Electron Stimulated Desorption Ion Research Articles

Electron stimulated desorption of oxygen atoms and ions from an oxidized AgZr alloy surface

Electron stimulated desorption (ESD) is a complex process in which the ion and neutral yields are functions of many variables. In order to examine some of these dependencies which have not been considered previously, a study of the composition and ESD behavior of a Ag 0.5 wt% Zr (Ag0.5Zr) alloy membrane has been conducted. Zr was added to the Ag primarily to increase the O concentration at the surface and thereby enhance the ESD signal. The surface was sputter cleaned, saturated with O 2, sputter cleaned again and then annealed at 250°C until no further changes occurred in the surface composition according to ion scattering spectroscopy (ISS) and Auger electron spectroscopy (AES). ISS data suggest that the resulting surface consists of islands of Zr oxide partially covering the Ag surface. Neutrals emitted in ESD are quite difficult to detect so a technique was developed to determine the O neutral-to-ion ratio of 1.1 × 10 7 for a primary beam energy of 1 keV. By monitoring the surface O concentration as a function of time with AES, a total ESD cross section of 3.9 × 10 −19 cm 2 was determined. The ESD O + energy distribution spectrum was obtained and compared with that obtained from O-charged Ag. Also, the effect of primary beam incidence angle on the ESD yield and ion energy distribution was examined and found to be significant.

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.

Ion spectroscopy gauge: Total pressure measurements down to 10−12 Pa with discrimination against electron-stimulated-desorption ions

The hot-cathode magnetron gauge by Lafferty (which presently holds the possibility of making the lowest-pressure measurements, 10−15 Pa) and the improved Helmer gauge by Benvenuti and Hauer (which carried out the lowest-pressure measurements to date, 2.5×10−12 Pa) have been examined regarding the measurement limit due to electron-stimulated-desorption (ESD) ions. On the basis of this examination, a new hot-cathode total pressure gauge, called an ion spectroscopy gauge, which substantially avoids errors caused by ESD ions down to 10−12 Pa, has been developed. Discrimination against ESD ions is accomplished by combining three technical elements: (i) a spherical grid ion source; (ii) a spherical 180° ion energy analyzer; and (iii) burying the ion source in an aluminum alloy flange. The ion source plays an important role in the generation of a large kinetic energy difference between gas-phase ions and ESD ions, by using electron space charge. The 180° analyzer, which has a high energy resolution, promotes ion focusing and tends to reduce the x-ray limit. The burying technique reduces the outgassing rate of the hot-cathode gauge. Performance measurements of the new gauge have been carried out in a new aluminum extreme high vacuum system. The effective separation of gas-phase ions from ESD ions for two variants of the gauge, one with a molybdenum grid and one with a platinum grid, was demonstrated using oxygen dosing. The performance of the platinum-grid gauge was confirmed down to 2×10−11 Pa, essentially free from errors caused by ESD ions. The x-ray limits and sensitivities of the new gauges were: below 2×10−12 Pa with a sensitivity of 4.5×10−4 A/Pa for the molybdenum grid and 5.6×10−12 Pa with a sensitivity of 1.8×10−4 A/Pa for the platinum grid.

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).

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.

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A new hot-cathode ionization pressure gauge, called an ion spectroscopy gauge, which avoids errors due to electron-stimulated desorption (ESD) ions down to 10-12 Pa has been developed. The elimination of ESD ion errors is accomplished by combining three technical elements, a spherical grid ion-source, a spherical 180° energy analyzer, and reduction of desorption from a hot-cathode gauge. The X-ray limit of the new gauge is below 1.3×10-12 Pa with a sensitivity of 3×10-4 (A/Pa). Performance measurements of the new gauge have been carried out in a new aluminum XHV system and confirmed down to 2×10-11 Pa.

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.

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.

Background effects in electron stimulated desorption ion angular distribution (ESDIAD) measurements on Si(111)-(7×7)

The background effect in electron stimulated desorption ion angular distribution (ESDIAD) measurements due to soft x-ray production on Si(111)-(7×7) is investigated. We find that the background intensity from a Si(111)-(7×7) surface varies linearly with incident electron beam energy and current density. It is also found that the elimination of the background effect (by subtraction) plays a crucial role in both quantitative and qualitative interpretations of digital ESDIAD measurements on silicon, as well as to similar measurements on other surfaces.

Coadsorption of water and chlorine on Ag(110): Evidence for adsorbate-induced hydrophilicity

Coadsorption of H2O and Cl was studied on a Ag(110) surface under conditions of ultrahigh vacuum with thermal desorption spectroscopy, low energy electron diffraction, and electron stimulated desorption ion angular distribution. The experiments were conducted over the temperature range of 100–650 K for water coverages ranging from zero to several multilayers and chlorine coverages θCl of 0–0.75 monolayers (ML). Water adsorption is stabilized by chlorine; the thermal desorption peak for water interacting with chlorine, called the α2 state, shifts to higher temperature by 25–40 K from the α1 state for desorption from the clean surface. A c(2×2) bilayer for H2O forms for coadsorption with less than 0.25 ML of Cl. The surface solvation number (SSN), defined as the number of stabilized water molecules per chlorine atom, varies from 13 to about 4 as θCl increases from 0 to 0.25. The unusually large SSN and the c(2×2) structure is evidence that Cl(a) promotes water adsorption to the metal surface itself in an effect called adsorbate-induced hydrophilicity. Coadsorption with higher chlorine coverages produces a p(4×3) structure for 0.25<θCl<0.4 and a c(4×4) structure for 0.4<θCl<0.5. Chlorine interacts directly with water in these structures in the form of surface solvation seen in previous studies of water coadsorption. The p(4×3) and c(4×4) patterns are evidence that coadsorbed water alters the distribution of chlorine on the surface. These results are interpreted in terms of the balance of forces among the two adsorbed species and the surface.

Water adsorption on Pt(110)(1 × 2)

Water adsorption on Pt(110)(1 × 2) has been studied at 100 K by thermal desorption spectroscopy, ultraviolet electron spectroscopy, photoemission of adsorbed xenon and electron stimulated desorption ion angular distribution. At this temperature water is molecularly adsorbed as small clusters with a peculiar orientation from the ridges. When oxygen is preadsorbed at 300 K, water is partially dissociated giving hydroxyl radicals which recombine at high temperature.

Adsorption and reaction of perchloric acid with clean and water-covered silver(110)

Adsorption of gaseous HClO{sub 4} on Ag(110) was studied under ultrahigh vacuum to investigate the surface chemistry of electrolytes. Surface reactions and adsorbate structure were studied over the temperature range of 100-900 K and coverage range of zero to several multilayers with thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), and electron-stimulated desorption ion angular distribution (ESDIAD). HClO{sub 4} adsorbed molecularly at 100 K with a sticking coefficient of {approx} 10{sup {minus}1} or less. The sticking coefficient increased to unity with increasing coverage due to hydrogen-bonding interactions with concurrently adsorbed H{sub 2}O. Once adsorbed, HClO{sub 4} deprotonated below 150 K to produce adsorbed perchlorate (ClO{sub 4}). Saturation coverage of ClO{sub 4} was 0.5 monolayer for which a c(2{times}2) LEED pattern was observed. Adsorbed perchlorate decomposed above 450 K to give adsorbed chlorine and gaseous oxygen in a Cl:O atomic ratio of 1:4.

Direct observation of adsorbate dynamics from low-frequency vibration on a step defect—CO on Pt(112)

The relative shape of the hindered translation potential energy well for CO chemisorbed on the step sites of a Pt(112) single crystal was determined with digital electron stimulated desorption–ion angular distribution (ESDIAD). The angular displacement of the CO molecule in its hindered translation well predominately determines the half-width at half-maximum (HWHM) of the neutral a3π CO species (CO*) produced by ESD, a species which may be imaged in an ESDIAD apparatus. Variations in the CO* ESDIAD HWHM as a function of temperature are due to population of higher levels of the hindered translational mode. By monitoring the CO* ESDIAD HWHM for CO coverages on the step below 50% step saturation (0.17 monolayer) as a function of temperature, we show that the steepness of the hindered translation potential well is different for CO vibrations up, down, and along the step edge, following the trend: down the steps>up the steps>along the steps. There is no coverage dependence in the CO* ESDIAD HWHM values up or down the steps, but the HWHM values along the steps at 0.17 monolayer CO are significantly broader than those from lower coverages in the temperature range of 150–350 K. We interpret this to indicate the production of transient structures of tilted CO on adjacent step sites formed from preferential diffusion of CO molecules laterally on the step sites. Thus, the experiments probe directly the anisotropy of lateral diffusion of the adsorbate on step sites.

Water adsorption on clean and caesium covered Cu{110}

The interaction of water on clean and caesium dosed Cu{110} has been investigated as a model for the simulation of electrochemical double layers in ultrahigh vacuum. We present new data from high resolution electron energy loss spectroscopy (HREELS) and electron stimulated desorption ion angular distributions which provide a detailed picture of the growth and structure of water on clean Cu{110}. Water adsorbs molecularly on this surface and forms hydrogen bonded clusters at all coverages. The presence of caesium has a strong influence on the adsorption of water. For ΘCs=0.06 water is molecularly adsorbed with an increase in the heat of adsorption. The strongly bound water is associated with an increase in the work function and with striking changes in the HREELS spectra: the libration mode at 750 cm−1, which dominates the clean surface water spectrum, is absent in the presence of caesium. These effects are attributed to a reorientation of water in the vicinity of the caesium adatom.

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.

Metal/electrolyte surface chemistry: The adsorption of nitric acid and water on Ag(110)

The adsorption of HNO 3/H 2O mixtures on Ag(110) was investigated to learn more about the chemistry of the metal/electrolyte interface. The experiments were performed in ultrahigh vacuum (UHV) using thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED), and electron stimulated desorption ion angular distribution (ESDIAD) over temperatures of 80–650 K and coverages of 0–10 monolayers (ML). As this is the first known study of HNO 3 in UHV, the mass spectrometer cracking pattern for HNO 3 is here reported. HNO 3 adsorbs irreversibly on the clean surface at 80 K and loses its acidic proton to form an adsorbed surface nitrate (NO 3) below 150 K. The saturation amount of adsorbed NO 3 is 0.4 ± 0.1 ML for which adsorption occurs in either a normal or split c(2 × 2) structure. N03 is stable on the surface up to 450 K beyond which it decomposes directly to gaseous NO 2 and NO and adsorbed atomic oxygen. NO 3 decomposition is first order with an activation energy E a = 151±4 kJ mol −1 and a pre-exponential factor of A = 10 15.4±0.4s −1. NO 3 stabilizes adsorbed H 2O by about 8 kJ mol −1 and is hydrated by as many as three H 2O molecules. Multilayers of HNO 3/H 2O desorb at 150–220 K and show evidence of extensive hydrogen bonding and hydration interactions. No evidence for HNO 3-induced corrosion or other surface damage was detected in any of these experiments.

Esdiad beam widths

In order to improve the understanding of the mechanisms leading to narrow ion beams in electronically stimulated desorption (ESDIAD: electron stimulated desorption ion angular distribution), the influences of surface temperature, coverage, primary energy of exciting electrons, azimuth, and isotope mass on the polar angle distributions of ions (O +, CO +) desorbed by electron impact from CO adsorbed on Ru(001) and Ni(110) have been studied. The results suggest that only part (one half to two thirds) of the observed widths is caused by thermal effects. Some conclusions about the general mechanisms of focused beam formation can also be drawn.

Recent developments in angle-resolved electron-stimulated desorption of ions from surfaces

The current status of research in angular distributions of ions desorbed from surfaces resulting from electron beam impact is reviewed. Topics discussed include theory, instrumentation, and selected examples. The present treatment is intended to augment previous reviews, emphasizing some recent developments, and should prove useful to both “seasoned” researchers and newcomers to the field of ESDIAD, electron-stimulated desorption ion angular distributions. Theoretical work discussed includes work on electronically excited states, image potential interactions of the desorbing species with the surface, and reneutralization effects. A complete review of instrumentation used since the first measurements in the early 1970's is presented including state-of-the-art methods which allow for detailed angular measurements of the mass and kinetic energy of desorbing ions. Selected examples are described which demostrate the utility of ESDIAD to determine local structure of adsorbate/surface complexes, to investigate the dynamical behavior of adsorbed species, and to examine highly insulating surfaces.

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).