Electron Stimulated Desorption Yields Research Articles

Electron conditioning of technical surfaces at cryogenic and room temperature in the 0–1 keV energy range

In the superconducting magnets of the Large Hadron Collider (LHC) at CERN, most of the beam-induced heat load is intercepted by a beam-screen (BS) cryogenically cooled to 5–20 K. When circulating the bunched proton beam, an electron cloud (EC) can form and bombard the BS copper surface with high doses of predominantly low-energy electrons, which desorb gas and consequently increase the pressure. The beam-induced pressure rise decreases during operation as the electron irradiation diminishes the secondary electron yield (SEY) and the electron-stimulated desorption (ESD) yield, a phenomenon referred to as ‘beam conditioning’. Low ESD and SEY values achieved rapidly are requisite to mitigate EC and maintain UHV in storage rings. We report data on ESD and SEY electron conditioning completed at cryogenic temperature with 0–1 keV electrons up to an electron dose of 5.10−3 C mm−2. Our results show that SEY conditioning depends on the primary electron energy and also that ESD yield significantly decreases with temperature. At 15 K, the amorphous-carbon coating and laser-treated copper present SEY below 1.1 and have initial ESD yields 3–6 times lower than OFE copper. Our results conform to the SEY and ESD's general understanding and extend it towards cryogenic temperatures.

Effect of the film thickness on electron stimulated desorption yield from Ti-Zr-V coating

With the benefits of evenly distributed pumping, low thermal outgassing rate and low photon-/electron-/ion stimulated desorption yields, non-evaporable getter (NEG) coating has been widely used in particle accelerators for many years. Our earlier work has demonstrated the different thickness of Ti-Zr-V coating in the range of 0.1–1 μm affects its pumping properties. In this study, the electron stimulated desorption (ESD) yields were studied for Ti-Zr-V coating deposited from a twisted target with its thickness less than 0.5 μm, while an uncoated sample was also measured as a reference. The ESD yields for H2, CH4, CO and CO2 were measured as a function of electron accumulated dose up to ∼2 × 1024 e-/m2 followed bakeout/activation by heating to 80, 120, 140, 180 and 250°C. After each ESD experiment the samples were full saturated with a mix of H2, CO and CO2. Both NEG coated samples demonstrate lower values in comparison to the uncoated one after heating/activation to temperatures up to 180°C for H2, up to 250°C for CH4, up to 140°C for CO and 120°C for CO2. The activation to 140°C is the most efficient way to reach lower pressure and avoid quick aging of the NEG coating. The thinner NEG coating provides a significantly lower ESD yield in comparison to the thicker one only after activation to 180°C, while their ESD yields are almost identical for each gas at the lower temperature.

Electron-stimulated desorption from molecular ices in the 0.15–2 keV regime

Electron-stimulated desorption (ESD) of cryosorbed molecules on surfaces is a process of relevance to fields as varied as vacuum dynamics in accelerators and astrochemistry. While desorption from such molecular systems induced by keV electrons and fast ions has been extensively studied, the sub-keV electron regime is comparatively little known. We measured and quantified electron-stimulated desorption from molecular ice systems (layers of N2, CO, CO2, Ar, and H2O/D2O condensed at cryogenic temperatures) in the 150–2000 eV electron energy range. In this regime, stopping power is no longer sufficient to explain the electron energy dependence of ESD yields. We introduce the notion of desorption-relevant depth, which characterizes the transition between two energy deposition regimes near the surface. We then apply this notion to the different systems, showing how ESD in the sub-keV regime can, for example, reveal the differences in species diffusion in crystalline and porous amorphous CO2 ices.

Single metal zirconium non-evaporable getter coating

Non-evaporable getter (NEG) coating has been used for years in many particle accelerator facilities due to its evenly distributed pumping speed, low thermal outgassing, and low photon and electron stimulated desorption yields. In our earlier work, it has been demonstrated that quaternary Ti–Zr-Hf-V coating deposited from an alloy target in a form of a rod has the lowest desorption yields, the highest sticking probability and sorption capacity. In this paper, a single element Zr target has been explored. This type of target is widely available and produced in a form of a wire that is easy to apply for a uniform coating of various shapes of vacuum chambers. Two samples coated with Zr film of dense and columnar structure were analysed and results of the pumping properties and electron stimulated desorption results are reported. The results show that pure Zr coating could be an economic solution, despite not being as effective as can be achieved with a quaternary NEG film. It shows that columnar Zr coating can be activated and reaches full pumping capacity at 160 °C. This is close to the activation temperature of Ti–Zr-Hf-V film and lower than that for the widely used ternary Ti–Zr–V film. Single metal Zr coatings demonstrated that it could be a more efficient and inexpensive way of producing the NEG-coated vacuum chambers.

Study of Ultra-High-Vacuum Properties of Carbon-Coated Stainless Steel Beam Pipes for High-Energy Particle Accelerators

Ultra-high vacuum (UHV) in high-energy particle accelerator is essential to ensure the required beam lifetime. In the present communication, the UHV characterization of vacuum chambers coated with a thin film of carbon is reported. Recently, such coatings have attracted the interest for their low secondary electron yield and, consequently, their capability to eradicate electron cloud in high-intensity proton beam accelerators. Carbon was coated on stainless steel beam pipes by magnetron sputtering. Coated and uncoated stainless steel pipes ( $$\hbox {C}_{\mathrm{ssp}}$$ and $$\hbox {UC}_{\mathrm{ssp}}$$ ) were compared based on their outgassing rates and electron-stimulated desorption yields. The study conclusively verified that magnetron-sputtered carbon coatings are compatible with the requirements of most of the modern particle accelerators.

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.

Note: Transient negative ions as initiators of oxygen fixation in <20 eV electron-irradiated DNA

This note represents a significant addition to our previous manuscript [N. Mirsaleh-Kohan, A. D. Bass, P. Cloutier, S. Massey, and L. Sanche, J. Chem. Phys. 136, 235104 (2012)] where we demonstrated that enhancements in the electron stimulated desorption yields of O− and OH− from O2 exposed DNA samples were due to the reactions of O2 with electron-induced damage sites within the DNA. Here, we investigate the attachment of O2 to electron-irradiated DNA as a function of electron energy and find a local maximum for O2 attachment to DNA at ∼11.4 eV. This reaction is likely initiated by the production of transient negative ions that dissociate to form RCHx−1• radicals. This work is thus relevant to our understanding of how O2 modulates radiation induced damage in cellular DNA and upon the question of the radio-sensitization of cells during, for example, radiotherapy.

Electron stimulated desorption from the 316 L stainless steel as a function of impact electron energy

The electron stimulated desorption (ESD) yields of vacuum chamber wall materials are required as one of the important input parameters in the design of vacuum systems for particle accelerators and many other vacuum systems where energetic electrons hit vacuum chamber walls. In the present study, the ESD yields were measured and analyzed (a) as a function of accumulated electron dose, (b) as a function of the total amount of desorbed gas, and (c) as a function of the energy of electrons bombarding. In this study, three separate 316 L stainless steel samples were bombarded with electrons up to an accumulated electron dose of ∼2 × 1023 e−/m2 at three different corresponding electron energies: 50 eV, 500 eV, and 5 keV. Once the required dose was reached, the ESD yield as a function of electron energy was measured between 10 eV and 6.5 keV. The results obtained by both methods were in agreement showing that ESD yield increases with electron energy. Experimental difficulties and problems with interpretation of data relating to such a study are described and discussed in the paper.

Effect of coating morphology on the electron stimulated desorption from Ti–Zr–Hf–V nonevaporable-getter-coated stainless steel

The electron stimulated desorption (ESD) was studied for quaternary Ti–Zr–Hf–V alloy coated stainless steel samples with different surface structures: dense on one sample and columnar on another. The ESD yields were measured as a function of electron accumulated dose up to ∼1023 e−/m2 or greater and three different NEG coating activation temperatures: 150, 180 and 250 °C. After each ESD experiment the samples were saturated with a mixture of H2, CO and CO2. Both samples depicted lower ESD yields for all desorbed species compared to a ternary Ti–Zr–V alloy. It was also shown that although the columnar NEG coating demonstrated better pumping properties and, for NEG activated at 150 °C, lower initial ESD yields, the higher activation temperature may result in a significant H2 yield increase with dose for the columnar NEG coating. This effect was demonstrated for the first time and should be considered for application in particle accelerators.

Electron stimulated desorption from stainless steel at temperatures between −15 and +70 °C

The electron stimulated desorption (ESD) yield from 316LN stainless steel was studied as a function of surface temperature in the range from −15 to +70 °C. The ESD yields as a function of electron dose for three samples kept at −5, +20 and +70 °C were found to be very similar. After long-term electron bombardment at fixed temperature, the ESD temperature dependence was also measured for each sample at temperatures between −15 and +70 °C. The results show that desorption yields increase with temperature, this dependence is small for H2 and increases with atomic mass number reaching the maximum difference of factor 3 for CO2. The main conclusion is that the vacuum chamber temperature in the range between −15 and +70 °C is not critical in most cases for accelerator vacuum system design. The change of ESD yield with temperature is relatively small compared to desorption yield uncertainties and the significant reduction observed with an accumulated electron dose.

Electron stimulated desorption from aluminium alloy and aluminium coated stainless steel

The electron stimulated desorption (ESD) yield was measured at the internal surface of tubular samples made of aluminium alloy and aluminium coated stainless steel. The ESD yields were measured as a function of electron accumulated dose to a maximum of 3 × 10 22 e −/m 2 at electron energy of 500 eV, then as a function of electron energy between 40 eV and 5 keV. The ESD yields as a function of electron energy were linear for all gasses except CH 4 and CO 2.

Electron stimulated desorption from bare and nonevaporable getter coated stainless steels

An installation for investigation of the electron stimulated desorption (ESD) from both pumping and nonpumping tubular samples was designed and built. This installation allows studying ESD and sample sticking probability as a function of electron dose up to about 1023 e−/m2, electron energy in the range 10 eV–6.5 keV, and sampling temperature in the range 0–80 °C. Two samples were investigated: bare and Ti–Zr–V coated stainless steels. The ESD yields were measured as a function of electron accumulated dose, electron energy, and different NEG coating activation temperatures. The effect of electron stimulated pumping of CO saturated NEG coating was demonstrated for the first time and is in a good agreement with the effect of photon stimulated NEG activation measured earlier.

Electron-stimulated reactions and O2 production in methanol-covered amorphous solid water films

The low-energy, electron-stimulated desorption (ESD) of molecular products from amorphous solid water (ASW) films capped with methanol is investigated versus methanol coverage (0-4x10(15) cm(-2)) at 50 K using 100 eV incident electrons. The major ESD products from a monolayer (ML) of methanol on ASW are quite similar to the ESD products from bulk methanol film: H(2), CH(4), H(2)O, C(2)H(6), CO, CH(2)O, and CH(3)OH. For 40 ML ASW films, the molecular oxygen, hydrogen, and water ESD yields from the ASW are suppressed with increasing methanol coverage, while the CH(3)OH ESD yield increases proportionally to the methanol coverage. The suppression of the water ESD products by methanol is consistent with the nonthermal reactions occurring preferentially at or near the ASW/vacuum interface and not in the interior of the film. The water and molecular hydrogen ESD yields from the water layer decrease exponentially with the methanol cap coverage with 1/e constants of approximately 6 x 10(14) and 1.6 x 10(15) cm(-2), respectively. In contrast, the O(2) ESD from the water layer is very efficiently quenched by small amounts of methanol (1/e approximately 6.5 x 10(13) cm(-2)). The rapid suppression of O(2) production by small amounts of methanol is due to reactions between CH(3)OH and the precursors for the O(2)-mainly OH radicals. A kinetic model for the O(2) ESD, which semiquantitatively accounts for the observations, is presented.

Nonthermal Water Splitting on Rutile TiO2: Electron-Stimulated Production of H2 and O2 in Amorphous Solid Water Films on TiO2(110)

The electron-stimulated reactions leading to H2 and O2 and the electron-stimulated desorption (ESD) of H2O from 0 − 60 ML films of amorphous solid water (ASW) adsorbed on TiO2(110) are investigated as a function of film thickness and isotopic composition at 100 K. For 100 eV incident electrons, both the H2 and O2 ESD yields have maxima when the ASW coverage is ∼20 monolayers (ML), while the H2O ESD yield increases monotonically with water coverage. All the products reach a coverage-independent yield above 40−50 ML. Experiments using layered films of H2O and D2O demonstrate that the molecular hydrogen is produced in reactions that occur preferentially at or near both the ASW/TiO2 interface and the ASW/vacuum interface. However, electronic excitations or ionic defects created within the interior of the ASW films by the energetic electrons can subsequently migrate to the interfaces where they initiate reactions. Electron irradiation of ASW films results in the formation of bridge-bonded hydroxyls on TiO2(110...

Site-dependent electron-stimulated reactions in water films on TiO2(110)

Electron-stimulated reactions in thin [<3 ML (monolayer)] water films adsorbed on TiO(2)(110) are investigated. Irradiation with 100 eV electrons results in electron-stimulated dissociation and electron-stimulated desorption (ESD) of adsorbed water molecules. The molecular water ESD yield increases linearly with water coverage theta for 0< or =theta< or =1 ML and 1<theta< or =2 ML. However, for theta>1 ML, the water ESD yield per additional water molecule adsorbed (i.e., the slope of the ESD yield versus coverage) is 3.5 times larger than for theta<1 ML. In contrast, the number of water molecules dissociated per incident electron increases linearly for theta< or =2 ML without changing slope at theta=1 ML. The total electron-stimulated sputtering rate, as measured by postirradiation temperature programmed desorption of the remaining water, is larger for theta>1 ML due to the increased water ESD for those coverages. The water ESD yields versus electron energy (for 5-50 eV) are qualitatively similar for 1, 2, and 40 ML water films. In each case, the observed ESD threshold is at approximately 10 eV and the yield increases monotonically with increasing electron energy. The results indicate that excitations in the adsorbed water layer are primarily responsible for the ESD in thin water films on TiO(2)(110). Experiments on "isotopically layered" films with D(2)O adsorbed on the Ti(4+) sites (D(2)O(Ti)) and H(2)O adsorbed on the bridging oxygen atoms (H(2)O(BBO)) demonstrate that increasing the water coverage above 1 ML rapidly suppresses the electron-stimulated desorption of D(2)O(Ti) and D atoms, despite the fact that the total water ESD and atomic hydrogen ESD yields increase with increasing coverage. The coverage dependence of the electron-stimulated reactions is probably related to the different bonding geometries for H(2)O(Ti) and H(2)O(BBO) and its influence on the desorption probability of the reaction products.

Electron-stimulated production of molecular oxygen in amorphous solid water on Pt(111): Precursor transport through the hydrogen bonding network

The low-energy, electron-stimulated production of molecular oxygen from thin amorphous solid water (ASW) films adsorbed on Pt(111) is investigated. For ASW coverages less than approximately 60 ML, the O(2) electron-stimulated desorption (ESD) yield depends on coverage in a manner that is very similar to the H(2) ESD yield. In particular, both the O(2) and H(2) ESD yields have a pronounced maximum at approximately 20 ML due to reactions at the Pt/water interface. The O(2) yield is dose dependent and several precursors (OH, H(2)O(2), and HO(2)) are involved in the O(2) production. Layered films of H(2) (16)O and H(2) (18)O are used to profile the spatial distribution of the electron-stimulated reactions leading to oxygen within the water films. Independent of the ASW film thickness, the final reactions leading to O(2) occur at or near the ASW/vacuum interface. However, for ASW coverages less than approximately 40 ML, the results indicate that dissociation of water molecules at the ASW/Pt interface contributes to the O(2) production at the ASW/vacuum interface presumably via the generation of OH radicals near the Pt substrate. The OH (or possibly OH(-)) segregates to the vacuum interface where it contributes to the reactions at that interface. The electron-stimulated migration of precursors to the vacuum interface occurs via transport through the hydrogen bond network of the ASW without motion of the oxygen atoms. A simple kinetic model of the nonthermal reactions leading to O(2), which was previously used to account for reactions in thick ASW films, is modified to account for the electron-stimulated migration of precursors.

Cleaning stainless steel for use in accelerators—Phase 1

Particle accelerators such as the SRS at Daresbury Laboratory generally require pressures lower than 10 −9 mbar for satisfactory stored electron beam lifetimes. To achieve this requires that particular attention be given to the cleanliness of the stainless-steel vacuum vessels and other components of the vacuum system. Pre-installation cleaning is an important step in the process. Currently, trichloroethylene is employed at Daresbury Laboratory as the primary cleaning solvent of choice, having been chosen as the outcome of earlier studies of the effects of various cleaning techniques on outgassing and electron- stimulated desorption from stainless steel [Herbert JD, Reid RJ, Groome AE. J Vac Sci Technol A 1994; 12(4): 1767; Herbert JD, Reid RJ. Vacuum 1996; 47(6–8): 693]. However, in light of the re-classification of trichloroethylene by the European Union as a class two carcinogen, with consequent stringent and expensive safety precautions required in order to continue using it, a suitable, more economical, replacement is sought after. This paper, details the assessment of different cleaning agents against the two criteria important to particle accelerator vacuum systems. The criteria being room temperature thermal outgassing rates and electron-stimulated desorption yields. A variety of different cleaning agents, ranging from halogenated solvents to aqueous-based cleaning agents have been tested. A comparison of the performance of a selection of such cleaning agents is presented and a satisfactory alternative to trichloroethylene is proposed.

An AES study of the room temperature conditioning of technological metal surfaces by electron irradiation

The modifications to technological copper and niobium surfaces induced by 2.5 keV electron irradiation have been investigated in the context of the conditioning process occurring in particle accelerator ultra high vacuum systems. Changes in the elemental surface composition have been found using Auger electron spectroscopy (AES) in the dose range of 10 −6–10 −1 C mm −2. The surface analysis results are correlated with electron dose dependent secondary electron yield (SEY) and electron stimulated desorption (ESD) yield measurements. Electron induced surface modifications are caused by a surface cleaning through ESD and a simultaneous accumulation of a carbonaceous contamination layer. The changes in the elemental surface composition are determined by the different kinetics of both processes. ESD yields decrease drastically with electron dose while carbon is accumulated on the surface with a constant rate. Initially a surface cleaning through ESD, in particular of hydrogen, is the dominant process. This surface cleaning causes a strong reduction of the SEY and of ESD yields. When the electron dose exceeds 10 −4 C mm −2 the electron beam induced carbon layer growth becomes the dominant process and the SEY is further reduced to values which are below those of an atomically clean metal surface.

Low-energy electron-induced processes in condensed CF2Cl2 films

We report measurements of electron stimulated desorption yields and kinetic energy distributions for F− and Cl− anions from thin films of CF2Cl2 condensed on polycrystalline platinum at 20 K. Dissociative electron attachment processes cause a peak in the F− yield at an electron energy of ∼4 eV and a peak in the Cl− yield at ∼7 eV. After electron irradiation, a new dominant structure appears in the Cl− yield at 5 eV incident electron energy, and Cl− ions having high kinetic energy (&amp;gt;1 eV) are observed in the kinetic energy distributions. These changes are attributed to the synthesis of Cl2 in the condensed phase induced by electron irradiation of the CF2Cl2 film. This Cl2 formation is demonstrated by the observation in the Cl− yield of the Cl2−/Cl2 2Πu resonance state, and by Cl− kinetic energy distributions that are similar to those obtained from condensed pure Cl2 films.

The effects of temperature and morphology on electron transmission and stimulated desorption of H− from thin hydrocarbon films

We have measured for the 0–20 eV incident energy range, the low-energy electron transmission (LEET) spectra and electron stimulated desorption (ESD) yields of H− from pure films of n-hexane and ethyl-benzene from 20 K to sublimation. The ESD yields from both dissociative electron attachment and dipolar dissociation initially increase as the films are heated. This effect is attributed to the collapse of pores within the films. For n-hexane, a marked decrease in ESD yields is observed at film temperatures ⩾80 K, which correlates to the onset of crystallization as evidenced in LEET spectra. This decrease in anion yield is associated with the formation of a well-defined band structure that reduces the time period an excitation and/or electron resides on a particular molecule. This behavior in the ESD yields from ethyl-benzene films is not observed prior to sublimation in agreement with LEET spectra which show negligible change with film temperature indicating that there is no change of phase prior to sublimation (i.e., the film remains amorphous at all temperatures).