Electron Stimulated Desorption Ion Angular Distribution Research Articles

NO adsorption and thermal behavior on Pd surfaces. A detailed comparative study

The adsorption and thermal behavior of NO on ‘flat’ Pd(111) and ‘stepped’ Pd(112) surfaces has been investigated by temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and electron stimulated desorption ion angular distribution (ESDIAD) techniques. NO is shown to molecularly adsorb on both Pd(111) and Pd(112) in the temperature range 100–373 K. NO thermally desorbs predominantly molecularly from Pd(111) near 500 K with an activation energy and pre-exponential factor of desorption which strongly depend on the initial NO surface coverage. In contrast, NO decomposes substantially on Pd(112) upon heating, with relatively large amounts of N 2 and N 2O desorbing near 500 K, in addition to NO. The fractional amount of NO dissociation on Pd(112) during heating is observed to be a strong function of the initial NO surface coverage. HREELS results indicate that the thermal dissociation of NO on both Pd(111) and Pd(112) occurs upon annealing to 490 K, forming surface-bound O on both surfaces. Evidence for the formation of sub-surface O via NO thermal dissociation is found only on Pd(112), and is verified by dissociative O 2 adsorption experiments. Both surface-bound O and sub-surface O dissolve into the Pd bulk upon annealing of both surfaces to 550 K. HREELS and ESDIAD data consistently indicate that NO preferentially adsorbs on the (111) terrace sites of Pd(112) at low coverages, filling the (001) step sites only at high coverage. This result was verified for adsorption temperatures in the range 100–373 K. In addition, the thermal dissociation of NO on Pd(112) is most prevalent at low coverages, where only terrace sites are occupied by NO. Thus, by direct comparison to NO Pd(111) , this study shows that the presence of steps on the Pd(112) surface enhances the thermal dissociation of NO, but that adsorption at the step sites is not the criterion for this decomposition.

Structure determinations at interfaces and oxide surfaces using ESDIAD

The technique of electron-stimulated desorption ion angular distribution (ESDIAD) has emerged in its own right over the last decade as a means of determining the geometry of surface, adsorbed species. The primciples underlying the operation and limits of the technique will be briefly discussed, and, in turn, (1) findings at interfaces of silicon with adsorbed water, and (2) recent work at oxide TiO 2 surfaces, both clean and with adsorbed gases, are outlined.

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.

Ge deposition on Pd(111): adsorption and decomposition of Ge 2H 6

The adsorption and decomposition of digermane (Ge 2H 6) on Pd(111) has been studied by Auger electron spectroscopy (AES), temperature-programmed desorption (TPD) and low energy electron diffraction (LEED) methods. Ge 2H 6 adsorbed on Pd(111) at 100 K thermally desorbs during TPD near 118 K from a weakly bound molecular state for Ge 2H 6 exposures near 1.0 × 10 15 cm −2 and higher. Below this exposure, the desorption of Ge 2H 6 is minimal, indicating that thermal dissociation of Ge 2H 6 occurs at the Pd(111) surface. AES results indicate the thermal dissociation of Ge 2H 6 on Pd(111) deposits Ge at the surface. However, no ordering of these Ge overlayers is observed by LEED. Electron bombardment of Ge 2H 6 adsorbed on Pd(111) at 100 K alters the desorption kinetics of Ge-containing species as reflected in TPD spectra. Electron bombardment leads to enhancement of Ge deposition at the surface as measured by AES. Nitric oxide (NO) adsorption has been used as a probe of the surface reactivity of Ge Pd(111) surfaces formed by thermal dissociation of Ge 2H 6 on clean Pd(111). TPD, AES and electron-stimulated desorption ion angular distribution (ESDIAD) measurements indicate that Ge pre-coverage inhibits the adsorption of NO on Pd(111) at 300 K.

Adsorption and desorption processes of Cl on a Si (111) 7 × 7 surface

The adsorption process and desorption mechanism of Cl on the Si(111) 7 × 7 surface were studied by means of LEED, AES, ESD (electron stimulated desorption) and ESDIAD (electron stimulated desorption ion angular distribution) methods. The saturation behavior in the adsorption process revealed by means of AES with a low electron flux was found to follow the Langmuir's adsorption equation. The LEED result suggested that the surface corrosion did not take place during the Cl exposure. In the ESD experiment, only Cl + was detected as the positive ion species. In the ESDIAD study, Cl + ions were predominantly observed in a cone of about 10° around the direction perpendicular to the surface. The dependence of the Cl + desorption yield on the incident electron energy showed two threshold energies (10–20 and 200 eV). These two thresholds correspond to two different excitation mechanisms for ion desorption. The higher (lower) threshold is ascribed to the Cl 2p (Cl3s) excitation. However, we could not detect any noticeable difference in the kinetic energy distribution of the desorbed ions through the whole excitation energy range. Hence, it is revealed that these two excitations result in the same final state for desorption as suggested by the KF model.

Long- and short-range stabilization of water by fluorine on Ag(110)

Water and fluorine were co-adsorbed on Ag(110) in an effort to model the metal/electrolyte interface and to compare the hydration of adsorbed fluorine with previous studies of hydroxyl and chlorine hydration. Thermal desorption of coadsorbed water reveals a series of fluorine-stabilized states with peak temperatures above that for the clean surface. At low water coverage water desorbs in a broad peak (B) at 260 K, representing strong hydration of fluorine, greater than that of Cl or OH. ESDIAD (electron stimulated desorption, ion angular distribution) measurements showed emission from this adsorption state along the [11̄0] direction. With increasing water coverage, a (1 × 2) low-energy electron diffraction pattern and ESDIAD emission along [001] coincide with a new water desorption peak (A 1) at 216–230 K. Two more thermal desorption peaks, A 2 at 190–200 K and A 3 at 165–175 K, develop with additional water coverage. The solvation number N s, representing the average number of stabilized water molecules per fluorine atom, ranges from 13 in the limit of low fluorine coverage to 2 at high fluorine coverage. For solvation numbers greater than 6, the fluorine/water interactions must extend beyond nearest neighbors forming either a second hydration shell or some form of ordered water layer. As fluorine coverage increases, the form of stabilization shifts from a long-range to a more direct, short-range interaction. The surface solvation of fluorine is compared with that of OH and Cl on Ag(110), as well as with the gas phase, single-ion hydration of F −, OH − and Cl −.

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.

Halogen surface chemistry on Si(100)−(2×1)

The surface chemistry of adsorbed halogen atoms on Si(100) has been studied using several surface-science methods. It has been found using electron-stimulated desorption ion angular distribution (ESDIAD) that Cl atoms bond to dangling bonds on symmetric Si 2 dimer sites, and that the Si-Cl bond angle is tilted (25°±4°) from the normal in the vertical plane containing the symmetric Si 2 dimer bond. The covalently bonded halogens Cl, Br, and I have been studied on Si(100) using atomic hydrogen bombardment at low substrate temperatures (300–630 K). In all cases, facile elimination of the hydrogen halide occurs, and the coverage of halogen may be driven to zero by moderate exposure to atomic hydrogen. The halogen extraction process is almost non-activated, suggesting that the chemical reaction to produce hydrogen-halide species is driven by the potential energy carried by the atomic hydrogen species. This is an example of an Eley-Rideal reaction process and provides a potentially useful new approach for controlling atomic layer chemistry on semiconductors.

Thermal disorder in adsorbed Cl on Si(100)

Thermal broadening of Cl+ electron stimulated desorption ion angular distribution (ESDIAD) patterns, related to the vibrational motion of the adsorbate, has been investigated by subtraction of patterns measured at different temperatures. In the case of Cl on Si(100), the observed difference pattern between 130 and 305 K indicates that azimuthally isotropic motion of Cl occurs about the most probable Si–Cl bond direction. Equal distributions from the in-plane and out-of-plane bending motions are observed, correlating with the ∼200 cm−1 frequencies associated with these motions.

ESDIAD studies of fluorine and chlorine adsorption at Si(100)

Abstract Electron stimulated desorption ion angular distribution (ESDIAD) patterns are reported for fluorine and chlorine adsorbed at the silicon (100) surface. Analytical and numerical methods of establishing the degree of angular compression imposed by the sample bias frequently employed in ESDIAD measurements, are described. Measurements of the two halogen adsorbates contrast strongly, particularly with regard to their response to annealing in the temperature range 300 to 1000 K. In the case of F + desorption, a minimum in ion intensity occurs along the surface normal. Maxima in the ion production are observed at a polar angle of ∼ 30°, and in registry with the principal crystal axes as reported by other workers. Cl + ion angular distributions however, indicate a mixture of normal and off-normal emission for adsorption at ≤ 300 K. This pattern transforms, irreversibly, to a single maximum directed along the surface normal in response to annealing to ∼ 650 K. The transformation is linked to the thermal depopulation of one adsorption state, leaving a higher binding energy state intact.

ESDIAD studies of fluorine and chlorine adsorption at Si(100)

Electron stimulated desorption ion angular distribution (ESDIAD) patterns are reported for fluorine and chlorine adsorbed at the silicon (100) surface. Analytical and numerical methods of establishing the degree of angular compression imposed by the sample bias frequently employed in ESDIAD measurements, are described. Measurements of the two halogen adsorbates contrast strongly, particularly with regard to their response to annealing in the temperature range 300 to 1000 K. In the case of F + desorption, a minimum in ion intensity occurs along the surface normal. Maxima in the ion production are observed at a polar angle of ∼ 30°, and in registry with the principal crystal axes as reported by other workers. Cl + ion angular distributions however, indicate a mixture of normal and off-normal emission for adsorption at ≤ 300 K. This pattern transforms, irreversibly, to a single maximum directed along the surface normal in response to annealing to ∼ 650 K. The transformation is linked to the thermal depopulation of one adsorption state, leaving a higher binding energy state intact.

Comparison of Cl 2 and HCl adsorption on Si(100)−(2 × 1)

The chemisorption and reaction of Cl 2 and HCl on Si(100)−(2 × 1) have been studied using Auger electron spectroscopy, high resolution electron energy loss spectroscopy (HREELS), digital electron-stimulated desorption ion angular distribution (ESDIAD), and temperature programmed desorption. At 100 K, both Cl 2 and HCl dissociatively chemisorb on the dangling bonds of Si(100). At saturation coverage, following an exposure of the Si(100) surface to Cl 2, the surface concentration of Cl is about one Cl atom per Si atom while HCl can produce a Cl coverage only about one-fourth of that achieved with Cl 2. This is the first report of a self-site-blocking effect in adsorption on a semiconductor surface. HREELS spectra indicate that both Cl 2 and HCl adsorption at 100 K give a monochloride surface species with an Si-Cl stretching frequency of about 550–600 cm −1. Digital ESDIAD measurements reveal that the Si-Cl bond angle for the monochloride from both Cl 2 and HCl adsorption is oriented on the vertical plane containing the Si-Si dimer bond and is inclined from the surface normal by 25° ± 4°. The etching products of Si(100) by Cl 2 at elevated temperatures are SiCl 2 at about 800 K and a small amount of SiCl 4 at about 500 K. For HCl, the only observed etching product is SiCl 2.

Quantum-resolved angular distributions of neutral products in electron-stimulated processes: desorption of CO ∗ from Pt(111)

The first quantum-resolved angular distribution measurements of electronically-excited neutral molecules which have undergone electron stimulated desorption (ESD) are presented. Two-dimensional imaging of laser resonance-enhanced multiphoton ionization (REMPI) is used to obtain angular distributions of CO ∗ in the v=0 vibrational level of the metastable a 3Π r state desorbed from CO/Pt(111) by 350 eV electrons. For saturation CO coverages at 90 K, sharp Gaussian distributions peaked about the surface normal (6° ± 0.5° half-width at half maximum) are observed, consistent with previously reported data acquired by ESDIAD (ESD ion angular distributions). The (1 + 1) photon REMPI scheme for state-specific CO ∗ detection involves the b 3Σ( v = 0) ← a 3Π( v = 0) transition at 283 nm, followed by photoionization at the same wavelength. In this paper, the overall experimental technique for REMPI imaging of products from electron stimulated processes is discussed. Thus specific CO ∗ data as a function of coverage and temperature is presented for comparison with the ESDIAD results.

Quantum-resolved angular distributions of neutral products in electron-stimulated processes: NO desorption from and NO 2 dissociation on Pt(111)

We present the first quantum-resolved angular distributions of ground-state neutral molecules which are products of electron stimulated desorption (ESD) and electron stimulated dissociation. Laser resonance-enhanced multiphoton ionization (REMPI) and two-dimensional imaging have been used to obtain angular distributions of NO desorbed by 350 eV electrons from O-precovered Pt(111). In a similar fashion, we have measured angular distributions for the NO product of NO 2 dissociation on clean and O-precovered Pt(111). In all cases, we observe narrow widths which are roughly the same as ion distributions determined by ESDIAD (ESD ion angular distributions). The angular distribution for NO ESD is sharply peaked (7° half-width at half maximum) along the surface normal for an O coverage (θ o) of 0.25 monolayer (ML). The angular distribution of the NO product from dissociation of side-bonded NO 2 on clean Pt(111) is unexpectedly peaked about the surface normal, and thus does not reflect dissociative forces parallel to the surface or the ∼ 25° off-normal ground-state bond direction. On O-precovered Pt(111), where NO 2 is N-bonded, ∼ 6° off-normal beams are observed. When the substrate is precovered with θ o > 0.5 ML, local disorder creates asymmetric site geometries which result in multiple peaked angular distributions with both normal and off-normal (∼9–10°) components; similar effects for NO ESD are observed. In all these studies, the NO angular distributions are invariant to rotational or vibrational state. This implies that the lateral translational degrees of freedom are essentially de-coupled from the internal modes of the molecule. The results are discussed in terms of desorption mechanisms, dissociative forces, site geometries, and disordered coadsorbate layers.

Width of particle beams desorbed in electron stimulated desorption: O+ and metastable CO from CO/Pt(111)

The role of adsorbate vibrations parallel to the surface on the width of particle beams escaping from the surface in electron stimulated desorption (ESD) has been studied. Chemisorbed CO on Pt(111) was employed as a model system. The angular distribution of O+ and of the metastable a 3π–CO (CO*) species desorbed in ESD was measured. A cluster calculation was used to model the CO vibration on Pt(111), from which it was possible to calculate the CO* and O+ ESDIAD (electron stimulated desorption-ion angular distribution) beam widths. For CO* we were able to calculate the width of the ESDIAD beams for θCO=0.15 ML with an accuracy better than the error bar of the measurement in the temperature range of 100–300 K. This calculation shows that the CO* angular distribution is related to both the bond direction and the parallel momentum of the surface oscillator at the time of excitation. For θCO=0.5 ML, the width of the measured and calculated values are in good agreement in the temperature range of 100–230 K; above 230 K the experimental results show a positive deviation from the calculated values, indicating that an extra broadening mechanism occurs above ∼230 K, probably because of the onset of an order–disorder transition. For the O+ ESDIAD beam widths the experimental results show a somewhat sharper distribution than the calculated values, most likely because of the angularly anisotropic neutralization rate that was not quantitatively considered in the calculation.

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.

Structural phenomena related to associative and dissociative adsorption of water on Ni(110)

The adsorption of water on Ni(110) has been studied by thermal desorption spectroscopy (TDS), work function (ΔΦ), Fourier transform infrared reflection–absorption spectroscopy (FTIR-RAS), low energy electron diffraction (LEED), and electron-stimulated desorption ion angular distribution (ESDIAD). The major findings of this study are the following: (1) Water molecules in the chemisorbed c(2×2) half-monolayer do not cluster and the plane of the molecules is highly inclined to the surface normal; (2) no ESDIAD evidence of oriented water dimers is observed at 130 K and no FTIR activity is observed following adsorption at 80 K until multilayers are populated; (3) water has been measured to partially dissociate at a minimum temperature of 205±2 K; (4)the binding energy of water to the Ni(110) surface is increased by H-bonding to adsorbed hydroxyls (produced by the partial dissociation) in a linear OH(ad)/H2O complex with planar symmetry in the [001] direction. This bonding results in a higher temperature desorption state (A1).

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.

Oxidation of CO by oxygen on a stepped platinum surface: Identification of the reaction site

The coadsorption of oxygen and carbon monoxide on the stepped Pt(112) surface has been studied using electron stimulated desorption–ion angular distribution (ESDIAD), temperature programmed desorption (TPD), and low energy electron diffraction (LEED). It has been possible to preferentially adsorb different isotopic CO molecules on step and terrace sites, respectively, following oxygen adsorption on step sites to partial coverage. Transient kinetic experiments show that below ∼200 K, isotopic CO present exclusively on terrace sites is more effectively involved in CO2 production, compared to less reactive CO on the step sites. Above ∼200 K, site exchange between step and terrace CO species prevents the measurement of the relative reactivity of the two kinds of chemisorbed CO. The results show that the elementary step producing CO2 from adsorbed CO and adsorbed oxygen is structure sensitive, even though the overall catalytic reaction between CO and O2 is generally classed as a structure insensitive reaction.

One-dimensional CO island formation in the coadsorption of H 2 and CO on the steps of Pt(112)

The interaction of coadsorbed CO and H 2 on the steps of Pt(112) [Pt(S)[3(111)×(001)]] was examined by digital electron-stimulated desorption ion angular distribution (ESDIAD), low-energy electron diffraction (LEED) and temperature-programmed desorption (TPD). On the clean surface, CO and H 2 both adsorb preferentially on step sites at low coverages; CO at stop sites on the step edge and H 2, as atoms, in the step 4-fold sites. For pure CO adsorption, an ordered (2× n) structure of step CO is observed at 0.19 ML (atop step sites half filled), and at least two CO structural transformations occur as the step sites fill further. These structural transformations, due to COCO repulsions, involve the production of tilted CO species whose tilt directions are related to the local CO coverage in one-dimensional chains of CO on the step edges. The structural transformations described above for pure CO layers on the step edges can also be produced by coadsorption of hydrogen on the (2× n) step CO structure. The CO structural transformations in the coadsorbed system are driven by H adsorption, due to the formation of immiscible one-dimensional CO and H islands at the steps. Similar results are observed for low coverages of CO adsorbed on the H atom saturated surface.