Electron Stimulated Desorption Signals Research Articles

Synthesis of complex organic molecules in simulated methane rich astrophysical ices.

It has been proposed that organic molecules required for life on earth may be formed by the radiation processing of molecular ices in space environments, e.g., within our solar system. Such processes can be studied in the laboratory with surface science analytical techniques and by using low-energy electron (LEE) irradiation to simulate the effects of the secondary electrons that are generated in great abundance whenever ionizing radiation interacts with matter. Here we present new measurements of 70 eV LEE irradiation of multilayer films of CH4, 18O2, and CH4/18O2 mixtures (3:1 ratio) at 22 K. The electron stimulated desorption (ESD) yields of cations and anions have been recorded as a function of electron fluence. At low fluence, the prompt desorption of more massive multi-carbon or C-O containing cationic fragments agrees with our earlier measurements. However, new anion ESD signals of C2-, C2H-, and C2H2- from CH4/18O2 mixtures increase with fluence, indicating the gradual synthesis (and subsequent electron-induced fragmentation) of new, more complex species containing several C and possibly O atoms. Comparisons between the temperature programed desorption (TPD) mass spectra of irradiated and unirradiated films show the electron-induced formation of new chemical species, the identities of which are confirmed by reference to the NIST database of electron impact mass spectra and by TPD measurements of films composed of the proposed products. New species observed in the TPD of irradiated mixture films include C3H6, C2H5OH, and C2H6. Furthermore, X-ray photoelectron spectroscopy of irradiated films confirms the formation of C-O, C=O, and O=C-O- bonds of newly formed molecules. Our experiments support the view that secondary LEEs produced by ionizing radiation drive the chemistry in irradiated ices in space, irrespective of the radiation type.

On the possibility of using low-energy electron stimulated desorption of ions as a surface probe: Analysis of Au substrates

We explore the possibility of using low-energy (0–50eV) electron stimulated desorption (ESD) of ions, analyzed by time-of-flight (TOF) mass spectrometry, as a surface analysis technique. As a first step, we measure the mass spectra from ions emanating from gold substrates prepared for molecular self-assembly by two different methods: cleaning with sulfochromic acid (CrO3/H2SO4) and exposure to UV/O3. Modifications of the yields of desorbed anions and cations as functions of incident electron energy provide information on the presence and the chemical nature of contaminants present on the Au surface, either before or after cleaning. Upon cleaning, the decrease in intensity of the yield functions of certain desorbed anions and cations associated with organic molecules confirms that both techniques are efficient in removing organic contaminants. On the other hand, other ESD signals increase or only appear by either cleaning procedure. This finding demonstrates how these cleaning techniques can contaminate Au surfaces with species, such as acid residues and nitrogenized organic molecules. Such observations have never been reported before with other methods of surface analysis. The present results strongly suggest that ESD of ions should be considered as a potential sensitive tool to complement surface analysis by other well-established methods.

Oxygen Attachment on Alkanethiolate SAMs Induced by Low-Energy Electron Irradiation

Reactions of (18)O2 with self-assembled monolayer (SAM) films of 1-dodecanethiol, 1-octadecanethiol, 1-butanethiol, and benzyl mercaptan chemisorbed on gold were studied by the electron stimulated desorption (ESD) of anionic fragments over the incident electron energy range 2-20 eV. Dosing the SAMs with (18)O2 at 50 K results in the ESD of (18)O(-) and (18)OH(-). Electron irradiation of samples prior to (18)O2 deposition demonstrates that intensity of subsequent (18)O(-) and (18)OH(-) desorption signals increase with electron fluence and that in the absence of electron preirradiation, no (18)O(-) and (18)OH(-) ESD signals are observed, since oxygen is unable to bind to the SAMs. A minimum incident electron energy of 6-7 eV is required to initiate the binding of (18)O2 to the SAMs. O2 binding is proposed to proceed by the formation of CHx-1(•) radicals via resonant dissociative electron attachment and nonresonant C-H dissociation processes. The weaker signals of (18)O(-) and (18)OH(-) from short-chain SAMs are related to the latter's resistance to electron-induced damage, due to the charge-image dipole quenching and electron delocalization. Comparison between the present results and those for DNA oligonucleotides self-assembled on Au (Mirsaleh-Kohan, N. et al. J. Chem. Phys. 2012, 136, 235104) indicates that the oxygen binding mechanism is common to both systems.

電子遷移誘起脱離研究の先駆者，石川義興博士および太田芳雄博士の業績紹介

Dr. Yoshioki Ishikawa is a pioneer who carried out systematic electron stimulated desorption (ESD) investigations for hydrogen adsorbed on a clean platinum plate (H2/Pt) under ultrahigh vacuum (UHV) conditions in 1942. (His name is spelled as Yosioki Isikawa in the original papers.) Although his papers are referred in several papers on desorption induced by electronic transitions (DIET), these seem to be not as well-known as famous ones by Menzel, Gomer, and Redhead in 1964. We describe his pioneering work in this article. He developed an UHV apparatus made of glass with a Pt plate and an oxide cathode pumped by a small mercury pump via a liquid air trap. Clear ESD signals were observed for H2/Pt at excitation energies of 8, 12, 14, 33-46, and 46-51 eV. He assigned the desorbate to atomic hydrogen (H), because it was pumped by a glass wall baked above 350°C. Based on these results and the potential curves of H2 in the gas phase, he proposed a model for the ESD of H from H2/Pt; that is, the first step is an electronic transition of adsorbed H2, and the second is the desorption of H along the repulsive potential surface of the excited state. This was the first introduction of the notion of DIET. He also carried out an ESD study of H2O/Pt in 1943. We also introduce achievements by Dr. Yoshio Ohta, who was another pioneer in the field of DIET. He measured electron-stimulated ion desorption from an oxidized Ni plate. He found that the threshold excitation energy for ion desorption is 22 eV, and that the kinetic energy of the desorbed ion is 1∼2 eV. To explain these results, he also proposed the concept of DIET.

HREELS, TPD and ESD study of electron‐induced decomposition of trimethylamine on Si(100) at 100 K

AbstractThe effects of 600 eV electron irradiation on adsorbed layers of trimethylamine (TMA) on a Si(100) surface at 100 K were studied using high‐resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), electron‐stimulated desorption (ESD) and x‐ray photoelectron spectroscopy (XPS). Experiments were performed at monolayer and higher coverages. The ESD results obtained on physisorbed TMA indicate electron‐induced decomposition of the parent molecule. The HREELS data also show cleavage of NC bonds on the chemisorbed layer of TMA and deposition of CHx groups on the surface, which is also supported by a dramatic enhancement of hydrogen desorption at 960 K following electron irradiation. This multiple‐step process is supported by ESD signal decay curves obtained during electron irradiation. Following electron irradiation of TMA/Si(100), the intensity of low temperature TPD peaks corresponding to TMA fragments decreases; however, the intensity of the mass 2 peak increases dramatically compared with the un‐irradiated surface. This is indicative of electron‐induced associative desorption of molecular hydrogen. The HREELS data from monolayer coverage of TMA on Si(100) after electron irradiation suggests the removal of CHx groups from the adsorbate layer and deposition of carbon and hydrogen on the surface. In the absence of physisorbed layers there is no indication of electron‐induced desorption of TMA fragments. Copyright © 2005 John Wiley & Sons, Ltd.

Photon-stimulated desorption of Na from a lunar sample: temperature-dependent effects

We report recent results in an investigation of source mechanisms for the origin of Na atoms in tenuous planetary atmospheres, focusing on non-thermal processes. Experiments include photon stimulated desorption (PSD), electron stimulated desorption (ESD), and ion sputtering of Na atoms from the surface of a lunar basalt sample. Bombardment of the sodium covered surface by 3 keV Ar + ions removes Na from the surface by sputtering into vacuum and by implantation into the sample bulk. Bombardment of the Na covered surface by ultraviolet photons or by low energy electrons ( E>3 to 4 eV) causes desorption of “hot” Na atoms. These results are consistent with our previous measurements of sodium and potassium desorption from a silica surface: electron- or photon-induced charge transfer from the substrate to the ionic adsorbate causes formation of a neutral alkali atom in a repulsive configuration, from which desorption occurs. There is a strong temperature-dependence of Na ESD and PSD signals, under conditions where the Na surface coverage is constant and thermal desorption is negligible. The yield of Na (atoms/photon) increases by 10× from 100 to 470 K; an activation energy of ∼20 meV is measured. This phenomenon may be attributed to thermally-induced changes in surface bonding sites, and will affect recent modeling of the sodium atmospheres of Mercury and the Moon.

Electron stimulated desorption of H−from thin films of 5-halouracils

We present measurements of low-energy (1–20 eV) electron stimulated desorption (ESD) of H− from thin films of 5-halouracils (5-XU, X=F, Cl, Br and I) condensed on polycrystalline Pt. The onset for H− desorption is ∼5.0 eV. For all 5-XU a single dissociative electron attachment (DEA) H− peak is observed, which is attributed to at least two dissociation channels at approximately 7.1 and 8.6 eV, resulting from N–H and C–H bond cleavage, respectively. Their relative contribution is parent-molecule dependent. Also, for a given molecule, these dissociation channels are affected differently by the metal, and the DEA maximum shifts by ∼0.5 eV as the film thickness increases. Above 10 eV, dipolar dissociation (DD) is the dominant H− formation mechanism. The yields of H− vary in the following relative order (T=thymine, U=uracil): T>U>5-FU>5-ClU>5-BrU>5-IU. A model is developed to explain the film thickness dependence of the desorbed anion signal: above a coverage of ∼1 monolayer (ML), the film thickness dependence of the H− ESD yield is interpreted in terms of incident electron and desorbing ion transmission within the films. The main difference in the thickness dependence of H− produced via DEA and DD is attributed to the fact that electron transfer to the metal from the transient molecular anion leads to dramatic decrease in the H− ESD signal below ∼1 ML, whereas it does not affect significantly DD, since its intermediate state is neutral.

Dominance of electron-stimulated desorption neutral species in ultra-high vacuum pressure measurements

An energy-dispersive residual gas analyzer (EDRGA) has been developed which enables us to separate the gas-phase ions from the electron-stimulated desorption (ESD) ions at each mass number. With this EDRGA, neutral ESD fragments from the grid, normally indistinguishable from the “gas phase” signals, were evaluated for the first time. Namely, the mass spectrum from the gas phase ions was compared with one synthesized from the fragmentation patterns of the existing parent molecules in the gas phase and normalized by partial pressure. It was clear that the fraction of neutral ESD fragments in the total ESD signal depends on the mass and the history of the evacuation, i.e. with or without baking. Neutral species dominate the ESD in UHV vacuums after bakeout, which suggests that even energy-dispersive total pressure gauges are unable to eliminate ESD errors.

Electron-stimulated desorption total cross-section determination for digermane on Si(100)

We have studied digermane-covered Si(100) using electron-stimulated desorption (ESD). Estimates are presented for the total H(a) ESD removal cross-section for digermane-exposed Si(100) substrates at 85 K using electrons incident at 150 eV energy. It is found that electron-enhanced deposition of Ge occurs only when physisorbed digermane is present. Auger electron spectroscopy provided the means for determining the relative amounts of germanium adsorbed on the Si(100) surface following digermane exposures, electron irradiation and surface reconstruction. It is found that two coverage regimes are important: initial dosing of digermane on Si(100) at 85 K results in overlayers consisting of both physisorbed digermane and chemisorbed GeH x (a) (x = 1, 2 or 3) species; and short anneals to 200 K following exposure of the Si(100) surface at 85 K lead to the presence of only chemisorbed GeH x (a). The two coverage regimes exhibit different ESD behavior. Two kinetic energy distribution (KED) peaks are seen when physisorbed digermane is present, and only one when it is absent. The ESD signal decay curves obtained from the two surfaces are also different: the presence of physisorbed digermane results in a two-component exponential signal decay; the absence of the physisorbed species results in a single-exponential decay. The total H removal cross-section from the physisorbed digermane overlayer was determined to be σ ∼ 1.4 x 10 -15 cm 2 , while that from Si(100) with only adsorbed GeH x present was found to be σ ∼ 2.6 x 10 -16 cm 2 . Our results suggest that adsorbed GeH x (a) species remain intact on the surface even when the Si(100) substrate is annealed to 200 K, indicating that hydrogen migration from surface GeH x (a) to Si surface sites does not occur at 200 K.

Electron‐stimulated desorption total cross‐section determination for digermane on Si(100)

We have studied digermane-covered Si(100) using electron-stimulated desorption (ESD). Estimates are presented for the total H(a) ESD removal cross-section for digermane-exposed Si(100) substrates at 85 K using electrons incident at 150 eV energy. It is found that electron-enhanced deposition of Ge occurs only when physisorbed digermane is present. Auger electron spectroscopy provided the means for determining the relative amounts of germanium adsorbed on the Si(100) surface following digermane exposures, electron irradiation and surface reconstruction. It is found that two coverage regimes are important: initial dosing of digermane on Si(100) at 85 K results in overlayers consisting of both physisorbed digermane and chemisorbed GeHx(a) (x=1, 2 or 3) species; and short anneals to 200 K following exposure of the Si(100) surface at 85 K lead to the presence of only chemisorbed GeHx(a). The two coverage regimes exhibit different ESD behavior. Two kinetic energy distribution (KED) peaks are seen when physisorbed digermane is present, and only one when it is absent. The ESD signal decay curves obtained from the two surfaces are also different: the presence of physisorbed digermane results in a two-component exponential signal decay; the absence of the physisorbed species results in a single-exponential decay. The total H removal cross-section from the physisorbed digermane overlayer was determined to be σ∽1.4×10−15 cm2, while that from Si(100) with only adsorbed GeHx present was found to be σ∽2.6×10−16 cm2. Our results suggest that adsorbed GeHx(a) species remain intact on the surface even when the Si(100) substrate is annealed to 200 K, indicating that hydrogen migration from surface GeHx(a) to Si surface sites does not occur at 200 K. © 1998 John Wiley & Sons, Ltd.

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

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

Effects of three-dimensional time evolution in kinetic-energy distributions for electron stimulated desorption

Quantum mechanical theory of electron stimulated desorption (ESD) of neutral particles from physisorbed layers is used to study how the kinetic-energy distributions of the desorption products are influenced by the time evolution in all spatial directions of the wave packet describing the adsorbed particle in the electronically excited state of the system. It is demonstrated that in case of the Antoniewicz scenario of desorption (properly generalized to three dimensions) different evolutions of the wave packet in the plane of the solid surface may result in drastically different distributions of particles desorbing in the direction of the surface normal even if the evolution in the latter direction is kept identical. In some cases the distributions may exhibit a multi-peak structure without different adsorption states being present - the effect never observed in a commonly used one-dimensional model. No such effects are expected for the recently proposed wave packet squeezing model of desorption. The behavior observed in the Antoniewicz model may also be present in the ESD signals from chemisorbed layers for which the Menzel-Gomer-Redhead model of desorption applies.

Time Sequential Measurement of the Precursor State of Oxygen on the Titanium Surface by Means of Time-of-Flight Spectroscopy for Electron Stimulated Desorption

An adsorption process of oxygen on the titanium single-crystal surface of (101̄0) has been investigated by means of a time-of-flight spectrometer for the electron stimulated desorption (ESD). An oxidation experiment has been performed at the oxygen pressure ranging from 1 to 8×10-8 Torr. TOF spectra of ESD signals were obtained sequentially by a digital acquisition system. In an initial adsorption stage, the O+ ESD signal intensity grew depending on the oxygen exposure. The O+ signal peaks have the most probable kinetic energies of 3.5±0.4 eV and 1.8±0.2 eV. The higher-energy peak is identified as the ESD signal from molecularly adsorbed oxygen on the surface, and the lower one as that from the chemically adsorbed oxygen.

Continuous electron stimulated desorption using a ZrO2/Ag permeation membrane

During the development of an atomic oxygen beam generator for laboratory simulation of the atmospheric conditions in low Earth orbit, a new technique for performing electron stimulated desorption (ESD) in a continuous manner has been developed. In this technique, oxygen permeates through a Ag membrane at elevated temperature thereby providing a continuous supply of oxygen atoms to the 1000-Å ZrO2 coating at the vacuum interface. ESD then results in a large peak of neutral O2 molecules which ultimately decays into steady-state desorption. The ESD signal is linear with respect to primary beam flux (3.5×10−2 O2 molecules per electron at a primary beam energy of 1 keV) but nonlinear with respect to primary beam energy.

Measurements of weakly bound hydrogen on Pd(111) between 5 and 80 K

The adsorption of hydrogen and deuterium onto a low-temperature surface of Pd(111) has been studied experimentally using electron-stimulated desorption (ESD) and temperature programmed desorption spectrometry (TPD). At a surface temperature TS=7.5±1 K, exposure to D2 leads to multilayer formation which is reflected in the desorption of deuterium ion clusters D+n by ESD (Ee−=100 eV). Cluster sizes up to n=21 have been observed which, except for n=2, consist exclusively of odd numbers of atoms. The temperature dependence of the ESD signal following exposure at TS=7.5±1 K suggests that by 9.5±1 K the multilayer desorbs leaving a diatomic molecular species which, in the high-coverage limit, is stable up to ∼20 K. Exposure to hydrogen at these low temperatures gives rise to two TPD peaks: the β (chemisorption) peak and a new TPD peak denoted γ (near 60 K at low coverage) which is assigned to desorption of a surface molecular hydrogen species. Mixed isotope experiments show that there is no mixing in the γ state consistent with this assignment. With increasing exposure, the γ peak grows in intensity and shifts rapidly towards lower temperature. TPD peak shape analysis gives a binding energy for the molecular state at low coverage of ∼0.020±0.010 eV and a corresponding preexponential of approximately 102 s−1.

Electron-stimulated desorption and thermal desorption spectrometry of H 2O on nickel (111)

The adsorption of H 2O at low temperature on clean and oxygen dosed nickel (111) has been studied using thermal desorption spectrometry (TDS) and electron-stimulated desorption (ESD). Both the single and multilayer H 2O regimes have been investigated. For first layer chemisorbed water, the thermal desorption data can be modeled well assuming first order kinetics with a desorption energy of about 9.8 kcal mol . For ice multilayers, a leading edge analysis gives a desorption energy of 11.5 ± 0.5 kcal mol . ESD measurements detected both H + and H 2O + ions, and several types of ESD measurements were made: determination of ion yield versus electron beam energy, determination of ion kinetic energy distribution curves (lEDC's), and determination of the temperature dependence of the ion yield (TDIY) at fixed excitation energy. The latter measurements show a correlation with the thermal desorption data: TDIY of H 2O + during desorption of multilayer H 2O films gives 11.0 ± 0.5 kcal mol for the desorption energy, in excellent agreement with the TDS results. Additional features are resolved in the temperature dependence of the ESD signal which are absent in the TDS spectra. Characteristic ion yield and ion kinetic energy distribution curves are given for adsorbed layers on the clean surface in both the single and ice multilayer regime. For the oxygen predosed surface, TDS was used to isolate a H 2O desorption state at higher temperature which is due to dissociative recombination, and ESD was used to characterize the ion yields and IEDC's of H + from hydroxyl groups adsorbed on this type of surface. Comparison of the data for H + ions from adsorbed water and hydroxyls indicates that ESD in this case is sensitive primarily to the local parent O-H bond, and insensitive to secondary chemical bonds.

Electron-stimulated desorption study of hydrogen-exposed aluminum films

H 2 adsorption of evaporated clean and H 2-exposed aluminum films is investigated by using the electron-stimulated desorption (ESD) method. A strong H + ESD signal is observed on a freshly evaporated aluminum surface which is clean according to previously proposed cleanliness criteria. An increased H + yield on H 2 exposure is also observed. However, the increasing rate of H + emission could be directly correlated with small increases in H 2O partial pressure during H 2 exposure. It is proposed thatthe oxidation of aluminum by water vapor and subsequent adsorption of H 2 or water is the primary process of the enhanced high H + yield during H 2 exposure.

Adsorption and coadsorption of carbon monoxide and hydrogen on Pd(111)

The adsorption and coadsorption of CO and H 2 have been studied by means of thermal desorption (TD) and electron stimulated desorption (ESD) at temperatures ranging from 250 to 400 K. Three CO TD states, labelled as β 2, β 1, and β 0 were detected after adsorption at 250 K. The population of β 2 and β 1 states which are the only ones observed upon adsorption at temperatures higher than 300 K was found to depend on adsorption temperature. The correlation between the binding states in the TD spectra and the ESD O + and CO + ions observed was discussed. Hydrogen is dissociatively adsorbed on Pd(111) and no ESD H + signal was recorded following H 2 adsorption on a clean Pd surface. The presence of CO was found to cause an appearance of a H + ESD signal, a decrease of hydrogen surface population and an arisement of a broad H 2 TD peak at about 450 K. An apparent influence of hydrogen on CO adsorption was detected at high hydrogen precoverages alone, leading to a decrease in the CO sticking coefficient and the relative population of CO β 2 state. The coadsorption results were interpreted assuming mutual interaction between CO and H at low and medium CO coverages, the “cooperative” species being responsible for the H + ESD signal. Besides, the presence of CO was proved to favour hydrogen penetration into the bulk even at high CO coverage when H atoms were completely displaced from the surface.

Adsorption and coadsorption of carbon monoxide and hydrogen on Pd(111)

Abstract The adsorption and coadsorption of CO and H 2 have been studied by means of thermal desorption (TD) and electron stimulated desorption (ESD) at temperatures ranging from 250 to 400 K. Three CO TD states, labelled as β 2 , β 1 , and β 0 were detected after adsorption at 250 K. The population of β 2 and β 1 states which are the only ones observed upon adsorption at temperatures higher than 300 K was found to depend on adsorption temperature. The correlation between the binding states in the TD spectra and the ESD O + and CO + ions observed was discussed. Hydrogen is dissociatively adsorbed on Pd(111) and no ESD H + signal was recorded following H 2 adsorption on a clean Pd surface. The presence of CO was found to cause an appearance of a H + ESD signal, a decrease of hydrogen surface population and an arisement of a broad H 2 TD peak at about 450 K. An apparent influence of hydrogen on CO adsorption was detected at high hydrogen precoverages alone, leading to a decrease in the CO sticking coefficient and the relative population of CO β 2 state. The coadsorption results were interpreted assuming mutual interaction between CO and H at low and medium CO coverages, the “cooperative” species being responsible for the H + ESD signal. Besides, the presence of CO was proved to favour hydrogen penetration into the bulk even at high CO coverage when H atoms were completely displaced from the surface.

Electron‐stimulated desorption from hydrogen exposed palladium 〈100〉

Measurements of electron‐stimulated desorption (ESD) of H+ have been carried out for a sputter‐cleaned and annealed palladium 〈100〉 surface exposed to hydrogen at low temperature. The dependence of the H+ yield on the energy of the incident electron beam energy from 15 to 400 eV exhibits thresholds for hydrogen desorption at the Pd 4p, 4s, and 3d core level binding energies. In addition, a low energy threshold is observed near 23 eV. The temperature dependence of the ESD signal has also been measured and shows that the H+ yield drops nearly two orders of magnitude upon warming from 130 to 250 K.