Electron Stimulated Desorption Ion Angular Distribution Pattern Research Articles

Chemisorption of HF on Silicon Surfaces

ABSTRACTWe have investigated the interaction of gaseous hydrofluoric acid (HF) with single crystal silicon surfaces using soft x-ray photoemission spectroscopy and Electron Stimulated Desorption Ion Angular Distributions (ESDIAD). Examination of the Si(2p) core level for surfaces saturated with HF shows the formation of silicon-fluoride bonds indicating the dissociative chemisorption of HF on both Si(111) and Si(100) surfaces. Inspection of the F(2s) and F(2p) valence levels at saturation coverage indicate that only one-half monolayer of fluorine bonds to the silicon. The primary ion desorbed by electron bombardment of these surfaces is F+ with only a minor contribution from H+. ESDIAD images from a saturation coverage of HF on stepped Si(100) surfaces reveal F+ desorption primarily along the direction of the terrace dimers. The ESDIAD patterns from HF adsorbed on Si(111) are characterized by strong normal F+ emission with a weak background component of off-normal emission. These results are consistent with the dissociative chemisorption of HF where the ion emission direction is determined by the Si-F bond directions.

Adsorption of H 2O on clean and oxygen-preposed Ni(110)

The adsorption of H 2O on both clean and modified Ni(110) surfaces has been studied using a variety of methods: electron stimulated desorption ion angular distribution (ESDIAD), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). Fractional monolayers, θ( H 2 O) < 0.5, of H 2O on clean Ni(110) are associated with a four-spot ESDIAD pattern suggesting that the H-ligands are in specific registry with the substrate. We postulate the formation of H 2O dimers bound via oxygen lone pair orbitals to Ni substrate atoms and oriented with the O … HO axis in [001] azimuthal directions. For θ( H 2 O) > 0.5–1 larger hydrogen bonded clusters with long range c(2 × 2) symmetry are formed. Upon heating to ⩾ 200 K, a fraction of the H 2O dissociates, forming OH(ad). TDS of H 2O from clean Ni(110) reveals four binding states having peak temperatures of 155, 210, 245 and 370 K. They are related to multilayer desorption (155 K), desorption from larger bilayer clusters (210 K), desorption from H 2O dimer clusters which might be stabilized by OH (245 K), and recombination of OH to yield H 2O(g) (360 K). Dissociation of H 2O is promoted by surface oxygen. For the adsorption of H 2O on oxygen-dosed Ni(110) at θ( O) > 0.08, a mixture of molecular and dissociative adsortion occurs immediately at 80 K, producing inclined OH. Isotropic exchange of H 2 16O with 18O(ad) is observed even for binding states in which dissociation is believed not to to occur and is related to a proton exchange involving H 2O(ad) hydrogen bonded to O(ad).

The direct observation of hindered rotation of a chemisorbed molecule: PF3 on Ni(111)

By using the bond direction imaging capabilities of the electron stimulated desorption ion angular distribution (ESDIAD) technique, we have observed a thermally induced azimuthal disorder effect due to the thermal population of unbound hindered rotor states in chemisorbed PF3 on Ni(111). The six beam F+ ESDIAD patterns observed for PF3 on Ni(111) are interpreted as evidence for a weak barrier that hinders the rotation of isolated PF3 molecules on the Ni(111) surface. At a coverage of ∼0.04 PF3/Ni, a barrier to rotation of ∼80±20 cm−1 is estimated from the observed temperature dependence of the F+ ESDIAD patterns using a two-dimensional hindered rotor model. The PF3 coverage dependence of the thermally induced azimuthal disorder effect indicates that intermolecular forces are responsible for an increase in the hindering potential at high PF3 coverages.

Oxygen chemisorption on Cr(110): II. Evidence for molecular O 2(ads)

Oxygen chemisorption and dissociation on Cr(110) at 120 K have been studied using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Dissociative adsorption dominates although vibrational and stimulated desorption data provide evidence for a coexisting minority molecular binding state. An O 2(ads) vibrational frequency of 1020 cm −1 and a six beam ESDIAD pattern are suggestive of super-oxo O 2(ads) bonding at six local sites each with the O-O molecular axis tilted away from the surface normal. These results are compared with data for chemisorbed oxygen on other transition metal surfaces.

Oxygen chemisorption on Cr(110) II. Evidence for molecular O 2(ads)

Abstract Oxygen chemisorption and dissociation on Cr(110) at 120 K have been studied using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Dissociative adsorption dominates although vibrational and stimulated desorption data provide evidence for a coexisting minority molecular binding state. An O2(ads) vibrational frequency of 1020 cm−1 and a six beam ESDIAD pattern are suggestive of super-oxo O2(ads) bonding at six local sites each with the O-O molecular axis tilted away from the surface normal. These results are compared with data for chemisorbed oxygen on other transition metal surfaces.

Coadsorption of water and lithium on the Ru(001) surface

The interactions between water and lithium have been studied on the surface of a Ru(001) crystal using thermal desorption spectroscopy, electron stimulated desorption ion angular distributions (ESDIAD), Auger spectroscopy and LEED. The presence of Li was found to influence strongly the H + ESD yield and the ESDIAD patterns from adsorbed water even at Li coverages of 0.02 monolayer; changes in the thermal desorption states for water were also observed at low Li coverages. For coadsorbed Li coverages above 0.05, ESDIAD measurements provided clear evidence of water decomposition, even at surface temperatures near 80 K; evidence for dissociation was also obtained from thermal desorption and Auger measurements. The present results are compared and contrasted to those reported previously for the H 2O/Na/Ru(001) system.

A simple model for the lowest excited states of α-CO/W (100): application to electron stimulated desorption: J Rubio et al,Vacuum, 32, (12). 1982, 719–722.

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 Nix(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, Nix(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 cm2, and is not strongly dependent on incident electron energies between 100 and 300 eV.

Laser-generated electron emission from surfaces: Effect of the pulse shape on temperature and transient phenomena: Jui-teng Lin and Thomas F George,J Appl Phys,54 (1), 1983, 382–387

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 Nix(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, Nix(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 cm2, and is not strongly dependent on incident electron energies between 100 and 300 eV.

Adsorption and orientation of NH 3 on Ru(001)

The interaction of NH 3 with clean Ru(001) surfaces has been studied using LEED (low energy electron diffraction), ESDIAD (electron stimulated desorption ion angular distribution), TDS (thermal desorption spectroscopy), and work function changes (Δφ). Four different binding states (denoted as α 1, α 2, β and γ) were detected with TDS. At low coverages, NH 3 desorbs from the α 1 state with a TDS peak maximum at ~ 310 K. The broadening of the TDS peaks and their shift to lower temperature with increasing NH 3 coverage are related to repulsive lateral interactions between neighboring NH 3 molecules. At higher NH 3 coverages (θ NH 3≳ 0.15), a second desorption peak (α 2) develops at T = 180 K, accompanied by a (2 × 2) LEED structure. With further increase of NH 3 exposure a sharp desorption peak (β state) is found at T = 140 K, and is interpreted as due to NH 3 species desorbing from a second adsorption layer. Finally a desorption peak due to multilayer adsorption (γ state) is found at 115 K. At low NH 3 coverages (α 1 state), a “halo”-like H + ESDIAD pattern gives evidence of randomly oriented or freely rotating NH 3, monomers, bounded via the N atoms to the surface with the H atoms pointing away from the surface. This orientation of NH 3 is supported by work function measurements showing a linear decrease of Δφ in the α 1 state. Structural information concerning the adsorption geometry of NH 3 in the β state has been obtained from LEED and ESDIAD. During the formation of the second NH 3 layer (β) a (2√3 × 2√3)R30° LEED pattern is observed and is accompanied by an ESDIAD pattern with a hexagonal outline. A structural model of the β-state bonding, in which second layer NH 3 molecules are bonded via threefold hydrogen bonds to the first layer NH 3, is proposed.

Adsorption of H 2O on Al(111)

The adsorption of H 2O on Al(111) has been studied by ESDIAD (electron stimulated desorption ion angular distributions), LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and thermal desorption in the temperature range 80–700 K. At 80 K, H 2O is adsorbed predominantly in molecular form, and the ESDIAD patterns indicate that bonding occurs through the O atom, with the molecular axis tilted away from the surface normal. Some of the H 2O adsorbed at 80 K on clean Al(111) can be desorbed in molecular form, but a considerable fraction dissociates upon heating into OH ads and hydrogen, which leaves the surface as H 2. Following adsorption of H 2O onto oxygen-precovered Al(111), additional OH ads is formed upon heating (perhaps via a hydrogen abstraction reaction), and H 2 desorbs at temperatures considerably higher than that seen for H 2O on clean Al(111). The general behavior of H 2O adsorption on clean and oxygen-precovered Al(111) (θ O ≲ monolayer) is rather similar at low temperature, but much higher reactivity for dissociative adsorption of H 2O to form OH adsis noted on the oxygen-dosed surface around room temperature.

Adsorption of H 2O on Al(111)

The adsorption of H 2O on Al(111) has been studied by ESDIAD (electron stimulated desorption ion angular distributions), LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and thermal desorption in the temperature range 80–700 K. At 80 K, H 2O is adsorbed predominantly in molecular form, and the ESDIAD patterns indicate that bonding occurs through the O atom, with the molecular axis tilted away from the surface normal. Some of the H 2O adsorbed at 80 K on clean Al(111) can be desorbed in molecular form, but a considerable fraction dissociates upon heating into OH ads and hydrogen, which leaves the surface as H 2. Following adsorption of H 2O onto oxygen-precovered Al(111), additional OH ads is formed upon heating (perhaps via a hydrogen abstraction reaction), and H 2 desorbs at temperatures considerably higher than that seen for H 2O on clean Al(111). The general behavior of H 2O adsorption on clean and oxygen-precovered Al(111) ( θ o⪅monolayer) is rather similar at low temperature, but much higher reactivity for dissociative adsorption of H 2O to form OH ads is noted on the oxygen-dosed surface around room temperature.

The adsorption of water on clean and oxygen-dosed Ru(011)

Water adsorption on clean and oxygen-dosed Ru(001) has been examined using thermal desorption spectroscopy (TDS), AES, LEED, and electron stimulated desorption ion angular distributions (ESDIAD). On the clean Ru(001) surface, three water TDS peaks were observed at 215 K (A 1), 180 K (A 2) and 155 K (C). The structure of the water on the surface was determined by ESDIAD and LEED as a function of coverage and temperature. The (√3 × √3)R30° LEED pattern indicated that water is in registry with the Ru substrate which is responsible for the close match of the ice and Ru lattice spacings. ESDIAD patterns were able to determine the presence of water monomers at low coverages and temperatures and to distinguish between water structures on the surface as a function of temperature. Both the A 1 and A 2 TDS peaks were found to be due to the desorption of water molecules from two-dimensional clusters. The energy separation of the two peaks was related to the dipole-dipole interactions from water bound into the clusters in two different orientations. A complex temperature- and coverage-dependent LEED structure was also observed which was attributed to an ordered domain structure of the water clusters. Preadsorption of oxygen on Ru(001) inhibited the azimuthal ordering of the adsorbed water as determined by ESDIAD and LEED for all water coverages and oxygen coverages. Oxygen pre-exposures also had a strong influence on the TDS peaks. The strong interaction of water with adsorbed oxygen induced an unfavorable water-substrate geometry.

The adsorption of H 2O on Ni(111); influence of preadsorbed oxygen on azimuthal ordering

ESDIAD (electron stimulated desorption ion angular distribution), LEED and TPD (temperature programmed thermal desorption) have been used to study the adsorption of H 2O on Ni(111), both clean and with preadsorbed oxygen. On the clean surface, a fractional H 2O monolayer adsorbed at 80 K exhibits no preferred azimuthal orientation for the H atoms; the local bonding configurations of H 2O have a nearly random distribution. Only one state is seen in TPD for a fractional H 2O monolayer ( T p ∼ 170K) and a weak ( 3 × 3 ) LEED pattern appears after saturation of the first H 2O layer. When a fractional monolayer of H 2O is coadsorbed with a low-coverage annealed oxygen layer ( θ O ≲0.05), a sharp, intense three-fold ESDIAD pattern of H + ions appears, suggesting that there are three configurations of H 2O having H atoms oriented azimuthally in [ 11 2] directions. We postulate that H 2O is bonded to the surface via the oxygen lone-pair orbitals, and that H 2O interacts with a neighboring O atom via a hydrogen bond. This leads to local azimuthal ordering in the absence of long-range order. Upon heating to 120 K, an H-abstraction reaction leads to the formation of OH(ads). Further heating leads to the formation of H 2O (which desorbs) and O(ads).

The structure of CO on Ni(111)

Electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and temperature programmed thermal desorption (TPD) have been used to study the adsorption of CO on Ni(111) in the temperature range 80−300 K. For low coverages, the CO layer is disordered; a c(4×2) pattern appears at coverages ϑ∼0.5, the maximum coverage at 300 K. At temperatures 220–240 K, a well-ordered (√7/2×√7/2)R 19° LEED pattern forms at saturation (ϑ∼0.57). At 80 K, the CO saturation layer is characterized by a ’’complex’’ LEED pattern. Only one binding state is seen in TPD for ϑ≲0.40 (peak temperatures 450–430 K); species having lower desorption temperatures populate at higher coverage. At 300 K adsorption the only ESD ion observed is O+, with desorption centered about the direction perpendicular to the surface. The O+ ion yield shows a maximum at intermediate coverages. CO+ ions are also observed at adsorption temperature &amp;lt; 300 K at higher coverages. The ESDIAD patterns for saturation coverage in the range 80–260 K indicate off-normal ion emission in addition to the normal component. The data suggest that for ϑ≲ 0.5, CO is adsorbed in multiply coordinated sites with the molecular axis perpendicular to the surface. At temperatures &amp;lt; 300 K, a fraction of CO can adsorb in singly coordinated sites as well. Upon heating the CO layer formed at 80 K (ϑ?0.57) the CO+ ion yield maximizes at ∼240 K when the (√7/2×√7/2)R 19° LEED pattern is well defined. The CO+ yield therefore reflects the ordering behavior in the adsorbate layer at temperatures below the onset of desorption and indicates a shift of a fraction of CO molecules from multiply coordinated to singly coordinated sites. Finally, a model is proposed involving resonance charge exchange between adsorbed CO molecules to account for the observed variations of the O+ yield with surface coverage.

The structure of NH3 on Ni(111)

In a recent study of the adsorption of NH3 on Ni(111) at T∠190 K using angle‐resolved UPS, it was concluded that NH3 is molecularly adsorbed, and is bonded to the surface via the N atom with the H atoms oriented away from the surface. To study the bonding configuration using a direct and independent technique, we have examined NH3 on Ni(111) using the electron stimulated desorption ion angular distribution (ESDIAD) method, coupled with temperature programmed desorption (TPD) and low‐energy electron diffraction (LEED). For NH3 coverages achievable at T≳150 K, the ESDIAD patterns are dominated by a ’’halo’’ of ion emission with little ion yield normal to the surface; the halo pattern is consistent with molecular NH3 bonded to Ni via the N atom. Whereas angle‐resolved UPS data indicate a specific azimuthal registry of NH3 with Ni(111), a well‐defined azimuthal orientation is not evident from the ESDIAD results. Possible reasons for the differences between these results are examined, including final state eff...

The role of steps and defects in electron stimulated desorption: Oxygen on stepped W(110) surfaces

In order to examine the role of atomic steps and defects on electron stimulated desorption (ESD) phenomena, we have studied the adsorption of oxygen on a polyhedral tungsten crystal containing a W(110) flat and 4 flats having orientations 6° and 10° off the (110) plane with rows of steps parallel to the [100] and [110] directions. Upon adsorption at 300 K, there is little or no ESD O + emission from the oxygen-covered (110) plane. In contrast, the stepped surfaces yield intense O + emission normal to the terraces and in “downstep” directions, as seen using electron stimulated desorption ion angular distributions (ESDIAD). The presence of atomic steps has a major influence on sticking probability, saturation oxygen coverage and the ESDIAD patterns at all adsorption temperatures in the range 100 K to ⪆1400 K. Adsorption at low coordination sites appears to be a key factor in producing high ESD ion yields.

The adsorption of cycloparaffins on Ru(001) as studied by temperature programmed desorption and electron stimulated desorption

The adsorption of C 2H 6, and the cycloparaffins C 3H 6, C 6H 12, and C 8H 16 on Ru(001) at 80 K has been studied using LEED, temperature programmed desorption, and ESDIAD (Electron Stimulated Desorption Ion Angular Distributions). An aim of these studies has been to examine the relationship between ESDIAD ion desorption angles and bond angles in weakly adsorbed species having known internal structure. Fractional monolayers of C 2H 6 and C 3H 6 both yield ESDIAD patterns due to H + ions desorbed in wide cones centered on the surface normal, consistent with adsorption into mobile, disordered layers. In contrast, a fractional monolayer of C 6H 12 yields a hexagonal H + ESDIAD pattern. These results indicate the azimuthal orientation of the CH bonds and are consistent with a simple model of C 6H 12 adsorption. In thermal desorption studies, the weakly adsorbed layer in contact with the substrate is easily distinguished from condensed multilayers. Mass analysis of ESD ions reveals that the only ESD ion product is H + for hydrocarbon coverages <1 monolayer. For multilayers of adsorbed C 6H 12 and C 8H 16, the ESD ion products have a mass distribution similar to the gas phase mass spectrometer cracking pattern. For all these fragments, the ESD cross sections are ≳ × 10 −17 cm 2.

Ion Angular Distributions in Electron Stimulated Desorption

Ions liberated from an adsorbed layer by electron stimulated desorption are shown to have strongly focused and symmetric angular distributions. We have examined the Electron Stimulated Desorption Ion Angular Distributions (ESDIAD) for O+ ions desorbed from oxygen adsorbed on W(100); the ESDIAD patterns were displayed on a phosphor screen and photographed. ESDIAD patterns have been examined as a function of oxygen coverage and surface heat treatment; in most cases, patterns of 4-fold symmetry were observed. Dependent on oxygen coverage and sample temperature, the patterns were oriented with the 4-fold axes either parallel to or at 45° to the rows of surface atoms in the W(100) plane, as determined by LEED in the same apparatus. In addition, systematic variations in the appearance of the patterns (sizes and intensities of 4-fold “lobes”) were seen for heat treatments at T\\lesssim1000 K and may be related to the formation of surface oxides. We propose that the details of symmetry and intensity observed in ESDIAD patterns may provide new insights into the determination of adsorption sites and the nature of the bonding of atoms and molecules to surfaces.