-step process and disappear at around 1200 K. Nitrogen atoms adsorb at the sites where the diffusion of the oxygen atoms into the bulk is suppressed, such as fourfold hollow sites. The NO- and O 2-exposed surfaces show the following structures with increasing temperature: a high background up to 850 K, a streak structure in the temperature range of 900–1100 K, (2 × 1) in the range of 1200–1300 K, (2 × 2 + 4 × 2) in the range of 1350–1500 K, (√5 × √5) R26°33′′ in the range of 1550–1650 K, c(4 × 4) in the range of 1700–1800 K and a clean structure above 1850 K. Oxygen atoms begin to form the surface oxide above 800 K on or under the surface.

Effects of electron irradiation on structure and bonding of SF6 on Ru(0001)

Electron-stimulated desorption ion angular distribution (ESDIAD) and temperature-programmed desorption (TPD) techniques have been employed to study radiation-induced decomposition of fractional monolayer SF6 films physisorbed on Ru(0001) at 25 K. Our focus is on the origin of F+ and F− ions, which dominate ESD from fractional monolayers. F− ions escape only in off-normal directions and originate from undissociated molecules. The origins of F+ ions are more complicated. The F+ ions from electron-stimulated desorption of molecularly adsorbed SF6 desorb in off-normal directions, in symmetric ESDIAD patterns. Electron beam exposure leads to formation of SFx (x=0–5) fragments, which become the source of positive ions in normal and off-normal directions. Electron exposure >1016 cm−2 results in decomposition of the entire adsorbed SF6 layer.

Structure of W(111) for NO and O 2 coadsorption studied by ESDIAD

Nitric oxide and oxygen coadsorbed on W(111) surface structures under various conditions have been studied by means of electron-stimulated desorption ion angular distribution (ESDIAD) and low energy electron diffraction (LEED) at room temperature. Only O + ions are detected by electron-stimulated desorption (ESD) from the NO- and/or the O 2-adsorbed W(111) surface. The ESDIAD patterns consist of three bright spots. With increasing temperature, the intensities of the ESDIAD pattern from the O 2/W(111) decrease monotonically, and disappear around 700 K. However, the intensities of the patterns from the NO+O 2/W(111) decrease rapidly above 700 K, and disappear around 850 K. The ESDIAD spots from the NO+O 2/W(111) become sharp up to 500 K, with increasing temperature. The surface, following the NO and/or the O 2 adsorption under any condition, has a (1×1) structure with a high background intensity, and the surface structure changes into a (3×3) structure after the disappearance of intensities of the ESDIAD patterns. The O atoms on the surface diffuse into the bulk with increasing temperature, but the diffusion is blocked by the N atoms, in the case of the NO-adsorbed surfaces under any condition.

Dynamics of the striped nanostructure of the oxidized Cu(110) surface: A momentum-resolved electron stimulated desorption ion angular distribution study

The striped structure of the partially oxidized Cu(110) surface has been studied using a novel technique, momentum-resolved electron stimulated desorption ion angular distribution (ESDIAD) Long …O–Cu–O… strings oriented in the 〈001〉 direction and exhibiting attractive interactions with each other form periodically arranged stripes with widths in the nanometer range [D. J. Coulman et al., Phys. Rev. Lett. 64, 1761 (1990); K. Kern et al., ibid. 67, 855 (1991)]. Two different oxygen sites were detected, leading to a twofold symmetrical four-beam O+ ESDIAD pattern with 22° tilting of the beams in the 〈11̄0〉 azimuth (A) direction and 8° in the 〈001〉 azimuth (B) direction. The relative intensities of the four beams have been compared for a wide range of oxygen coverages leading to a model in which the A beams correspond to O+ ions desorbing from the edges of the stripes and the B beams from the internal regions of the stripes. This model is confirmed by studying the effect of the coadsorption of Ar at 32 K on the oxidized structure where the Ar is preferentially adsorbed on the edges of the stripes. The dynamical motion of the one-dimensional …O–Cu–O… oscillator chains situated at the edges of the stripes has been studied by momentum-resolved ESDIAD measurements over a broad range of temperatures (80–650 K).

Momentum resolved electron stimulated desorption ion angular distribution, a new technique, probing the low frequency motion of adsorbed molecules on single crystal surfaces

A new technique, momentum resolved electron stimulated desorption ion angular distribution (ESDIAD), provides a method for taking snapshots of the zero-point position and lateral momentum of particles adsorbed on crystalline surfaces. By employing state-of-the-art electronics and computer technology it is possible to record for each desorbing particle the desorption direction together with the flight time. High momentum and directional resolved images are obtained, with time-of-flight resolution in the picosecond range and data acquisition rates up to 100 kHz. This enables us to deconvolute spatial and momentum contributions to the ESDIAD pattern and to map the low frequency motion of the adsorbed particles. These maps reflect the adsorbate interactions with the substrate and with neighboring species on the substrate. For selected examples it is demonstrated that by measuring the three dimensional momentum vector for each desorbing particle it is possible to probe the lowest energy states of adsorbed species, as well as to measure the momentum distribution when the adsorbed species gains thermal energy. Such information can be used as a basis for thinking about anisotropies in lateral motion of particles on surfaces. One major opportunity involves the study of dissimilar chemisorbed species which, when imaged together in momentum and real space, give new insights into the first stages of interaction between the species, leading ultimately to a chemical reaction.

ESDIAD studies of the structure of TiO 2(110)(1 × 1) and (1 × 2) surfaces and interfaces in conjunction with LEED and STM

The TiO 2(110)-(1 × 1) surface and its reconstruction as a (1 × 2) form have been studied with low energy electron diffraction (LEED), electron stimulated desorption ion angular distribution (ESDIAD) and scanning tunnelling microscopy (STM). Oxygen ion desorption occurs within a lobe perpendicular to the (1 × 1) surface, changing to two off-normal lobes for the (1 × 2) reconstruction. This transformation in the ESDIAD pattern is consistent with the added Ti 2O 3 row model of the (1 × 2) reconstruction proposed by Onishi and Iwasawa. STM studies of the stoichiometric and electron irradiated surfaces reinforce the association of the O + ESD contribution with majority sites at the surface. Adsorption of acetic acid on the (1 × 1) surface produces a (2 × 1) overlayed and induces a reconstruction of the underlying substrate. ESDIAD reveals H + ions emitted off-normally from dissociatively adsorbed acetate, and along the surface normal from surface hydroxyls. Adsorption of acetic acid on the (1 × 2) surface does not modify the LEED pattern, but ESDIAD reveals H + desorption with a weaker off-normal contribution consistent with the Ti 2O 3 model of the reconstruction.

Measurement of Anisotropy in the Lateral Momentum of a Vibrating Adsorbed Molecule: CO/Cu(110)

Anisotropies in the frustrated translational mode of CO chemisorbed on Cu(110) have been measured for the first time, with energies of 4.9meV in the {l_angle}1{bar 1}0{r_angle} directions and 3.1meV in the {l_angle}001{r_angle} directions. The measurements are made using a new time-of-flight ESDIAD (electron stimulated desorption ion angular distribution) method, permitting the resolution of bond directional effects and initial state lateral momentum effects in ESDIAD patterns. These measurements provide a more detailed understanding of the librational potential energy well shape than has been possible previously. {copyright} {ital 1997} {ital The American Physical Society}

The orientation of acetate on a TiO2(110) surface

The adsorption of acetic acid on a TiO2(110) surface has been studied using electron stimulated desorption ion angular distribution (ESDIAD) and low energy electron diffraction (LEED). The acetate intermediates arising from the dissociative adsorption of acetic acid form an ordered (2×1) overlayer at saturation coverage. The H+ ESD ion angular distributions can be resolved into two contributions: Those ions desorbing from hydrogen atoms bonded at the oxide substrate, and those ions desorbed via the rupture of the C–H bonds of the acetate. The geometry of the ESDIAD pattern led us to propose that the acetates are bridge bonded with the five-fold coordinated Ti 4+ ions, with their molecular plane perpendicular to the surface. Decomposition of acetate at room temperature occurs under electron beam irradiation, resulting in the desorption of CH2CO and CH3/CH4.

Adsorbate–adsorbate repulsions—the coverage dependence of the adsorption structure of CO on Cu(110) as studied by electron-stimulated desorption ion angular distribution

The coverage dependent orientation of CO adsorbed on a Cu(110) surface was studied by the electron-stimulated desorption ion angular distribution (ESDIAD) technique. A neutral excited (CO*) species is imaged and in addition positive ions are measured. The adsorption temperature was varied between 32 K and 150 K. By applying the ESDIAD technique at a temperature below 80 K it was possible to decrease the beamwidths drastically, to determine the angular distributions better than ±0.5°, and to study the adsorption of CO chemisorbed and physisorbed on the surface. With increasing CO coverage we observe three distinct ESDIAD patterns. Starting from a normal beam pattern with an elliptical cross section with the major axis oriented in the 〈11̄0〉 direction for coverages up to 0.2 monolayer (ML), a transformation of the ESDIAD pattern into a pattern of two separated beams is observed for a coverage of about 0.5 ML, indicating a tilting of the molecules in the 〈11̄0〉 directions by ∼9°. With further increasing CO coverage an additional central peak develops with an elliptical broadening now in the 〈001〉 direction. The changes of the pattern are reversible as shown by decreasing the coverage by thermal desorption. Based on these ESDIAD and digital low energy electron diffraction results, a linear-chain model for CO adsorption is proposed. Temperature programmed desorption measurements also indicate the presence of repulsive CO–CO interactions in the adlayer.

Structure of W(100) for NO adsorption studied by ESDIAD

The adsorption state of nitric oxide on a W(100) surface at room temperature has been studied by means of electron stimulated desorption ion angular distribution (ESDIAD) and low energy electron diffraction (LEED). Only O + ions are detected by ESD from the NO adsorbed W(100) surface. The ESDIAD pattern from the NO-adsorbed surface has a bright center spot and 4-symmetry [100] and [110] lobes with very weak intensity. At low NO exposure (below 0.4 L), the center spot is a hazy square in shape, and becomes circular in shape with increasing exposure. The [100] and [110] lobes appear around the center spot above 0.4 L. With increasing temperature, the intensities of the center spot and the [100] lobes exhibit similar variation, and the [110] lobe intensities decrease monotonically. The surface after NO adsorption has a diffuse (4 × 1) structure, which changes to the sharp (4 × 1) structure around the temperature at which the center spot intensity decreases (∼900–1100 K), and a (2 × 1) structure appears above 1100 K. There are 4 adsorption sites for O atoms on the NO-adsorbed surface.

Dynamics and structure of chemisorbed CO on Cu(110): An electron-stimulated desorption ion angular distribution study

The adsorption of CO on a Cu(110) surface has been studied in the temperature range between 32 K and 200 K by the ESDIAD (electron-stimulated desorption ion angular distribution) technique. Here a desorbing neutral excited CO* species is spatially imaged. At coverages up to θ=0.2, a pronounced anisotropy in the librational motion of CO adsorbed in the atop configuration has been observed. The higher amplitude of libration is along the copper atom rows ([11̄0]-direction). Large changes in the observed CO* pattern in the low temperature range between 32 K and 70 K give an indication of a shallow potential well and the thermal population of the higher librational levels. With increasing CO coverage three distinct ESDIAD patterns are observed. Starting from a single normal beam with an elliptical cross-section having the major axis in the [11̄0]-direction for coverages up to 0.2 monolayer (ML), a transformation of the ESDIAD pattern into two separated beams is observed for a coverage of about 0.5 ML, indicating a tilting of the molecules in the [11̄0]-directions. With further CO coverage increase, an additional central beam grows with a slight elliptical broadening in the [001]-direction. Based on ESDIAD and digital low-energy electron diffraction measurements, a linear-chain model for CO adsorption on atop sites of Cu(110) is proposed.

NO Adsorption and Thermal Behavior on Pd(112): The Effect of Surface Modification by O and Ge

The effect of O(a) and Ge(a) on the adsorption of NO on the stepped Pd(112) surface has been studied by temperature programmed desorption (TPD), Auger electron spectroscopy (AES), secondary electron emission crystal current (SEECC), low energy electron diffraction (LEED), and electron stimulated desorption ion angular distribution (ESDIAD) methods. The presence of either O or Ge at the Pd(112) surface alters the kinetics of NO thermal desorption, destablizing the NO which subsequently desorbs at lower temperatures. The dissociative NO channels, which produce N[sub 2] and N[sub 2]O as products, are simultaneously suppressed by the preadsorption of O or Ge. The similarities between the NO/O/Pd(112) and NO/Ge/Pd(112) results lead to the conclusion that steric blocking is more important than electronic poisoning effects in these systems. O[sup +] ESDIAD patterns from NO/Pd(112) are essentially identical to those from NO/O/Pd(112) and NO/Ge/Pd(112), implying that the surface modifiers do not preferentially block the step sites toward NO adsorption. The ESDIAD data are interpreted in terms of the random adsorption of O and Ge on the surface. 24 refs., 12 figs.

Direct images of isotropic and anisotropic vibrations in the Cl-Si and H-O-Si chemisorption bonds on Si(100)

The electron-stimulated desorption ion angular distribution (ESDIAD) technique combined with a digital subtraction method have been employed to generate difference ESDIAD patterns between measurements made at 130 and 305 K for both adsorbed Cl and OH on Si(100). The difference ESDIAD patterns reveal, in real space, images qualitatively related to the thermally excited vibrational amplitudes of the adsorbates. This permits a qualitative visualization of the potential energy surface associated with an adsorbate on its adsorption site. For Cl Si(100) , isotropic thermal broadening of the Cl + beams from 130 to 305 K is observed. In contrast, OH Si(100) exhibits anisotropic thermal broadening of the H + beams on heating from 130 to 305 K. These results are correlated with the vibrational frequencies of the surface oscillators.

The adsorption and surface reaction of SiCl 4 on Si(100)-(2 × 1)

The adsorption and surface reaction of SiCl 4 on Si(100)-(2 × 1) have been investigated in the temperature range of 100–1000 K. Adsorption of monolayer SiCl 4 and multilayer SiCl 4 are observed by temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and electron-stimulated desorption ion angular distribution (ESDIAD). Upon heating to ~ 200 K and above, Si-Cl bond scission in adsorbed SiCl 4 occurs, depositing SiCl x species. Heating to 673 K leaves the surface with only silicon monochloride species, SiCl(a), exhibiting a Si-Cl stretching mode, vSiCl, at ~ 560 cm −1, and a Cl + ESDIAD pattern indicative of inclined Si-Cl bond directions from Cl adsorbed on Si 2 dimer sites. Silicon substrate etching occurs above 800 K producing SiCl 2(g) as the desorption product.

Transformation of Cl bonding structures on Si(100)-(2 x 1).

The digital electron-stimulated-desorption ion angular distribution (ESDIAD) method has been used to observe the existence and the irreversible thermal transformation between different bonding structures for Cl on the Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surface. The ESDIAD pattern produced from dissociative chemisorption of ${\mathrm{Cl}}_{2}$ at 120 K shows intense normal emission of ${\mathrm{Cl}}^{+}$, plus lower intensity of ${\mathrm{Cl}}^{+}$ emission focused along the [011] and [011\ifmmode\bar\else\textasciimacron\fi{}] axes (the axes of ${\mathrm{Si}}_{2}$ dimer orientations in two domains). Upon annealing (T\ensuremath{\le}673 K), the ${\mathrm{Cl}}^{+}$ ESDIAD pattern is irreversibly transformed into a four-beam pattern with a twofold azi- muthal symmetry in the plane of each dimer axis, indicative of the formation of energetically stable Si-Cl bonds inclined 25\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}4\ifmmode^\circ\else\textdegree\fi{} from the surface normal.

Influence of coadsorbed potassium on the electron-stimulated desorption of F+, F-, and F* from PF3 on Ru(0001).

The electron-stimulated-desorption ion angular distributions (ESDIAD) of positive ions, negative ions, and metastable species from ${\mathrm{PF}}_{3}$ and K coadsorbed on a Ru(0001) surface are measured. In the absence of K, only positive- and negative-ion ESDIAD is observed, showing highly anisotropic, off-normal fluorine-ion emission (${\mathrm{F}}^{+}$, ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$) from ${\mathrm{PF}}_{3}$, which demonstrates that PF bonds are inclined away from the surface normal. In the presence of K, ESDIAD patterns of ${\mathrm{F}}^{+}$, ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$, and metastable ${\mathrm{F}}^{\mathrm{*}}$ display only normal emission, suggesting a reorientation or chemical reaction which results in fluorine bonding perpendicular to the surface. Thermal desorption spectrometry, Auger electron spectroscopy, and soft-x-ray photoemission (SXPS) are used to determine the surface chemistry as an aid in intepreting the ESDIAD results. The SXPS data show that in the presence of potassium, ${\mathrm{PF}}_{3}$ dissociates, possibly forming KF-like species, which suggests that an alkali-metal-induced chemical reaction is reponsible for the changes observed in ESDIAD. The yields of the various desorbed fluorine species vary strongly with potassium coverage; the results for ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{F}}^{\mathrm{*}}$ are interpreted in terms of a charge-transfer model.

Electron induced decomposition of Ni(CO) 4 adsorbed on Ag(111)

Nickel tetracarbonyl (Ni(CO) 4) adsorption on Ag(111) was studied with temperature-programmed desorption (TPD), digital electron-stimulated desorption ion angular distribution (ESDIAD) and Auger electron spectroscopy (AES). Ni(CO) 4 physisorbs molecularly at 90 K on Ag(111), with thermal desorption maxima observed near 170 and 150 K. Analysis of AES spectra indicates no Ni, C or O is present at the surface following TPD, showing that Ni(CO) 4 does not thermally decompose on Ag(111). Bombardment by 100–300 eV electrons induces positive ion and excited neutral particle desorption, and conversion of molecularly adsorbed Ni(CO) 4 into unidentified Ni x (CO) y surface species. CO thermally desorbs from these surface-bound species near 240, 300 and 400 K, leaving a Ni deposit on the Ag surface. This Ni diffuses below the surface above 450 K. ESDIAD patterns from molecularly adsorbed Ni(CO) 4 are composed of ESD fragments (CO +, O +, and neutrals) ejected normal to the surface, suggesting that Ni(CO) 4 on Ag(111) orients one CO ligand nearly perpendicular to the substrate. In contrast, Ni x (CO) y surface species do not produce directed ESDIAD beams, possibly reflecting a non-normal or random orientation of CO ligands in these adspecies. The total cross section for electron induced Ni(CO) 4 fragmentation is about 2 × 10 −16 cm 2, and is not strongly dependent on incident electron energies between 100 and 300 eV.

Carbon monoxide–oxygen interaction on the Pt(111) surface: An electron stimulated desorption ion angular distribution (ESDIAD) study

CO adsorption on the p(2×2)O–Pt(111) surface was studied by the digital ESDIAD (electron stimulated desorption ion angular distribution) method in combination with TPD, LEED and work function measurements. Three ESD products were detected: CO+, O+ and metastable CO. The ESDIAD patterns of each of these species were measured. The most significant difference in the ESD behavior of chemisorbed CO on the oxygen-covered surface from that of CO adsorbed on clean platinum surface was found at low CO coverages. This indicates that there is no preferential adsorption on the surface sites nonaffected by oxygen. A small tilting of CO was found.

CO adsorption on Pt(111) modified with sulfur

CO adsorption on clean and S-covered Pt(111) was studied using temperature programmed desorption (TPD), electron stimulated desorption ion angular distribution (ESDIAD), LEED, and work function measurements. Special attention was paid to comparing the CO adsorption rate, binding energy, and soft bending modes on a clean surface and on p(2×2) S/Pt(111) with S coverage =0.25 S/Pt. It was found that on p(2×2) 0.25 S/Pt(111), the CO adsorption rate is decreased by a factor of 2 and only one CO adsorption state with maximum coverage, θCO ≂0.25 CO/Pt is detected. On the basis of the ESD data the CO adsorption state on p(2×2) 0.25 S/Pt(111) is assigned to terminal-CO residing on the next nearest Pt atom and separated from S by 3.72 Å. When compared with the same CO configuration on clean Pt(111) in the limit of low θCO, the adsorption binding energy of the terminal CO on sulfided Pt(111) is decreased by 8 kcal/mol. For this same overlayer, the cross sections for production of all ESD products (CO+ , O+ , and metastable CO*) is increased by 30%–50%. This result is interpreted considering the possible S-induced perturbations of the CO–5σ/metal and metal/CO–2π* coupling. An important result in the present study is the observation of a substantial decrease of the polar angle of the ESDIAD patterns of all CO ESD products from the sulfided surface which indicates a decrease of the amplitude of the CO bending modes. This appears to be direct experimental evidence for S-induced stiffening of the soft CO-bending vibrations. Approximate estimations (on the basis of the measured polar angles of the ESDIAD patterns) showed that the frequency of the CO frustrated translational modes increases by about a factor of 2—from 48 cm−1 for CO/Pt(111) to ≃100 cm−1 for CO/p(2×2) 0.25 S/Pt(111).

The interaction of NF3 with Ru(0001): Order at steps

As part of a larger study of small fluorinated molecules on metal and semiconductor surfaces we investigated the adsorption of NF3 on Ru(0001) using measurements of electron-stimulated desorption ion angular distribution (ESDIAD), low-energy electron diffraction (LEED), thermal desorption spectroscopy, and Auger electron spectroscopy (AES). At 100 K, NF3 adsorbs with a high sticking probability. Some molecular NF3 desorbs on heating, with a peak maximum at 200 K. All molecular desorption is complete by 400 K, but AES shows that nitrogen and fluorine remain on the surface up to 1100 K. The LEED pattern of the majority of the surface remains (1×1) at all coverages and annealing conditions. At most locations on the Ru(0001) surface (planar regions), ESDIAD is dominated by F+ emission normal to the surface over the temperature range 100–1100 K. On stepped regions of the surface (near edges and defects) highly directional and intense hexagonal patterns of F+ emission are observed (200–1100 K). The off-normal beams desorb at 67°±3° away from the normal. It appears that most ESDIAD patterns in this system are derived from F or NFx (x<3) fragments.