Auger Stimulated Ion Desorption Research Articles

Direct evidence for low-energy electron emission following O LVV Auger transitions at oxide surfaces

Oxygen, the third most abundant element in the universe, plays a key role in the chemistry of condensed matter and biological systems. Here, we report evidence for a hitherto unexplored Auger transition in oxides, where a valence band electron fills a vacancy in the 2s state of oxygen, transferring sufficient energy to allow electron emission. We used a beam of positrons with kinetic energies of sim 1 eV to create O 2s holes via matter-antimatter annihilation. This made possible the elimination of the large secondary electron background that has precluded definitive measurements of the low-energy electrons emitted through this process. Our experiments indicate that low-energy electron emission following the Auger decay of O 2s holes from adsorbed oxygen and oxide surfaces are very efficient. Specifically, our results indicate that the low energy electron emission following the Auger decay of O 2s hole is nearly as efficient as electron emission following the relaxation of O 1s holes in hbox {TiO}_2. This has important implications for the understanding of Auger-stimulated ion desorption, Coulombic decay, photodynamic cancer therapies, and may yield important insights into the radiation-induced reactive sites for corrosion and catalysis.

Desorption from Methanol and Ethanol Ices by High Energy Electrons: Relevance to Astrochemical Models

Methanol and ethanol, precursors of prebiotic molecules, are found in interstellar and circumstellar environments. At low temperatures, electrons may interact with these frozen molecules on dust grain surfaces stimulating desorption of atomic and molecular ions and charged clusters. These heavy fragments released from the icy mantles could contribute to the abundance in the gas phase of organic molecules in such environments. In this work, we investigate the ionic fragments desorbed from methanol and ethanol pure ices due to high energy electron impact. Absolute desorption yields (ions/impact) for each fragment desorbed from the ice surface were determined. Several clusters and heavier molecular ions were observed at higher electron energies. Two mechanisms seem to be involved in the desorption process, namely, the Auger stimulated ion desorption and that of secondary electrons. These data may provide support to establish more accurate astrochemical models and contribute to explain the influence of solar wind on condensed alcohols.

Photoabsorption and desorption studies on thiophene-based polymers following sulphur K-shell excitation

Photon stimulated ion desorption (PSID) and NEXAFS studies have been performed on thiophene-based polymers at the Brazilian Synchrotron Light Source following sulphur K-shell photoexcitation. For poly(thiophene) (PT) and poly(3-methylthiophene) (P3MT) it was found that the S 1 s → π *, σ * (S–C) excitation produces S + desorption efficiently. On the other hand, S 2+ desorption is enhanced at higher energy excitations. These results are interpreted in terms of the Auger-stimulated ion desorption mechanism. For poly(3-hexylthiophene) (P3HT) S + desorption seems to be suppressed, which may be due to the hexyl side-chains. Desorption ion yield curves for molecular fragments reproduce the photoabsorption spectrum, being dominated by the indirect process.

Spectroscopic investigation and characterization of polypyrrole film doped with [MeN 4] 2[Ni(dmit) 2] complex salt

Polypyrrole doped with bis(1,3-dithiole-2-thione-4,5-dithiolate) nickellate (II) (PPy/[Ni(dmit) 2] 2−) was electropolymerized in acetonitrile solution and characterized by Fourier transform infrared (FTIR), ultraviolet–visible (UV–vis), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopies, as well as cyclic voltammetry (CV), conductivity, and scanning electron microscopy (SEM) measurements. Ionic desorption studies conducted by photon stimulated ion desorption (PSID) were also performed using synchrotron radiation at the S K-edge and time-of-flight mass spectrometry (ToF-MS) for ion analysis. The spectroscopic techniques indicated that the [Ni(dmit) 2] 2− was inserted onto the PPy backbone without modification during the electropolymerization process. Loss of electroactivity during the oxi-reduction processes indicated that the [Ni(dmit) 2] 2− leaves the film, undergoing anion exchange during the redox cycle. PPy/[Ni(dmit) 2] 2− exhibited higher intrinsic conductivity value than other PPy/M-dmit films studied previously. SEM measurements showed an irregular and heterogeneous surface with steps formation. PSID results showed that the species desorb through the X-ray induced electron stimulated desorption (XESD) mechanism while atomic sulfur species are mainly formed by Auger stimulated ion desorption (ASID). These results confirmed the presence of [Ni(dmit) 2] 2− at the topmost layers of the hybrid film.

Positive and negative ionic desorption from condensed formic acid photoexcited around the O 1s-edge: Relevance to cometary and planetary surfaces

Photon stimulated ion desorption (PSID) studies have been performed in condensed formic acid using oxygen 1s-edge synchrotron radiation from the Brazilian synchrotron light source (LNLS), operating in a single-bunch mode. Ion formation was discussed in terms of the Auger stimulated ion desorption (ASID) and X-ray induced electron stimulated desorption (XESD) mechanisms. It is found that O 1s(C–OH) → π*(C O) and O 1s(C O) → 3s/σ*(HCO) transitions favored the production of C +, CH +, O +, O − and H − ions. The hydroxyl anion has not been observed while the hydroxyl cation showed low intensity or was absent. Some anion formation routes from dissociative reactions are suggested taking into account the positive ion yields.

Ionic desorption from intrinsically conducting polymer films based on polypyrrole induced by high energy electrons and soft X-ray synchrotron radiation

Ionic desorption from polypyrrole films doped with dodecylsulfate as counter anion (PPyDS) deposited onto fluorine tin oxide glass (FTO) was studied through electron (ESID) and photon (PSID) stimulated ion desorption techniques, employing time-of-flight mass spectrometry (TOF-MS) for ion analysis. ESID results suggested strong contribution from DS fragments to the mass spectra. In the 650–1500 eV energy range the relative desorption ion yield curves show maxima at about 700, 1200 and 1400 eV, which may be related to carbon, nitrogen and oxygen containing fragments, respectively, and interpreted in terms of the Auger stimulated ion desorption (ASID) mechanism. For energies greater than 1500 eV, the secondary electrons play an important role in the ion formation. PSID studies were performed at the Brazilian Synchrotron Light Source (LNLS) covering the sulphur K-shell photoexcitation during a single-bunch operation mode of the storage ring. Ionic desorption of atomic and molecular fragments were also observed. Relative desorption ion yield curves have been determined as a function of the photon energy, indicating that at the photon energy region covered by the PSID studies the indirect XESD (X-ray induced electron stimulated desorption) process plays an important role in the ionic desorption of PPyDS irradiated by soft X-rays.

Dessorção iônica e degradação de filmes de polipirrol dopado com dodecilsulfato induzidas por elétrons de alta energia

Electron stimulated ion desorption (ESID) and degradation studies of polypyrrole doped with dodecylsulfate (PPy/DS) deposited on FTO were performed using time-of-flight mass spectrometry (TOF-MS) for ion analysis. The results suggest a strong contribution from fragments of the dodecylsulfate hydrocarbon chain to the mass spectra. In the 650-1500 eV energy range the ion yield curves show maxima at about 600, 1200 and 1400 eV, which can be related to carbon, nitrogen and oxygen-containing fragments, respectively, and interpreted in terms of the Auger Stimulated Ion Desorption (ASID) mechanism. Degradation studies indicate rapid loss of heavier hydrocarbons and an increase of bulk and substrate fragments. Some degradation profiles suggest formation of new species.

Photon stimulated ion desorption from intrinsically conducting polymer films based on polypyrrole doped with [Ni(dmit) 2] 2−

Photon stimulated ion desorption and near-edge X-ray absorption fine structure studies have been performed on polypyrrole (PPy) films doped with [Ni(dmit) 2] 2− as counter ion. The experiments were taken at the Brazilian synchrotron light source (LNLS) operating in a single-bunch mode following sulphur K-shell photoexcitation and using time-of-flight mass spectrometry for ion analysis. At the sulphur 1s-edge ionic desorption of atomic and molecular fragments can be observed. Desorption ion yield curves have been determined as a function of the photon energy. The results showed that while the S + ion yield reproduces the photoabsorption spectrum, the S 2+ desorption is enhanced at higher energy excitations. These results are discussed in terms of the Auger stimulated ion desorption (ASID) mechanism, since these ions are not observed below the sulphur K-shell excitation region. Furthermore, for the other ionic species the indirect XESD (X-ray induced electron stimulated desorption) and the ASID processes seem to play an important role.

Photon stimulated ion desorption studies from poly(sulphone) using synchrotron radiation in a single-bunch mode

Photon stimulated ion desorption (PSID) studies from poly(sulphone) (PS) have been carried out by using high-resolution time-of-flight mass spectrometry. Synchrotron radiation at the Brazilian Synchrotron Light Source (LNLS) operating in a single-bunch mode was used as excitation source. PS was excited at the sulphur 1s-edge and the desorption of small and large fragments has been observed. The results are discussed in terms of the X-ray induced electron stimulated desorption mechanism. The present results contrast with previous ones reported for poly(3-methylthiophene) (PMeT), in which the observation of S + and S 2+ was interpreted solely in terms of an Auger-stimulated ion desorption mechanism.

Kinetic energy distribution of H + desorbed by core-level excitations of condensed ammonia using a miniature cylindrical mirror analyzer (CMA)

We have developed a miniature cylindrical mirror analyzer (CMA) mounted on a 70-mm-diameter conflat flange for measurements of kinetic energy distribution of ions desorbed in the polar angles of 24°–28° from the surface normal. The acceptance solid angle is 0.16 sr and the energy resolution (FWHM) is estimated as E/Δ E = 20. We have applied it for measurements of kinetic energy distributions of ions desorbed from condensed ammonia (NH 3) at the surface 4a 1 ← N 1s resonant transition ( hν = 400.7 eV) and at N 1s ionization ( hν = 412.5 eV). The results support the two H + desorption mechanisms proposed previously, that is, a four-step mechanism involving ultrafast H 2N–H extension within the lifetime of the N 1s core hole at 4a 1 ← N 1s, and a normal Auger stimulated ion desorption mechanism at N 1s ionization.

Photon stimulated ion desorption from poly(3-methylthiophene) following sulphur K-shell excitation

Photon stimulated ion desorption (PSID) studies have been performed in poly(3-methylthiophene) (PMeT) using synchrotron radiation at the sulphur 1s-edge. The PSID studies were performed at the Brazilian Synchrotron Light Source (LNLS), operating in a single-bunch mode. It was found that S 1s → π ∗ , σ ∗ (S–C) and higher energy excitations produce S + desorption with a partial ion yield curve similar to the photoabsorption spectrum. On the other hand, S 2+ desorption is enhanced at higher energy excitations. These results are interpreted solely in terms of the Auger-stimulated ion desorption mechanism, since no S + or S 2+ desorption was observed outside the sulphur 1s-edge. In addition, desorption of H + and other minor fragments of PMeT desorb mainly by XESD (X-ray induced electron stimulated desorption). To our knowledge, it is the first observation of multiple charged ions in PSID studies of polymers.

Ionic desorption in poly(methyl methacrylate) induced by fast electrons

Electron stimulated ion desorption (ESID) studies have been performed in poly(methyl methacrylate) (PMMA) using a time-of-flight mass spectrometer and a pulsed electron beam source with pulse duration of 0.18 μs (FWHM) and frequency of 3 kHz. The incident electron energy was 350–1200 eV. Several ionic fragments have been observed, H + being the main ionic species desorbed. Partial ion yields have been determined for the main fragments as a function of the electron energy. They show a maximum at 500–600 eV. Ionic desorption from PMMA is tentatively discussed in terms of the Auger stimulated ion desorption (ASID) mechanism. This is, to our knowledge, the first description of ESID on polymers.

Auger Stimulated Ion Desorption of Negative Ions viaK-Capture Radioactive Decay

We report on Auger stimulated ion desorption via Coulomb explosion from surface self-assembled alkylthiol and fluorocarbon molecular layers, triggered by K-capture decay of an imbedded radioactive 55Fe atom. The charge state of the ejecta is determined by charge exchange in binary atomic collisions in bulk and electron tunneling outside the solid, as well as by fragmentation of electronically excited molecules or molecular fragments. We describe the first nonbeam experiments documenting positive and abundant negative ion desorption due solely to core electron excitation after radioactive decay.

Photon-stimulated ion desorption for PMMA thin film in the oxygen K-edge region studied by Auger electron-photoion coincidence spectroscopy

Auger electron-photoion coincidence (AEPICO) measurements have been performed for poly-methylmethacrylate 1 (PMMA) thin films induced by oxygen core excitation. The dominant productions of CH ( n51-3) ions were observed at n 1 resonant excitation to O-CH anti-bonding state, and OCH ion was observed at resonant excitation to C-OCH 3 3 anti-bonding state. Their AEPICO yields were strongly dependent on Auger electron kinetic energy and shows enhancement around 505 eV Auger electron energy for each ion. This seems to be related to the spectator resonant Auger-stimulated ion desorption. The correlation between ion desorption and Auger final state for PMMA thin films in the oxygen K-edge region is discussed. © 2001 Elsevier Science B.V. All rights reserved.

Mechanism of ion desorption induced by core-electron transitions of condensed molecules and adsorbates studied by electron ion coincidence spectroscopy

Abstract Mechanism of ion desorption induced by core-electron transitions of condensed molecules and adsorbates on surfaces is studied by Auger electron photoion coincidence (AEPICO) spectroscopy combined with synchrotron radiation. In the case of O:1s ionization of condensed H 2 O ( hν =564 eV), H + desorption is attributed to the normal Auger stimulated ion desorption (ASID) mechanism, that is normal Auger final states are concluded to be responsible for H + desorption. One of the driving forces for the H + desorption is concluded to be the electron missing in the orbitals with O–H bonding character. At the 4a 1 ←O:1s resonance ( hν =533.4 eV), on the other hand, the ultrafast ion desorption mechanism is suggested to be favorable, that is, the repulsive potential energy surface of the (O:1s) −1 (4a 1 ) 1 state is responsible for the H + desorption. For H + desorption at 3p←O:1s ( hν =537 eV), the spectator Auger stimulated ion desorption mechanism is concluded to be probable. In the case of H 2 O chemisorbed on a Si(100) surface, the lifetime of the excited electron is found to be short in comparison with the time scale of H + desorption. The dominant H + desorption channels are attributed to multi-(more than two) hole states created by minor Auger processes, such as shake-up/off-like or cascade Auger decays. These investigations demonstrate the power of AEPICO spectroscopy to clarify the mechanism of ion desorption induced by core-electron excitations.

Ion Desorption Reaction of Surface Admolecules Induced by Synchrotron Radiation.

Studies of surface photochemical reactions induced by core excitation have developed with the advance of synchrotron radiation technique over a range extending from vacuum ultraviolet to soft X-ray. Studies of the photon stimulated ion desorption (PSID) have been carried out mainly on the basis of the initial excitation energy dependence of the desorption yield. In particular, site-specific reactions have been found in PSID following core electron excitation. This is very interesting from the viewpoint of the control of chemical reactions using monoenergetic photo-excitation. The Auger stimulated ion desorption (ASID) mechanism is widely proposed as a model of PSID. As the Auger final state is directly related to the ion desorption in this model to elucidate the PSID mechanism, it is essential to get the information about the Auger final state. In this article, we have described about PSID of formic acid induced by carbon core excitation using ion TOF method and about PSID of condensed H2O induced by oxygen core excitation using Auger electronphotoion coincidence (AEPICO) method.

Desorption of H ions from water chemisorbed on Si(100) by O 1s excitation - an Auger electron-photoion coincidence spectroscopy study

Auger electron-photoion coincidence (AEPICO) spectroscopy, which has been recently developed and proved to be a very powerful technique for investigating the dynamics of desorption induced by the core-level excitation, is applied to the investigation of Auger-stimulated ion desorption from the chemisorbed-water-Si(100) surface induced by O 1s excitation. It is shown that the fast relaxation of the excited state with a core hole and an excited electron takes place before the core hole decay, and that the desorption yield is enhanced by the shake-up (and/or shake-off) excitation. The relative cross-section for Auger-stimulated ion desorption is estimated, and is shown to increase as holes are created at deeper levels of the valence bands as the final state of the Auger decay. A comparison is also made with condensed H 2O.

Auger-electron-ion coincidence study of photon-stimulated ion desorption for condensed acetonitrile

Auger-electron-photoion coincidence (AEPICO) studies of photon-stimulated ion desorption (PSID) for condensed acetonitrile induced by carbon core excitation have been performed to elucidate the desorption mechanism related to the Auger process. We have detected only the H + ion in AEPICO spectra. The total ion yield spectrum divided by the total electron yield shows that the desorption efficiency is largely increased at the resonant excitation to CH∗. We have also measured the Auger electron spectrum and the AEPICO yield spectrum at the CH∗ excitation. The AEPICO yield spectrum shows enhancement at 245–250 eV electron energy. This seems to be related to the spectator resonant Auger stimulated ion desorption. That is, H + desorption is enhanced due to a two-hole-one-electron state at which the electron is in an anti-bonding CH∗ orbital and the two holes can be in a 1π bonding orbital localized on the CH 3 group. We have also measured similar spectra for other resonant excitation ( π∗ CN , σ∗ CC , σ∗ CN ). The results are discussed in connection with the bonding/anti-bonding character and localized character of the excited state.

Auger-final-state selected ion desorption study of condensed NH 3 and ND 3 by using Auger electron-photoion coincidence spectroscopy

Photostimulated ion desorption induced by core-level excitations was studied for condensed NH 3 and ND 3 by using Auger electron-photoion coincidence (AEPICO) spectroscopy with synchrotron radiation. Series of AEPICO spectra for condensed NH 3 and ND 3 were measured as a function of the electron kinetic energy around the N KVV Auger electron energy range. AEPICO yields of both H + and D + were enhanced at the electron kinetic energies corresponding to the N KVV Auger transitions, which provide clear evidence of an Auger-stimulated ion desorption mechanism. We analyzed the fine structure of the AEPICO yield spectra on the basis of the assignment of Auger final states, and suggested that Auger final states containing two holes in the same valence orbital efficiently lead to ion desorption. The AEPICO yield of D + was found to be one-third to one-half the yield of H +. We concluded that the isotope effect is derived from the reneutralization and recapture of D + on the surface because of the slow desorption velocity of D + compared with that of H +.

Study of ion desorption induced by core-level excitations of condensed Si(CH 3) 4 by using photoelectron-photoion coincidence spectroscopy (PEPICO) combined with synchrotron radiation

Photoelectron-photoion coincidence (PEPICO) spectra of condensed Si(CH 3) 4 were measured with C 1s and the Si 2p photoelectrons using synchrotron radiation. H + was found to be the only species desorbed in both cases. Furthermore, the H + yield in coincidence with the C 1s photoemission was found to be three times as large as that with the Si 2p photoemission. These results indicate that the Auger-stimulated ion desorption mechanism is responsible for the H + desorption, that the neutralization and recapture probabilities of the polyatomic ions are much larger than those of H +, and that the C 1s photoemission leads more efficiently to H + desorption than the Si 2p photoemission.