Ion Desorption Mechanism Research Articles

Adsorption of mercury ions from synthetic aqueous solution using polydopamine decorated SWCNTs

Mercury is one of the most toxic and unnecessary elements for human body in a way that only slight amount of it can cause serious hygienic problems and environmental pollution. In the present study, magnetized single walled carbon nanotubes (SWCNTs) functionalized by polydopamine was employed for elimination of mercury heavy metal ions from aqueous media. In order to characterize the physical and chemical properties of the SWCNT and to confirm the functionalization steps, different analyses including TEM, SEM, VSM, EDS, FT-IR were utilized. In the present paper, the effects of experimental parameters such as adsorbent dosage, solution pH, metallic ion initial concentration, process thermodynamics and desorption mechanism on batch system were widely studied. In addition, mercury adsorption trend in continuous system was investigated by a fixed bed reactor. Comparison of adsorption process kinetics were done with models of Freundlich, Langmuir and Dubinin–Radushkevich in batch system and consistency with Yan and Thomas models in continuous system by Sigma plot and MATLAB software packs. Isotherm studies and adsorption kinetics revealed that mercury ions follow Freundlich adsorption isotherm and pseudo-second-order kinetic model. Moreover, thermodynamic studies showed that mercury ion adsorption is an exothermic and spontaneous reaction. Although Thomas model was found to predict more exact results for maximum mercury ion adsorption, both Thomas and Yan models had good fitness with fixed bed reactor data. Finally, adsorption process demonstrated that removal efficiency of the prepared adsorbent was so much higher than raw SWCNTs.

New ion desorption mechanism from rare gas solids by multiply charged ion impacts

This study measured the absolute ion desorption yield from the surface of solid Ne and Ar by low-energy Neq+ (q = 1–4), Arq+ (q = 1–6), and Krq+ (q = 2–6) ion impacts. Results revealed the dependence of ion desorption yield on the potential and the kinetic energies of the incident ions, as well as its linear proportionality with the former. Present results could not be explained by existing models for ion desorption from insulator surfaces. We proposed a new mechanism of ion desorption that considers the contribution of both energy types to ion desorption.

New mechanism of ion desorption from rare gas solids by multiply-charged ion impact

SynopsisWe have measured the absolute ion desorption yield from the surface of solid Ne and Ar by low-energy multiply-charged ion impact. Present results could not be explained by well-known Coulomb explosion model. We here propose a new mechanism of ion desorption.

On the mechanism of ion desorption in the process of laser desorption/ionization from silicon surfaces

The mechanism of ion desorption in the process of laser desorption/ionization from silicon surfaces is studied using pyridine and N,N-dimethyl-1-phenylethylamine as examples. Based on the experimental and theoretical results, dependences of ion signal on the surface temperature are obtained for two different wavelengths of laser radiation, 355 nm and 532 nm. The theoretical part of the work includes numerical calculations of surface temperatures of amorphous silicon using the SLIM software package and quantum-chemical calculations of binding energy between the ions and silicon surface using the Hartree-Fock method and Firefly software package. It is demonstrated that the ions are desorbed via a thermal mechanism at temperatures much lower than the melting point of amorphous silicon.

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.

Potential sputtering of ionic species from rare gas solids by multiply charged ionimpact

The interaction of low-energy multiply chargedArq + (q ≤ 7) ions with a solid Ne surface is experimentally studied. Desorption of very large cluster ionsNen + (n > 100) is observed. The size distribution of smaller (n = 1–3) cluster ions depends strongly on the charge state of the incident ion, whereas that of larger (n > 7) cluster ions exhibits no dependence on the charge state, indicating that desorption oflarge cluster ions is due to kinetic sputtering. The potential sputtering yield is estimated byanalyzing the size distribution of the desorbed cluster ions. The results suggest that the iondesorption mechanism, which is known as desorption induced by electronic transitions, canalso be applied to explain the present results.

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.

Recent progress in coincidence studies on ion desorption induced by core excitation

This paper reports on recent studies of photostimulated ion desorption (PSID) usingelectron ion coincidence (EICO) spectroscopy combined with synchrotron radiation.H+ desorption fromH2O dissociatively adsorbedon Si(111) and SiO2/Si(111) surfaces (H2O/Si(111) and H2O/SiO2/Si(111)) was studied for Si L-edge excitation. The Si2p–H+ photoelectron photoion coincidence (PEPICO) and Si 2p photoelectron spectra ofH2O/Si(111) andH2O/SiO2/Si(111) showthat H+ desorption probability increases as the number of positive charges at the Si site increases. TheH+ desorption probability per Si 2p ionization for theSi4+ site was estimated andfound to be 5–7 × 10−5. Weproposed a mechanism that H+ desorption is induced by Si 2p photoionization accompanied by a Si LVVV double-Augertransition. This article also reviews recent EICO work on site-specific ion desorption of1,1,1-trifluoro-2-propanol-d1 (CF3CD(OH)CH3)adsorbed on Si(100) surfaces, and on the mechanisms of PSID of poly(tetrafluoroethylene) (PTFE)and TiO2(110). Clear site-specific ion desorption was observed for the C 1s core excitation of aCF3CD(OH)CH3 sub-monolayer on Si(100). A spectator-Auger-stimulated ion-desorption mechanism was proposedfor F+ desorption induced by a transition from F 1s toσ(C–F)* of PTFE.O+ desorption inducedby O 1s excitation of TiO2(110) was attributed mainly to three-hole final states resulting from multi-electron excitation/decay. ForO+ desorption induced byTi core excitation of TiO2(110), on the other hand, charge transfer from an O 2p orbital to a Ti 3d orbital, instead of theinteratomic Auger transition, was proposed to be responsible for the desorption. Theseinvestigations demonstrate that EICO spectroscopy combined with synchrotron radiation isa useful tool for studying PSID.

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.

Isotope effects in H +(D +) desorption induced by 4a 1 ← O 1s resonant transition of condensed H 2O (D 2O)

Auger electron–photoion coincidence (AEPICO) experiments have been performed for condensed water (H 2O and D 2O) at the 4a 1 ← O 1s resonant transition. Several kinds of isotope effects were observed in H + (D +) desorption, they were, (1) the differences in ion kinetic energy for the selected Auger-final-states, (2) the relative intensity differences in AEPICO spectra, (3) peak shifts in AEPICO and resonant Auger electron spectra and (4) the intensity difference in prominent peak in ion yield spectra. Those isotope effects can be explained by a four-step ion desorption mechanism involving O–H (O–D) bond extension within the lifetime of the O 1s core-hole.

Polarization-dependent dissociation selectively induced by core-electron excitation in methyl ester terminated self-assembled monolayer

Polarization dependence of ion desorption for methyl ester terminated self-assembled monolayer was investigated in carbon and oxygen core-excitations in order to clarify ion desorption mechanism. Site-selective ionic dissociation is induced by the C 1s,O 1s(OCH 3) → σ *(O CH 3) and O 1s(OCH 3) → σ *(C OCH 3) excitations. Ions site-selectively desorbing in these excitations strongly depend on SR incident angle. On the other hand, ion yields in non-selective excitations indicate almost no dependence. Difference in the polarization dependence is considered due to the dissociation mechanisms characterized as direct and indirect dissociation processes.

Ion Desorption by Inner-shell Excitation and Photodegradation of Poly(vinylidene fluoride)

ABSTRACTAuger electron spectra (AES) and Auger electron photo-ion coincidence (AEPICO) spectra of poly(vinlylidene fluoride) (PVDF) were observed to study the mechanism of effective fluorine ion (F+) desorption by irradiation of photons corresponding to the transition from F 1s to s(C-F). In the AES at photon energy (hυ) = 690.3 eV (from F1s to |σ|(C-F)), the spectator-Auger shift is about 3 eV. At hn = 690.3 eV the contribution of spectator Auger electron to the observed Auger spectrum is large. In the F+ AEPICO spectra at hυ = 690.3 eV an intense peak appears. The peak positions of the F+ AEPICO spectra are in agreement with those of the Auger spectra. These results indicate that the mechanism for effective F+ ion desorption induced by the transition from F1s to σ(C-F) occurs through the spectator-Auger processes. The efficiency of ion desorption depends on the electronic structure of the spectator-Auger final state.

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.

極角分解ミニチュア飛行時間型イオン質量分析器の製作と評価，電子−イオンコインシデンス分光への応用

Coincidence measurement of energy-selected electrons and mass-selected ions (electron-ion coincidence (EICO) spectroscopy) is a powerful technique to clarify ion desorption mechanism induced by electron transitions, because excited states leading to ion desorption is directly identified. Information on the coincidence ions, however, is limited to the mass and the yield so far. In order to obtain information on kinetic energy and desorption polar angle of ions, we have developed a new polar-angle-resolved miniature time-of-flight ion mass spectrometer (TOF-MS), which can be used for EICO spectroscopy. The TOF-MS consists of a shield for electric field, an ion drift electrode with three meshes, and microchannel plates with three concentric anodes. By using SIMION 3D version 7.0 we simulated ion trajectories of H+ for the TOF-MS with a drift bias of -30 V. The results show that the desorption angles of H+ with a kinetic energy of 3 eV detected by the innermost anode, the intermediate anode, and the outermost anode are 0°17°, 22°48°, and 57°90°, respectively. The performance of the TOF-MS was evaluated by measuring H+ yield spectra and Auger electron-H+coincidence spectra of condensed water (H2O).

Ion desorption from molecules condensed at low temperature: A study with electron-ion coincidence spectroscopy combined with synchrotron radiation (Review)

This article reviews our recent work on photostimulated ion desorption (PSID) from molecules condensed at low temperature. We have used electron–ion coincidence (EICO) spectroscopy combined with synchrotron radiation. The history and present status of the EICO apparatus is described, as well as our recent investigations of condensed H2O, NH3, CH3CN, and CF3CH3. Auger electron photon coincidence (AEPICO) spectra of condensed H2O at the O:1s ionization showed that H+ desorption was stimulated by O:KVV Auger processes leading to two-hole states (normal-Auger stimulated ion desorption (ASID) mechanism). The driving forces for H+ desorption were attributed to the electron missing in the O–H bonding orbitals and the effective hole–hole Coulomb repulsion. The normal ASID mechanism was also demonstrated for condensed NH3. The H+ desorption at the 4a1←O(N):1s resonance of both condensed H2O and condensed NH3 was found to be greatly enhanced. Based on the AEPICO spectra the following four-step mechanism was proposed: (1) the 4a1←1s transition, (2) extension of the HO–H (H2N–H) distance within the lifetime of the (1s)−1(4a1)1 state, (3) spectator Auger transitions leading to (valence)−2(4a1)1 states, and (4) H+ desorption. The enhancement of the H+ desorption yield was attributed to the repulsive potential surface of the (1s)−1(4a1)1 state. At the 3p←O:1s resonance of condensed H2O, on the other hand, the H+ yield was found to be decreased. The AEPICO spectra showed that the H+ desorption was stimulated by spectator Auger transitions leading to (valence)−2(3p)1 states. The decrease in the H+ yield was attributed to a reduction in the effective hole–hole Coulomb repulsion due to shielding by the 3p electron. Photoelectron photon coincidence (PEPICO) spectra of condensed H2O showed that the core level of the surface H2O responsible for the H+ desorption was shifted by 0.7 eV from that of the bulk H2O. The H+ desorption from condensed CH3CN was also investigated. In a study of condensed CF3CH3 using PEPICO spectroscopy, site-specific ion desorption was directly verified; that is, H+ and CH3+ desorption was predominant for the C:1s photoionization at the -CH3 site, while C2Hn+, CFCHm+, and CF3+ desorption was predominantly induced by the C:1s photoionization at the -CF3 site. These investigations demonstrate that EICO spectroscopy combined with synchrotron radiation is a powerful tool for studying PSID of molecules condensed at low temperature.

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.

Femtosecond time-resolved photo-stimulated desorption from ionic crystals

We have used the pump–probe technique to measure the positive ion yield, from ionic crystals, as a function of time delay between two femtosecond laser pulses. The two-pulse technique allows direct observation of solid state and surface dynamics on a femtosecond timescale. We find the ion yield, from 265 nm irradiated MgO and KBr, depends critically on the time delay between pulses. For example, the Mg + desorption yield displays three distinct features; a coherence peak followed by a fast rise and decay features. In contrast, the yield of K + from KBr displays only the coherence peak and picosecond decay features. The observed ion detection thresholds suggest that, although the nanosecond laser ion desorption mechanism may be dominated by defect photoabsorption, significant electron–hole pair production may contribute to the femtosecond laser desorption mechanism. By determining the ultrafast time-dependence of positive ion emission, we hope to reveal the mechanism of laser ion desorption for both regimes.