Electron Stimulated Desorption Ion Research Articles

Supramolecular clusters and chains of 2,6-dimethylpyridine on Cu(110): Observation of dynamic configuration change with real-space surface science techniques and DFT calculations

The adsorption of 2,6-dimethylpyridine (2,6-DMP) on Cu(110) has been studied using low temperature scanning tunneling microscopy (LT-STM), time-of-flight electron stimulated desorption ion angular distribution (TOF-ESDIAD), and density functional theory (DFT) calculations. At low temperatures (T<~150K), the 2,6-DMP adsorbs in a flat configuration on Cu(110) producing clusters and extended domains via weak hydrogen bonding (C—H···N) with the molecular symmetry axis aligned along the <001> surface direction. At near-saturation coverage, a c(6×2) long-range ordered structure was observed. Upon annealing to T=200K, the 2,6-DMP molecules adopt an upright configuration with their pyridine ring plane oriented parallel to the <11̅0> azimuth. These upright 2,6-DMP molecules produce extended molecular chains where the repulsive interactions between the molecular chains give rise to coverage-dependent interchain distances. At near-saturation coverage, a 1260 surface structure is observed for the upright configuration. The DFT calculations suggest that the Cu adatom plays an important role in the adsorption configuration change of the 2,6-DMP molecule.

Electron transport and precipitation at Mercury during the MESSENGER flybys: Implications for electron-stimulated desorption

To examine electron transport, energization, and precipitation in Mercury's magnetosphere, a hybrid simulation study has been carried out that follows electron trajectories within the global magnetospheric electric and magnetic field configuration of Mercury. We report analysis for two solar-wind parameter conditions corresponding to the first two MESSENGER Mercury flybys on January 14, 2008, and October 6, 2008, which occurred for similar solar wind speed and density but contrasting interplanetary magnetic field (IMF) directions. During the first flyby the IMF had a northward component, while during the second flyby the IMF was southward. Electron trajectories are traced in the fields of global hybrid simulations for the two flybys. Some solar wind electrons follow complex trajectories at or near where dayside reconnection occurs and enter the magnetosphere at these locations. The entry locations depend on the IMF orientation (north or south). As the electrons move through the entry regions they can be energized as they execute non-adiabatic (demagnetized) motion. Some electrons become magnetically trapped and drift around the planet with energies on the order of 1–10 keV. The highest energy of electrons anywhere in the magnetosphere is about 25 keV, consistent with the absence of high-energy (>35 keV) electrons observed during either MESSENGER flyby. Once within the magnetosphere, a fraction of the electrons precipitates at the planetary surface with fluxes on the order of 10 9 cm −2 s −1 and with energies of hundreds of eV. This finding has important implications for the viability of electron-stimulated desorption (ESD) as a mechanism for contributing to the formation of the exosphere and heavy ion cloud around Mercury. From laboratory estimates of ESD ion yields, a calculated ion production rate due to ESD at Mercury is found to be on par with ion sputtering yields.

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.

Obsevation of Hydrogen Behaviours on the Surface using Hydrogen Microscopy

A principle and applications of “hydrogen microscope” have been described. An electron stimulated desorption (ESD) method is useful tool to detect hydrogen on the solid surfaces using a time-of-flight technology with slow electron pulsed-beam. Scanning ESD Ion microscope (SESDIM) is simply called the “hydrogen microscope” which highly sensitive to detect proton. After description of some characteristics and applications of the ESD, imaging method and examples of hydrogen 2-D maps are introduced as results of hydrogen storage alloys and ion implanted device.

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.

四極子形質量分析計のイオン源グリッド表面から放出された電子励起脱離イオン：脱離イオン種と圧力依存性

The pressure dependence of the electron stimulated desorption (ESD) ions were investigated by the quadrupole mass spectrometer with a Bessel-box type energy analyzer. The mass spectra of the ESD ions were measured under introduction of H2O, CO, O2 and CO2. The ESD ion peaks for those gasses were observed at O+ (m/z=16). The mass spectra of the gas phase ions were also measured and compared with those of ESD ions during the pumping down process. The origin of the main species in ESD ions, O+, was the adsorbed H2O on the grid surfaces.

Analysis of hydrogen distribution on Mg–Ni alloy surface by scanning electron-stimulated desorption ion microscope (SESDIM)

Hydrogen distribution and behavior on a Mg–Ni alloy surface are studied by using a time-of-flight electron-stimulated desorption (TOF-ESD) microscopy and a scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDX). The desorbed hydrogen ions are energy-discriminated and distinguished into two characters in the adsorbed states, which belong to Mg 2Ni grains and the other to oxygen-contaminated Mg phase at the grain boundaries. Adsorbed hydrogen is found to be stable up to 150 °C, but becomes thermally unstable around at 200 °C.

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.

Edge-On Bonding of Benzene Molecules in the Second Adsorbed Layer on Cu(110)

The bilayer of benzene on Cu(110) was studied with temperature-programmed desorption (TPD), time-of-flight electron stimulated desorption ion angular distribution (TOF-ESDIAD), and scanning tunneling microscopy (STM). TPD spectra show that three well-defined adsorption states exist. The alpha layer corresponds to the first layer containing flat-lying benzene molecules. As coverage increases, the beta layer forms on top of the alpha layer, and eventually, a multilayer, gamma, forms. TPD measurements show that the number of benzene molecules in the beta layer is equal to the number of benzene molecules in the alpha layer. ESDIAD measurements establish that the orientation of the benzene molecules in the beta layer is edge-on, with two C-H bonds directed toward the surface. STM images of the beta layer reveal closely spaced edge-on benzene molecules arranged in repeating hexagons, as well as loosely spaced benzene molecules with greater apparent height, which are also edge-on species. Correlation between the different measurements suggests a structural model for the benzene bilayer.

Fabrication of an electron-stimulated desorption apparatus and a study on desorbed hydrogen on the cold surface of Cu and rare-gas solid

An electron-stimulated desorption (ESD) system was assembled in order to study ESD ions desorbed from the cold surface of materials. We have investigated electronically stimulated ions desorbed from physisorbed hydrogen on Cu and rare-gas solid. The desorption yield showed a dependence on incident electron energy. Desorbed H+ and H+2 ions of exposed H2 on the surface of rare-gas solid were much higher than H2 exposed directly to the surface of Cu. The kinetic energy of desorbed H+ and H+2 ions depends on incident electron energy. When the incident electron energy was 200 eV, the kinetic energies of H+ and H+2 were 5.0044 eV and 0.8405 eV, respectively.

The electron-stimulated desorption ions from the ionizer of a quadrupole mass spectrometer

The species and the pressure dependence of the electron-stimulated desorption (ESD) ions from the grid surface of the ionizer of the quadrupole mass spectrometer were measured by the QMS with the Bessel-box-type energy analyzer (BB-QMS). The BB-QMS was developed in order to measure true mass spectra of gas phase without ESD ion effect in high to extreme high vacuum. The ESD ions observed in typical residual gasses such as water, carbon monooxide, oxygen and carbon dioxide were H + (H 2O), O + (H 2O, CO, O 2, CO 2), OH + (H 2O). The intensity of the ESD ion signals showed saturation upon increasing coverage of the water molecules on the grid surface.

Generation of extremely high vacuum by ordinary procedures with a SUS304 vacuum chamber and total and partial pressure measurements by hot cathode ionization gauges and a quadrupole mass spectrometer

The present study generated an extremely high vacuum of 10−10 Pa by very common methods such as using SUS304 chamber material, electropolishing surface treatment and turbomolecular pumping technology. We examined the gas phase and electron-stimulated desorption ions of the ion gauges, the effect of ambient temperature, total pressure and H2 partial pressure. One of the results showed that, as is well known, most of the residual gas in the extremely high vacuum chamber was H2 but there was also a little F, which is not so well known.

CO adsorption on the stepped Ag(2 2 1) surface

The adsorption and geometry of CO on the stepped Ag(2 2 1) surface, Ag(S)[4(1 1 1) × (1 1 1)], has been studied by temperature programmed desorption (TPD) and time-of-flight electron-stimulated desorption ion angular desorption (TOF-ESDIAD) methods. The adsorbed CO molecule exhibits two distinct desorption processes near 80 K (α 1) and near 90 K (α 2). At low coverage, only the α 2 desorption feature is observed while at higher coverage the α 1 feature can be seen as well. Upon electron bombardment, three desorption species can be identified by TOF-ESDIAD, O +, CO +, and CO* (electronically excited neutral CO species). The coverage-dependent CO* angular distribution pattern was studied by annealing the surface. At low coverage, a two-beam pattern aligned along the 〈 1 ¯ 1 0 〉 azimuth is obtained indicating that CO molecules tilt in both directions along the step edges. At higher coverage, an additional feature is observed, producing a three beam pattern, which contains contributions both from terrace and step sites. This suggests that CO preferentially adsorbs at the step sites of Ag(2 2 1) at low coverages, and that both terrace and step sites are occupied at higher coverages. It is also concluded that the CO molecules adsorb perpendicular to the terrace surface.

Axial-symmetric transmission gauge: Extension of its pressure measuring range and reduction of the electron stimulated desorption ion effect in ultrahigh vacuum

The basic characteristics of the axial-symmetric transmission gauge (AT gauge), that were modified for measurement over a wide pressure range, are reported. A Faraday cup ion collector is used instead of a secondary electron multiplier to extend the measurable pressure range to 10−3Pa. The sensitivity of the gauge was determined by a conductance modulation method, and by direct comparison against an extractor gauge and a spinning rotor gauge as well. When operating in O2, there was systematic disagreement between pressure readings with the AT gauge and the extractor gauge. This is due to the difference in the elimination efficiency of the electron stimulated desorption ion.

Adsorption and geometry of the chemisorbed benzoate species on Cu(1 1 0)

The adsorption of benzoic acid onto the Cu(1 1 0) surface has been studied by temperature programmed desorption (TPD) and by the time-of-flight electron stimulated desorption ion angular distribution (TOF-ESDIAD) technique. Condensed multilayers of undissociated benzoic acid form at high benzoic acid exposures at 81 K surface temperature, and desorption of the multilayer has been observed at 270 K. Adsorbed benzoic acid in the first layer decomposes to chemisorbed benzoate upon heating. The H + ion emission pattern observed upon electron stimulated desorption indicates that the benzoate species is bound by its carboxylate moiety to two close-packed Cu atoms, with the molecular plane parallel to the 〈 1 1 ¯ 0 〉 crystal azimuth. Three prominent H + beams are emitted from the 3-, 4-, and 5-positions on the phenyl ring, and these beam orientations indicate that the axis of the benzoate molecule is normal to the crystal face. The angle analysis shows that the initial angle of H + ion emission from the 3- and 5-positions in the phenyl ring is 64° with respect to the surface normal.

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.