Electron-stimulated Desorption Research Articles

Reaction Kinetics of Water Molecules with Oxygen Vacancies on Rutile TiO2(110)

The formation of bridging hydroxyls (OHb) via reactions of water molecules with oxygen vacancies (VO) on reduced TiO2(110) surfaces is studied using polarized infrared reflection–absorption spectroscopy (IRAS), electron-stimulated desorption (ESD), and photon-stimulated desorption (PSD). Narrow IR peaks at 2737 and 3711 cm−1 are observed for the stretching vibrations of ODb and OHb, respectively. The IRAS spectra indicate that the bridging hydroxyls are oriented normal to the TiO2(110) surface. Using IRAS, we have studied the kinetics of water reacting with the vacancies by monitoring the formation of bridging hydroxyls as a function of the annealing temperature on the TiO2(110). Separate experiments have also monitored the loss of water molecules (using water ESD) and vacancies (using the CO photooxidation reaction) due to the reactions of water molecules with the vacancies. All three techniques show that the reaction rate becomes appreciable for T > 150 K and that the reactions are largely complete for ...

Low-energy Electrons Interactions with Chemisorbed and Physisorbed Films of L-cysteine/Au(111)

SynopsisWe investigate electron attachment to chemisorbed and physisorbed L-cysteine/Au(111) films and its subsequent dissociation via electron stimulated desorption (ESD) measurements; More specifically, dissociative electron attachment (DEA) which in essentially, molecular dissociation into a neutral and anionic fragment following initial electron capture into short-lived molecular anion or resonance leads to the rupture of specific bonds, if the incident electron energies are within characteristic ranges (below 20 eV).

Electron induced degradation of condensed Fe(CO)5 studied by electron stimulated desorption

SynopsisWe have measured the electron stimulated desorption (ESD) yields of anions and cations from thin films of Fe(CO)5 deposited onto both Xe or Pt surfaces. Measurements were made as functions of incident electron energy, incident current, and the thickness of both the Fe(CO)5 and Xe layers. The desorption of C- and O- occur exclusively via dipolar dissociation (DD), while the desorption of Fe(CO)x- (x = 0-4) proceeds via DD and dissociative electron attachment (DEA). Threshold energies for the desorption of cationic fragments C+, O+, CO+, and Fe(CO)+x (x = 0-4) were measured. Only CO+ appears to be produced by DD as opposed to dissociative ionization.

Electron-Induced Surface Reactions of η3-Allyl Ruthenium Tricarbonyl Bromide [(η3-C3H5)Ru(CO)3Br]: Contrasting the Behavior of Different Ligands

Toward the goal of better understanding the elementary steps involved in the electron beam-induced deposition (EBID) of organometallic precursors, the present study is aimed at understanding the sequence of electron-stimulated reactions of surface-bound η3-allyl ruthenium tricarbonyl bromide [(η3-C3H5)Ru(CO)3Br], an organometallic complex with three different ligands: carbonyl (CO), halide (Br), and η3-allyl (η3-C3H5). X-ray photoelectron spectroscopy and mass spectrometry were used in situ to probe the effects of 500 eV electrons on nanometer scale films of [(η3-C3H5)Ru(CO)3Br]. Initially, electron irradiation decomposes the precursor, reducing the central Ru atom and causing the ejection of CO ligands into the gas phase. Experimental evidence points to the inability of electron irradiation to remove the carbon atoms of the η3-allyl (η3-C3H5) ligand from the resulting EBID deposits. Although the Br atoms are not labile in the initial molecular decomposition step, they are removed from the film after exposure to higher electron doses as a result of a slower, electron-stimulated desorption process. Comparative studies with [(η3-C3H5)Ru(CO)3Cl] reveal that the identity of the halogen does not influence the elementary reaction steps involved in the decomposition process. Collectively, results from these studies suggest that sufficiently volatile organometallic precursors with a small number of carbonyl and halide ligands could be used to generate deposits in EBID with significantly higher metal concentrations (and correspondingly lower levels of organic contamination) compared to existing EBID precursors.

Reactivity pattern of bromonucleosides induced by 2-hydroxypropyl radicals: photochemical, radiation chemical, and computational studies.

The bromonucleosides (BrdX's) 5-bromo-2'-deoxyuridine (BrdU), 5-bromo-2'-deoxycytidine (BrdC), 8-bromo-2'-deoxyadenosine (BrdA), and 5-bromo-2'-deoxyguanosine (BrdG) may substitute for ordinary nucleosides in DNA. As indicated by electron-stimulated desorption experiments, such a modified biopolymer is greater than 2-3-fold more sensitive to damage induced by excess electrons. The other major product of water radiolysis, the (•)OH radical, may form a number of other radicals in chemical reactions with the complex content of the cell. Thus, the well-proved BrdU-labeled DNA radiosensitivity may be, at least in part, related to secondary organic radicals. Therefore, in the current study, the propensity of BrdX's to damage induced by 2-hydroxypropyl radical (OHisop(•))-a prototype radical species-was investigated. The HPLC and LC-MS analyses revealed the formation of two major products from the brominated pyrimidine nucleosides, a native nucleoside and an adduct of BrdX and OHisop(•) , and only an adduct of BrdX from the bromopurine nucleosides. Quantum chemical calculations ascribed this evident difference between purines and pyrimidines to the electron transfer from OHisop(•) to BrdX that is especially favorable in pyrimidines.

Role of Low-Energy Electrons (<35 eV) in the Degradation of Fe(CO)5 for Focused Electron Beam Induced Deposition Applications: Study by Electron Stimulated Desorption of Negative and Positive Ions

Fe(CO)5 is one of the most common precursors used in focused electron beam induced deposition (FEBID). Since high-energy electrons interacting with matter produce large amounts of secondary electrons with energies <50 eV, those generated within the substrate in FEBID are expected to play a major role in the dissociation process of the precursor molecules. The aim of this study is to identify the role of the secondary electrons in the deposition process of Fe(CO)5 and the relevant dissociation mechanisms, using an electron stimulated desorption system with 4–33 eV electrons. The desorption of charged fragments from thin films of Fe(CO)5, condensed on Xe or onto a Pt foil, was measured as a function of incident electron energy, incident current, and thickness of both Fe(CO)5 and Xe films. Both dissociative electron attachment (DEA) and dipolar dissociation (DD) are involved in the production of anions, specifically, C– and O–, and Fe(CO)x– (x = 0–4). Cations C+, O+, CO+, and Fe(CO)x+ (x = 0–4) were detected with desorption thresholds in the ∼15–25 eV range. These fragments are produced via direct dissociative processes. This study reveals the detailed dissociation mechanisms of Fe(CO)5 induced by low-energy electron impact by the direct observation of Fe(CO)x fragments under different conditions, thus confirming the frequently proposed mechanisms of deposition of Fe by FEBID.

Electron-Induced Decomposition of Condensed Acetone Studied by Quantifying Desorption and Retention of Volatile Products

The electron-induced decomposition of thin condensed layers of acetone was studied by a combination of electron-stimulated desorption (ESD) experiments and thermal desorption spectrometry (TDS) monitoring both the decay of acetone and the formation of volatile products. A reaction mechanism for the decay is proposed and its validity verified by modeling the TDS and ESD data on the basis of a set of resulting elementary rate equations. The results show that ESD data as a function of electron exposure also give indirect evidence of the presence of nonvolatile species. In particular, formation and desorption of CO is delayed because part of the carbonyl groups become incorporated in larger nonvolatile compounds. These, in turn, decompose upon further electron exposure to release CO. ESD curves obtained below the desorption temperature of the small volatile products CO and CH4 have also been reproduced well assuming that acetone layers have a limited capacity for uptake of volatile species and become saturated with these products during decomposition. Finally, we show that the quantitative evaluation of ESD data obtained above and below the desorption temperature of small volatile products in principle offers an approach to a quantitative comparison of product amounts desorbing in ESD and postirradiation TDS.

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Fragmentation and Ion Desorption from Condensed Pyrimidine by Electron Impact: Implications for Cometary and Interstellar Heterocyclic Chemistry

The desorption process induced by photons, electrons, or ions onto icy surfaces is an important mechanism for leading neutral or ionized molecular species to the gas phase in interstellar and circumstellar environments. In this work, we report the results of high energy electron impact onto a condensed pyrimidine ice by means of electron-stimulated ion desorption (ESID). Desorbed cations from the icy surface were analyzed by time-of-flight mass spectrometry (TOF-MS). The most abundant desorbed ions are H+ and H2+, although several other fragments, such as C2H2+, HCN+, C3H3+, CHN2+, C2H2N2H+, and C4H4N2H+, have also been identified. The fragmentation pattern of pyrimidine was found to be characterized by six distinct regions on its desorption spectra, containing ionic fragments bearing one to six members of the original pyrimidine ring. Five-membered fragment ions were observed for the first time in electron impact experiments. Absolute values of desorption yield for several ions were determined. Desorbed ions observed in this experiment are in agreement with observations of the comet Halley coma composition and may act as markers of the presence of pyrimidine in the interstellar medium.

Electron-stimulated desorption from polished and vacuum fired 316LN stainless steel coated with Ti-Zr-Hf-V

In this study, two identical 316LN stainless steel tubular samples, which had previously been polished and vacuum-fired and then used for the electron-stimulated desorption (ESD) experiments, were coated with Ti-Zr-Hf-V with different morphologies: columnar and dense. ESD measurement results after nonevaporable getter (NEG) activation to 150, 180, 250, and 350 °C indicated that the values for the ESD yields are significantly (2–20 times) lower than the data from our previous study with similar coatings on nonvacuum-fired samples. Based on these results, the lowest pressure and best long-term performance in particle accelerators will be achieved with a vacuum-fired vacuum chamber coated with dense Ti-Zr-Hf-V coating activated at 180 °C. This is likely due to the following facts: after NEG activation, the hydrogen concentration inside the NEG was lower than in the bulk stainless steel substrate; the NEG coating created a barrier for gas diffusion from the sample bulk to vacuum; the dense NEG coating performed better as a barrier than the columnar NEG coating.

Electronic Structure of the SrTiO3(001) Surfaces: Effects of the Oxygen Vacancy and Hydrogen Adsorption

The influence of electron irradiation and hydrogen adsorption on the electronic structure of the SrTiO3 (001) surface was investigated by ultraviolet photoemission spectroscopy (UPS). Upon electron irradiation of the surface, UPS revealed an electronic state within the band gap (in-gap state: IGS) with the surface kept at 1×1. This is considered to originate from oxygen vacancies at the topmost surface formed by electron-stimulated desorption of oxygen. Electron irradiation also caused a downward shift of the valence band maximum indicating downward band-bending and formation of a conductive layer on the surface. With oxygen dosage on the electron-irradiated surface, on the other hand, the IGS intensity was decreased along with upward band-bending, which points to disappearance of the conductive layer. The results indicate that electron irradiation and oxygen dosage allow us to control the surface electronic structure between semiconducting (nearly-vacancy free: NVF) and metallic (oxygen de cient: OD) regimes by changing the density of the oxygen vacancy. When the NVF surface was exposed to atomic hydrogen, in-gap states were induced along with downward band bending. The hydrogen saturation coverage was evaluated to be 3.1±0.8×10 14 cm ?2 with nuclear reaction analysis. From the IGS intensity and H coverage, we argue that H is positively charged as H ~0:3+ on the NVF surface. On the OD surface, on the other hand, the IGS intensity due to oxygen vacancies was found to decrease to half the initial value with molecular hydrogen dosage. H is expected to be negatively charged as H? on the OD surface by occupying the oxygen vacancy site.

Dynamics of Dissociative Electron–Molecule Interactions in Condensed Methanol

We have investigated the dynamics of low-energy (1–20 eV) electron-induced reactions in condensed thin films of methanol (CH3OH) through both electron-stimulated desorption (ESD) and postirradiation temperature-programmed desorption (TPD) experiments conducted under ultrahigh vacuum conditions. Results of ESD experiments, involving a high-sensitivity time-of-flight mass spectrometer, indicate that anion (H–, CH–, CH2–, CH3–, O–, OH–, and CH3O–) desorption from the methanol thin film at incident electron energies below about 15 eV is dominated by processes initiated by the dissociation of temporary negative ions of methanol formed via electron capture, a resonant process known as dissociative electron attachment (DEA). However, postirradiation TPD investigation of radicals, especially •CH2OH and CH3O• remaining in the methanol thin film, demonstrates that electron impact excitation, not DEA, is the primary mechanism by which the radical–radical reaction products methoxymethanol (CH3OCH2OH) and ethylene glycol (HOCH2CH2OH) are formed. This apparent dichotomy between the results of ESD and postirradiation experiments is attributed to the low DEA cross section for methanol compared to that of species such as halomethanes. Our results suggest that for molecules such as methanol, low-energy electron-induced electronic excitation, rather than DEA, plays a dominant role in ionizing radiation-induced chemical synthesis in environments such as the interstellar medium.

Effect of surface polishing and vacuum firing on electron stimulated desorption from 316LN stainless steel

The reduction of thermal outgassing from stainless steel by surface polishing or vacuum firing is well-known in vacuum technology, and the consequent use of both techniques allows an even further reduction of outgassing. The aim of this study was to identify the effectiveness of surface polishing and vacuum firing for reducing electron-stimulated desorption (ESD) from 316LN stainless steel, which is a frequently used material for particle accelerator vacuum chambers and components. It was found that, unlike for thermal outgassing, surface polishing does not reduce the ESD yield and may even increase it, while vacuum firing of nonpolished sample reduces only the H2 ESD yield by a factor 2.

Graphene Coatings for the Mitigation of Electron Stimulated Desorption and Fullerene Cap Formation

Graphene already has numerous applications in transmission electron microscopy. Here, we extend its application in electron microscopy by demonstrating its potential to stop electron induced desorption in nonconducting samples, where in essence charge build-up leads to surface atom desorption. Graphene films provide a conduction pathway to prevent charge build-up and do not interfere with the imaging process allowing the direct imaging of specimens sensitive to electron induced desorption. We also show that small graphene flakes on the surface of MgO transform to fullerenes or hemispherical fullerenes. The hemispherical fullerenes anchor to the MgO surface and are of particular interest as they suggest it should be possible to nucleate single walled carbon nanotubes on the surface of oxide supports without the need of a catalyst particle.

An Insight into Grain Refinement Mechanism of Ultrananocrystalline Diamond Films Obtained by Direct Current Plasma-Assisted Chemical Vapor Deposition

The grain-refinement mechanisms involved during the deposition of diamond films by direct-current plasma assisted chemical vapor deposition (DC-PACVD) were investigated as a function of the inter-electrode electric field (IEEF). As IEEF was increased from 260 to 940 V cm−1, the local electron temperatures near the growth front increased strongly; as a result, a strong grain refinement occurred ultimately yielding ultrananocrystalline diamond films (UNCD). Such observations were attributed to novel features of the DC-PACVD, including the electron-stimulated desorption (ESD) of the hydrogen-terminated moieties located at the surface, and the consequently enhanced generation of bi-radical sites at the growing diamond surface.

Ethanol adsorption on rutile TiO2(110)

We investigated ethanol adsorption on TiO2(110) surfaces with scanning tunneling microscopy (STM) at 78 K. The ethanol was deposited by pulse injection. The STM images show that the ethanol molecules adsorbed above Ti4+ sites formed rows in the [001] direction even in the case of multilayer adsorption. Ethanol desorbed from the surface when the sample was annealed to room temperature, and the multilayer was reduced to a monolayer. Mainly ethoxy groups remained attached to the surface, and the majority of them were bound to the Ti rows. Furthermore, electron-stimulated desorption experiments revealed that only a few molecules desorbed from the surface when the scans were conducted with a sample bias ≥+3 V. In contrast, when scans were conducted at ≥+4 V, the majority of the hydrogen and covalent bonds between the adsorbates and the rutile were broken and only a small amount of species remained adsorbed on the surface. In the case of the cold dose without further annealing, the remaining ethoxy groups preferentially adsorbed on the Ti rows, whereas in the case of the annealed surface, many of the ethoxys shifted to oxygen bridging sites. This change in the adsorption behavior can be explained by the different oxidation states of the surface, which play an important role in the adsorption-desorption process.

Effect of deposition of samarium atoms on the electron-stimulated desorption of cesium and samarium atoms from the surface of tungsten coated by gold and cesium

It has been shown that deposition of Sm atoms on W(100) surface coated by several monolayers of gold and cesium affects noticeably the yield of Cs atoms in electron-stimulated desorption (ESD) from this surface. The measurements have been performed by the time-of-flight method with a surface-ionization detector. The paper reports on the first observation of ESD of Sm atoms from the tungsten surface coated by layers of gold and cesium. The ESD threshold for Sm atoms, E e = 57 eV, coincides with that for Cs atoms and corresponds to the energy of the Au 5p 3/2 core level. The dependence of the ESD yield of Sm atoms on the bombarding electron energy E e follows a resonance pattern in the form of a narrow peak located in the range 57 ≤ E e ≤ 66 eV. Deposition of Sm atoms at room temperature (∼300 K) reduces (by a factor of about two) the ESD yield of Cs atoms for 600 s, and deposition of Sm atoms at 160 K reduces the ESD of Cs atoms down to zero already for 270 s. This difference finds explanation in the study of the change the structure of the top layer of the (Au + Cs)/W surface coating undergoes under cooling of the surface from 300 to 160 K.

The role of electron-stimulated desorption in focused electron beam induced deposition

We present the results of our study about the deposition rate of focused electron beam induced processing (FEBIP) as a function of the substrate temperature with the substrate being an electron-transparent amorphous carbon membrane. When W(CO)6 is used as a precursor it is observed that the growth rate is lower at higher substrate temperatures. From Arrhenius plots we calculated the activation energy for desorption, Edes, of W(CO)6. We found an average value for Edes of 20.3 kJ or 0.21 eV, which is 2.5–3.0 times lower than literature values. This difference between estimates for Edes from FEBIP experiments compared to literature values is consistent with earlier findings by other authors. The discrepancy is attributed to electron-stimulated desorption, which is known to occur during electron irradiation. The data suggest that, of the W(CO)6 molecules that are affected by the electron irradiation, the majority desorbs from the surface rather than dissociates to contribute to the deposit. It is important to take this into account during FEBIP experiments, for instance when determining fundamental process parameters such as the activation energy for desorption.

Intensity distributions of reflected surface channeling protons scattered on surfaces of electron-bombarded alkali halide crystals

We have examined the surface-channeling of 550keV protons on electron-bombarded KBr(001) surfaces at grazing incidence. On the surface, electron-stimulated desorption (ESD) resulting from the irradiation of 5keV electrons changes the surface morphology. In order to investigate the change of the surface morphology, the luminous intensity distributions observed on a fluorescent screen (scattering patterns) of the reflected protons under the surface-channeling conditions are measured. Normalized specular intensity of the protons oscillates, and the results of computer simulations show that the period of the intensity oscillation agrees with the period of layer-by-layer desorption. The measured period of the oscillation is comparable to the simulated one, i.e., the period of the desorption, however, the measured amplitude of the oscillation is weak. This shows that the layer-by-layer desorption of the experimental surface is observed but is not as remarkable as that of the perfect surface introduced in the simulation.

Dynamics Research on Electron-Stimulated Desorption of Fluorine from Fluorinated Graphene Surface

The experiment of stimulating fluorine to desorb from fluorinated graphene was carried out in order to make clear the dynamical mechanism of the electron-stimulated desorption (ESD) of fluorine. X-ray photoelectron spectroscopy (XPS) with an electron beam gun accessory was used not only to stimulate fluorine to desorb, but also to characterize the concentration of fluorine after ESD. The concentration of fluorine dropped quickly following electron beam irritation. The desorption cross section of fluorine is about 1.5×10-17 cm2. It was also found that electron beam with low current density did not damage the structure of fluorinated graphene. These results confirm that it is feasible to fabricate all graphene electronics by ESD technology.