Electron Stimulated Desorption Ion Angular Distribution Research Articles

IR Spectroscopic Measurement of Diffusion Kinetics of Chemisorbed Pyridine through TiO2Particles

The chemisorption and surface diffusion/desorption of pyridine from TiO2 powder (P25) have been measured as a function of temperature using transmission IR spectroscopy. Two classes of diffusion have been measured with activation energies of 36 kJ/mol (fast) and 90 kJ/mol (slow). By comparing density functional theory (DFT) calculations of bonding energies on TiO2 rutile (110) and anatase (101), the dominant crystal planes expected in P25 TiO2, it is found that fast diffusion would be expected on the rutile phase and that much slower diffusion would be expected on the anatase phase. In addition, the presence of oxygen defect sites will produce more strongly bound pyridine than either of the model crystal planes selected for investigation. To both types of TiO2 surfaces, pyridine bonding occurs to coordinatively unsaturated Ti cation sites through the N lone pair of the pyridine molecule. More than 85% of the molecules exhibit slow surface diffusion, and this is attributed to diffusion on the dominant anatase crystallites as well as to diffusion on defective sites. Diffusing molecules exhibit a ν19b ring-breathing mode, with the rapidly diffusing molecules exhibiting a mode frequency of ∼1438 cm−1 and the slowly diffusing species exhibiting a mode frequency of ∼1445 cm−1. Electron stimulated desorption ion angular distribution (ESDIAD) studies of the C−D bond directions for pyridine-d5 on the rutile TiO2(110)-1 × 1 surface show that the ring plane is rotated by 37 ± 1° with respect to the [001] azimuth on the crystal surface, in good agreement with the 39° rotational angle calculated by DFT. Several ring-breathing modes of pyridine are slightly broadened on rutile sites compared to anatase sites, and this may be due to more freedom for librational vibrations of the more weakly bound pyridine molecules.

Hydroxyl Chain Formation on the Cu(110) Surface: Watching Water Dissociation

The formation of hydroxyl chains from water dissociation on the Cu(110) surface has been studied by using a combination of scanning tunneling microscopy (STM), electron stimulated desorption ion angular distribution (ESDIAD), temperature programmed desorption (TPD), and density functional theory (DFT) calculations. Annealing the D2O-covered surface to a temperature of ∼200 K leads to desorption of D2O molecules and produces a zigzag structure due to adsorbed OD groups with a periodicity of 5 A along the direction in the STM image. Coadsorption of O2 promotes the water dissociation reaction and produces hydroxyl chains with much higher coverage. ESDIAD measurements show a two-beam pattern consistent with OD(a) species inclined ∼40° with respect to the surface normal and orientated along the azimuth. The calculations reveal the existence of stable chain structures comprised solely of hydroxyl groups as well as of interacting water and hydroxyl groups that are consistent with the observed STM image.

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

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

Electron-Stimulated Desorption Study of Oxygen Adsorbed on Ag(110). Observation of Inclined Physisorbed Species

Physisorbed molecular oxygen adsorbed on the Ag(110) surface was studied by temperature-programmed desorption (TPD) and time-of-flight electron-stimulated desorption ion angular distribution (TOF-ESDIAD). Oxygen molecules in the first physisorbed layer are tilted strongly from the surface normal and are oriented parallel to either the or the azimuthal crystallographic directions of the Ag(110) surface, based on measurements of the trajectory of O+ ions produced by electron impact. In the multilayer regime, oxygen molecules are mainly aligned parallel to the azimuth with an O−O bond direction close to the surface normal. ESD cross sections for both O+ ion formation and for O2 desorption have been measured for the physisorbed species.

電子衝撃脱離法によるＭｇＯ（００１）壁開面へのメタノール吸着の観察

Adsorption of methanol to the cleaved MgO(001)surface is studied using electron stimulated desorption (ESD) technique. CH2+, CO+ and CH3OH+ are identified as the desorbed ions originated to methanol. However, the yield of these ESD ions is few compared to OH+ in water adsorption for the sample just after cleavage in methanol. The yield of ESD ions from the adsorbed methanol is increased by immersion in methanol for 6 hours. By mixing water into methanol, the yield of ESD ions increases in the sample just after cleavage. From the results of electron stimulated desorption ion angular distribution (ESDIAD), it is found that the CH2+ and CO+ desorb from the step site and the CH3OH+ ions desorb from the step and terrace sites.

Structural analysis of Pt(1 1 1)c(√3 × 5)rect.–CO using photoelectron diffraction

Core level shift scanned-energy mode photoelectron diffraction using the two distinct components of the C 1s emission has been used to determine the structure of the Pt(1 1 1)c(√3 × 5)rect.–CO phase formed by 0.6 ML of adsorbed CO. The results confirm earlier assignments of these components to CO in atop and bridging sites, further confirm that the best structural model involves a 2:1 occupation ratio of these two sites, and provides quantitative structural parameter values. In particular the Pt–C chemisorption bondlengths for the atop and bridging sites are, respectively, 1.86 ± 0.02 Å and 2.02 ± 0.04 Å. These values are closely similar to those found in the 0.5 ML coverage c(4 × 2) phase, involving an atop:bridge occupation ratio of 1:1, obtained in earlier quantitative low energy electron diffraction studies. The results also indicate a clear tilt of the molecular axis of atop CO species in this compression phase, consistent with the finding of an earlier electron-stimulated desorption ion angular distribution investigation.

Dynamics of NF 3 in a condensed film on Au(1 1 1) as studied by electron-stimulated desorption

We report a low-temperature dynamics study of condensed layers of NF 3 on Au(1 1 1) by time-of-flight electron-stimulated desorption ion angular distribution (TOF-ESDIAD), temperature-programmed desorption (TPD) and low-temperature scanning tunneling microscopy (LT-STM). Upon adsorption at 30 K, molecular NF 3 adsorption occurs first at the step edges and at minor terrace defect sites with the formation of 2D islands. Within the islands, NF 3 is adsorbed in an upright conformation via the nitrogen lone pair electrons projecting fluorine atoms away from the surface as judged by the presence of only a sharp F + central beam in the ESDIAD pattern. At higher coverages, 3D islands start to populate the surface. Electron bombardment of a thick NF 3 (∼6 ML) layer adsorbed on the Au(1 1 1) surface leads to emission of F +, N +, NF +, NF 2 + and NF 3 + ions as observed in the TOF-ESD distribution. Upon heating to ∼37 K, a sudden decrease of the NF 2 + and NF 3 + ion yield, which is not related to thermal desorption, is observed which reflects the surface migration of NF 3 molecules, leading to local thinning of the film. The thinning process occurs at the temperature of onset of molecular rotations and self-diffusion in the bulk NF 3 crystal. In this process, some NF 3 molecules move closer to the surface which results in higher efficiency for ion neutralization by the underlying metal surface. In the TPD spectra, the monolayer desorption is observed to begin at ∼65 K, exhibiting zero-order kinetics with an activation energy of 21 kJ/mol.

Surface-aligned ion-molecule reaction on the surface of a molecular crystal CD3+ + CD3I --> C2D5+ + DI.

An ion-molecule reaction has been observed from a condensed molecular crystal of CD(3)I using the time-of-flight electron-stimulated desorption ion angular distribution technique. The CD(3)I multilayer is produced by growth on an ordered substrate. The reaction occurs between CD(3)(+) ions produced by electron-stimulated desorption and neighbor CD(3)I molecules in the topmost layer of the molecular crystal of CD(3)I, forming product C(2)D(5)(+) ions whose desorption dynamics have been measured. The normal momentum of the product ion is close to that of the reactant ion, suggesting that the reaction is dominated by a two-body collision, i.e., the momentum of the reactant CD(3)(+) ion governs the momentum of the product C(2)D(5)(+) ion. The ion-molecule reaction is of high cross section since the C(2)D(5)(+) yield is comparable to the CD(3)(+) yield. It is found that the yield and directionality of the emission of the C(2)D(5)(+) product ion is governed by the molecular order that is characteristic of the molecular crystal of CD(3)I. Destroying or modifying this order by using a spacer layer of H(2)O diminishes the C(2)D(5)(+) product ion yield relative to the reactant CD(3)(+) yield and broadens the ion emission directions.

Negative ion formation in electron-stimulated desorption of CF2Cl2 coadsorbed with polar NH3 on Ru(0001).

Photon-induced dissociation of CF2Cl2 (freon-12) in the stratosphere contributes substantially to atmospheric ozone depletion. We report recent results on dissociation and negative ion formation in electron-stimulated desorption (ESD) of CF2Cl2 on Ru(0001), when CF2Cl2 is coadsorbed with a polar molecule (NH3), for electron energies ranging from 50 to 300 eV. Two different time-of-flight methods are used in this investigation: (a) an ESD ion angular distribution detector with wide collection angle and (b) a quadrupole mass spectrometer with narrow collection angle and high mass resolution. Many negative ESD fragments are seen (F-, Cl-, FCl-, CF-, F2-, and Cl2-), whose intensities depend on the surface preparation. Using both detectors we observe a giant enhancement of Cl- and F- yields for ESD of CF2Cl2 coadsorbed with approximately 1 ML of NH3; this enhancement (>10(3) for Cl-) is specific to certain ions, and is attributed to an increased probability of dissociative electron attachment due to "trapped" low-energy secondary electrons, i.e., precursor states of the solvated electron in NH3. In further studies, the influence of polar NH3 spacer layers (1-10 ML) on ESD of top-layer CF2Cl2 is determined, and compared with thick films of condensed CF2Cl2. The magnitudes and energy dependences of the Cl- yields are different in these cases, due to several contributing factors.

Methanethiol Chemisorption on Cu(110): Chemical and Geometrical Issues Related to Self-Assembly

The adsorption of methanethiol on the clean Cu(110) and the sulfur-passivated Cu(110) surface has been studied with the time-of-flight electron-stimulated desorption ion angular distribution (TOF-ESDIAD) method, temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). Methanethiol on the clean Cu(110) surface thermally decomposes below 320 K, producing CH4(g) and C2H6(g), leaving the sulfur atom on the surface. A c(2 × 2) LEED pattern was observed at a low coverage of sulfur, and a c(8 × 2) structure developed from the c(2 × 2) phase at the saturation coverage of sulfur. Chemisorbed sulfur acts as a poison for further methanethiol decomposition on the surface. On the sulfur-passivated Cu(110) surface, methanethiol adsorbs dissociatively and produces a methanethiolate layer on the surface. The molecular orientation of CD3S on the sulfur-passivated Cu(110) surface was determined by assessing the C−D bond direction with respect to the surface normal and the crystal azimuths using the TOF-ESDIAD method. The C−S bond direction in the methanethiolate on the sulfur-passivated Cu(110) surface was oriented parallel to the 〈001〉 azimuthal plane. The C−S bond axis of methanethiolate is tilted 45° away from the surface normal.

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

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

OXIDATION OF Mg(0001) STUDIED BY LEED AND ESDIAD

The oxidation of Mg(0001) was studied using LEED (Low Energy Electron Diffraction) and ESDIAD (Electron Stimulated Desorption Ion Angular Distributions) with supplemental information from STM (Scanning Tunneling Microscopy). In agreement with previous work, two kinds of oxide were observed: a sub-oxide consisting of subsurface oxygen and ionic MgO. From 0 to 8 L (Langmuir=1×10-6 Torr Sec) oxygen exposure, the 1×1 LEED pattern obtained for clean Mg(0001) becomes diffuse and almost disappears, indicating that the initial stages of oxidation lead to a disordering of the surface. At higher oxygen exposure (~12 L) a diffuse 1×1 returns, showing that this new growth is in registry with the Mg substrate. The ESD O + ion yield, which is initially near zero for substrate oxygen, increases dramatically at this point, indicating that the oxide is now on the surface. The desorbing O + forms a normal emission pattern in the ESDIAD experiment at still higher oxygen exposures.

Molecular triangulation – finding the conformation of adsorbed self-assembled organic monolayers

We have developed a method that images two or more C–H bond directions within a complex adsorbed molecule on a single-crystal substrate. Since most surface-science methods are insensitive to H atoms in adsorbed molecules, this method offers unique capabilities for determining molecular conformations of relatively complex molecules at surfaces, permitting studies of structural changes over a wide range of coverage. This procedure, called molecular triangulation, is a new development based on the electron stimulated desorption ion angular distribution (ESDIAD) phenomenon used to measure individual bond directions in simple adsorbed molecules. The new ESD-molecular triangulation (ESD-MT) method provides incisive conformational information for large adsorbed organic species.

Real-time observation of the dehydrogenation processes of methanol on clean Ru(001) and Ru(001)-p(2×2)–O surfaces by a temperature-programmed electron-stimulated desorption ion angular distribution/time-of-flight system

Decomposition processes of methanol on clean and oxygen-precovered Ru(001) surfaces have been visualized in real time with a temperature-programmed (TP) electron-stimulated desorption ion angular distribution (ESDIAD)/time-of-flight (TOF) system. The mass of desorbed ions during temperature-programmed surface processes was identified by TOF measurements. In the case of methanol (CH 3OD) adsorption on Ru(001)-p(2×2)–O, a halo pattern of H + from the methyl group of methoxy species was observed at 100–200 K, followed by a broad pattern from the methyl group at 230–250 K and by a near-center pattern from O + ions originating from adsorbed CO above 300 K. The halo pattern is attributed to a perpendicular conformation of the CO bond axis of the methoxy species, leading to off-normal CH bond scission. On the other hand, methanol adsorbed on clean Ru(001) did not give any halo pattern but a broad pattern was observed along the surface normal, indicating that the conformation of the methoxy species is not ordered on the clean surface. Comparison between the ESDIAD images of the oxygen-precovered surface and the clean surface suggests that the precovered oxygen adatoms induce ordering of the methoxy species. Real-time ESDIAD measurements revealed that the oxygen atoms at the Ru(001)-p(2×2)–O surface have a positive effect on selective dehydrogenation of the methoxy species to CO+H 2 and a blocking effect on CO bond breaking of the methoxy species.

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

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

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

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

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

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

Temperature-programmed ESDIAD/TOF system as a new technique for characterization of adsorbed molecules and reaction intermediates

A temperature-programmed (TP-) electron stimulated desorption ion angular distribution (ESDIAD)/time-of-flight (TOF) system was developed to monitor changes in adsorbed layers in real time. This system can measure ESDIAD images and TOF spectra of desorbing ions stimulated by pulsed electron beam in temperature programmed surface processes. The coadsorption layers of CO and ammonia, the coadsorption layers of CO and methylamine, and the coadsorption layer of CO and acetylene on Ru(001), which were previously studied by our group using LEED, TPD and HREELS, have been observed by this system. The change in configuration of admolecules, the change in site occupation and the thermal evolution of adspecies can be monitored.

Development of a temperature-programed electron-stimulated desorption ion angular distribution/time-of-flight system for real-time observation of surface processes and its application to adsorbed layers on Ru(001)

A temperature-programed (TP) electron-stimulated desorption ion angular distribution (ESDIAD)/time-of-flight (TOF) system was developed in order to observe surface processes in real time by ESDIAD images and to measure TOF spectra of desorbing ions for identification of the mass and the kinetic-energy distribution of ions. The instrumentation of this system is described. This system was applied to (∛×∛)R30°-CO/Ru(001) (0.33 ML) and CO-saturated Ru(001) surfaces. As for the (∛×∛)R30°-CO/Ru(001), the increase of the half width at half maximum of the ESDIAD images upon annealing was found corresponding to the thermal excitation of the bending mode and/or hindered translation. On the other hand, as for the CO-saturated surface, the static disorder of the molecular axis of CO was larger, and apparent thermal excitation was not observed. After partial desorption of CO from the CO-saturated surface where the surface changes into the ∛×∛ structure at 400–430 K, the yield of O+ increased due to the change in the adsorption site of CO. TOF spectra for ammonia adlayers (NH3 and ND3) were also measured by the developed system and the isotopic ratios for ESD yields depending on the adsorption states (chemisorbed first layer and physisorbed second layer) were obtained.

Measuring dynamical behavior of admolecules — ESDIAD studies near the zero point

The Electron Stimulated Desorption Ion Angular Distribution (ESDIAD) method has been used to measure the dynamical behavior of admolecules on a Cu(110) surface. Two studies are reported: First a precursor NH 3 species to the chemisorbed NH 3 state was observed by adsorption at 32 K and the conversion to the chemisorbed state was witnessed at 55 K. ESDIAD was used to measure the configurational change, which occurs during the conversion. Second, anisotropic zero-point frustrated translational energy of chemisorbed CO was measured by time-of-flight ESDIAD.