Electron Stimulated Desorption Ion Angular Distribution Research Articles

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.

NH 3 adsorption and dissociation on a stepped Fe(s)(100) surface

The interaction of NH3 with a stepped Fe(100) surface has been studied using electron stimulated desorption ion angular distributions (ESDIAD), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). The goal has been to provide insights into the molecular structure of adsorbed NH3 and the influence of steps on NH3 orientation and dissociation. For low NH3 coverages, θ(NH3)≈0.02, both molecular and dissociative adsorption occurs at step sites. The adsorbed species are believed to include oriented NHx fragments as well as NH3 species which are inclined due to interaction between the NH3 dipole and the dipolar charge distribution at the steps. For higher coverages, up to saturation of the first monolayer (θ(NH3)≈0.34), ESDIAD is dominated by a “halo” H+ pattern indicating the NH3 is bonded via the N atom and the H atoms (with random azimuthal orientation) are pointed away from the surface. The ESDIAD data correlate well with TDS data, where different binding states due to fractional monolayer, second layer and multilayer adsorption are observed. Using Auger electron spectroscopy (AES), it is found that only a small amount of residual nitrogen, θ(N)

Adsorption of H2O on planar and stepped Si(100): Structural aspects

The adsorption of water on two silicon surfaces [reconstructed planar (100)2×1 and single domain stepped (100)2×1 cut 5° towards (011)] was studied using low-energy electron diffraction and digital imaging electron stimulated desorption ion angular distributions (ESDIAD) as a function of temperature (145–700 K) and coverage. Water has been shown previously to chemisorb dissociatively to form surface OH groups. At 300 K the H+ ESDIAD pattern for the planar surface is a nearly symmetric halo, indicating that OH is oriented with its bond vector inclined away from the surface normal, while at low temperatures (≤200 K) a four-lobed pattern that preserves substrate symmetry is observed. This reversible temperature dependence is related to librations and rotations of the OH complexes. ESDIAD from the stepped surface exhibits a two-lobed pattern, with enhanced emission towards the steps, consistent with bonding of OH to single-domain terrace sites. An interpretation is presented based on the dimer model of the Si(100) reconstruction in which the OH bond axis azimuths are oriented nearly perpendicular to the dimer azimuths.

The adsorption of water and oxygen on Ag(110): A study of the interactions among water molecules, hydroxyl groups, and oxygen atoms

The adsorption of water on clean and oxygen covered Ag(110) in ultrahigh vacuum was studied with thermal desorption spectroscopy (TDS), electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED), work function measurements (Δ), and Auger electron spectroscopy (AES). The ranges of experimental conditions were θ(O) between 0 and 0.5, θ(H2O) between 0 and 2, and temperature between 80 and 400 K. Water adsorbs on the clean surface at 80 K with no long-range order and little, if any, short-range order as determined by LEED and ESDIAD, respectively. Electron bombardment of water adlayers on clean Ag(110) results in dissociation to form OH groups at 80 K. Water also reacts with preadsorbed oxygen, either in atomic or molecular form, to form OH groups at 80 K. The O-H bond axis of the OH groups is tilted with respect to the surface normal and its thermodynamically stable, azimuthal orientation is along the [001] direction. In the absenoe of excess, molecular H2O, the OH groups exhibit no hydrogen bonding. The OH groups form (1×m) (m=2,3) LEED compression patterns upon annealing to 200 K. The γ adsorption state, which evolves water in a desorption peak at 225 K, was carefully studied. This state consists of coadsorbed H2O and OH in a 1:2 ratio. The results of this work are combined with earlier TDS and vibrational measurements for this system to propose an adsorption model for the γ state, in which an H2O molecule is hydrogen bonded to two OH groups to form an (HO)2-H2O surface complex.

Mechanisms and applications of electron stimulated desorption in surface science

Bombardment of surfaces by low energy (20–1000 eV) electrons causes electronic excitations that can result in the desorption of ionic and neutral atoms and molecules from the surface layer. The mechanisms of electron stimulated desorption of ions and neutrals will be discussed, and the uses and limitations of electron stimulated desorption ion angular distributions (ESDIAD) for determining local structure of surface molecules will also be described. ESDIAD is useful as a structural tool because the direction of ion desorption is determined by the orientation of the surface bond which is ruptured by the excitation. Examples to be discussed include the influence of surface ‘additive’ atoms (e.g. O, Br, K) on the local structure of NH 3, H 2O and CO adsorbed on transition metal surfaces.

The effect of potassium on the molecular orientation of CO chemisorbed on Ni(111): An esdiad study

Using electron stimulated desorption (ESD) and electron stimulated desorption ion angular distribution (ESDIAD) techniques, we have determined that coadsorbed potassium systematically quenches the O + ion yield from CO on the Ni(111) surface for 1000 eV electron excitation energies. The quenching appears to be a short range K-CO interaction; 3 or 4 CO molecules are affected for each K atom adsorbed on the surface. The quenching effect of K on CO indicates that a significant electronic perturbation of CO is caused by its local interaction with K. This effect prevents ESDIAD observation of the K-quenched CO species. In addition, the CO molecules that are not quenched at a potassium coverage of 0.02 K/Ni exhibit a normally oriented C-O bond similar to that found for CO adsorbed on a K-free Ni(111) surface.

Molecular orientation on metal surfaces by electrostatic interactions: The adsorption of cyclopentene on a stepped (221) silver surface

Step defects at metal surfaces are known to cause a local dipole at the surface, thereby creating an enhanced surface electric field in their vicinity. Using the electron stimulated desorption ion angular distribution (ESDIAD) technique, cyclopentene is observed to be ordered by the interaction of its permanent dipole moment with the electrostatic field at the steps on the Ag (221) surface. This ordering is not seen for cyclopentene on Ag (111). The experimental results agree with estimates of the interactional energy of the dipole with the electric field at the steps, and the direction of orientation is consistent with calculations of the sign of the cyclopentene dipole moment.

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.

Water adsorption on Pt(110)

Water adsorption on Pt(110) has been studied by electron stimulated desorption (ESD), electron stimulated desorption ion angular distribution (ESDIAD), thermal desorption spectroscopy (TDS), and work function (ΔΦ) measurements. Between 100 and 200 K water is molecularly adsorbed. The first fractions are adsorbed as small clusters before an ice layer starts to build up.

Oxygen chemisorption on Cr(110) I. Dissociative adsorption

The initial stages of oxygen chemisorption on Cr(110) at 300 K have been investigated using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Dissociative chemisorption occurs with near unit sticking probability, leading to an ordered p(4 × 2)O overlayer at θo ⋍18. A model with atomic oxygen located in the two-fold symmetric hollow sites is proposed. The previously reported c(3 × 1)O overlayer was not observed in this study. A disordered oxygen adlayer is found as θo increases. ESDIAD and HREELS data suggest that chemisorbed oxygen atoms occupy inequivalent local binding sites for θ > 0.25; subsurface oxygen is not found under these mild oxidation conditions.

Oxygen chemisorption on Cr(110): I. Dissociative adsorption

The initial stages of oxygen chemisorption on Cr(110) at 300 K have been investigated using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Dissociative chemisorption occurs with near unit sticking probability, leading to an ordered p(4 × 2)O overlayer at θ o ⋍ 1 8 . A model with atomic oxygen located in the two-fold symmetric hollow sites is proposed. The previously reported c(3 × 1)O overlayer was not observed in this study. A disordered oxygen adlayer is found as θ o increases. ESDIAD and HREELS data suggest that chemisorbed oxygen atoms occupy inequivalent local binding sites for θ > 0.25; subsurface oxygen is not found under these mild oxidation conditions.

An enhanced electron-stimulated-desorption ion angular distribution method for imaging molecular orientations of adsorbed species

A striking improvement has been made in the quality of positive ion angular distribution data obtained in electron-stimulated-desorption ion angular distribution (ESDIAD). The improved method has been used to study configurational changes in chemisorbed CO on Ni(110) which are induced by intermolecular interactions at high coverage. CO molecules adsorbed with normal C–O bond orientations on ridge Ni sites tilt away from the normal by ±19° in directions perpendicular to the ridges for CO coverages above 0.75 CO/Ni. Evidence for the C–O tilting has also been studied as a function of electron energy. This has demonstrated that an O(1s) excitation process leads to intense and sharp O+ ESDIAD beams. The improved ESDIAD method involves the removal of a soft x-ray background signal which originates from electron impact on the metal surface, and is therefore present in all photographic ESDIAD measurements made to date. The angular distribution of the background effect is independent of electron energy and current density. The resultant ‘‘pure ESDIAD’’ patterns show more quantitative details in high and low signal regions than has been seen before.

Electron stimulated desorption and its relation to molecular structure at surfaces

Recent examples of the use of electron stimulated desorption ion angular distribution (ESDIAD) in determining the structures of molecules on surfaces are discussed. These include the adsorption and decomposition of NH3 on Fe(100), and the stabilization of H2O on Ag(110) by impurity O and Br atoms. ESDIAD evidence for ‘‘lying down’’ CO on several surfaces is presented, and the influence of beam damage in producing new surface structures for H2O on Ni(110) is discussed. Calculations of the influence of the surface image potential and reneutralization effects on ion trajectories are described briefly.

Calculations of reneutralization effects in ESDIAD

Calculations are presented which describe the influence of ion reneutralization processes on measured electron stimulated desorption ion angular distributions (ESDIAD). The results indicate that reneutralization effects generally act in an opposite sense to the image field in affecting ion angular distributions, and that these counterbalancing effects tend to cancel one another partially over a wide range of polar angles.

Chemisorption and dissociation of O 2 on Cr(110): An EELS/ESDIAD study

Electron stimulated desorption ion angular distributions (ESDIAD) and high resolution electron energy loss spectroscopy (EELS) have been used in conjunction with Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) to identify molecular and atomic binding states for oxygen chemisorbed on Cr(110). Dissociative chemisorption occurs at and above 120 K, although below 200 K an additional minority molecular binding state is simultaneously populated. A six-fold symmetric ESDIAD pattern for the molecular state and an O 2 vibrational frequency of 1020 cm −1 are suggestive ofsuper-oxy O 2(ads) bonding to Cr(110) at six equivalent local sites.

CO chemisorption on Cr(110): Evidence for a precursor to dissociation

High resolution electron energy loss spectroscopy (EELS), electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) have been combined to study CO chemisorption on the Cr(110) surface. At 120 K, two molecular binding modes are sequentially populated. The first, α1CO, is a novel binding mode for a clean metal surface in which the molecules are bound with the C–O axis nearly parallel to the Cr(110) surface. These α1CO molecules exhibit a νCO of 1150–1330 cm−1, no detectable ion emission in ESDIAD, and form a c(4×2) α1CO overlayer at saturation (θCO≈0.25). A model is proposed in which the α1CO are π bonded to the Cr(110) surface at twofold symmetric surface hollow sites. Annealing experiments demonstrate that the α1 binding mode is the precursor to dissociation on Cr(110). At higher CO coverages, 0.25<θCO<0.35, the α1CO overlayer is disordered by the addition of the second binding mode, α2CO. Three new vibrations, at 495, 1865, and 1975 cm−1, are observed by EELS while O+ ion emission along the [110] direction is seen in ESDIAD. These α2CO results are indicative of terminally bound molecules adsorbed at atop and bridge sites and aligned approximately perpendicular to the surface. Annealing studies of CO and O/CO adlayers provide evidence for a surface reaction in addition to direct dissociation above 170 K. In contrast, adsorption at 300 K leads to direct dissociative chemisorption with no detectable intermediate molecular adsorbate. The implications of these results to catalytic reactions of CO are considered and comparisons to CO adsorption on clean and ‘‘promoted’’ transition metals are made.

Interactions of CO + K on Ru(001): Structure and bonding

Recent studies of CO + K on Ru(001) revealed an anomalously low CO streaching frequency of 1460 cm −1 for low CO and K coverages. Hoffmann and de Paola proposed a side-on bound molecule with the CO molecular axis parallel to the metal surface: Weimer and Umbach suggested that CO is bound perpendicular to the surface, as on clean Ru(001), but that the CO is sp 2 hybridized in the presence of K(ads). The main objectives of the present work were to search for configurational changes of adsorbed CO on K + Ru(001) and to compare these results with CO + O(ad). We used the ESDIAD (electron stimulated desorption ion angular distribution) method in combination with LEED (low energy electron diffraction) and TDS (thermal desorption spectroscopy). The ESDIAD data for low coverages ( θ K × 0.1) of CO + K on Ru(001) show that the O + ESD signal from CO is suppressed by coadsorption with K; this can arise from a reorientation of the CO molecular axis, from perpendicular to inclined, or from increased reneutralization rates due to coadsorption with K. TDS provides evidence for strong interaction between CO and K, and a combination of LEED and ESD measurements suggest the formation of ordered species having K x (CO) 2 x stoichiometry.

The structure of the surface hydration shell of bromide on Ag(110)

The interaction of water and bromine on Ag(110) has been studied in the temperature range 80–300 K, using ESDIAD (electron stimulated desorption ion angular distribution), LEED (low energy electron diffraction) and TDS (thermal desorption spectroscopy). Compared to the clean surface, water adjacent to surface bromide was characterized by a higher binding energy and pronounced orientational order. The mixed adlayers resulting from this surface hydration of bromide exhibited long-range order. The surface hydration number ( n H 2O / n Br) θ Br ≲ 0.25 and decreased to a few tenths at the bromine saturation coverage θ Br ⋍ 0.75 . Comparison is made to the results of a recent similar study on Cu(110) and electrochemical aspects are briefly considered.