Photon Stimulated Ion Desorption Research Articles

Site-selective photofragmentation of chlorinated polymeric films observed around the chlorine K-edge

Photon stimulated ion desorption (PSID) and Near-edge X-ray absorption fine structure (NEXAFS) studies have been performed on poly(vinyl chloride) (PVC) and poly(vinyl dichloride) (PVDC) around the chlorine 1s-edge. Experiments were performed using a synchrotron source operating in the single-bunch mode and a time-of-flight mass spectrometry for ion analysis. Cl+ ion yields, as a function of the photon energy, reproduce the photoabsorption spectrum, showing significant increase at the 1s-resonance. Edge-jump ratios, defined as the ratio between edge-jumps (intensity ratio of the yields between above and below the absorption edge) of two different transitions, for Cl+ ion yields were much higher than the equivalent electron yields, indicating site-selectivity in C–Cl bond breaking for both polymers, as a result of efficient spectator Auger decay. The expected isotope ratio of 3:1 for chlorine was measured for PVC. The interpretation of the NEXAFS spectrum was assisted by quantum mechanical calculations at a multireference perturbation theory level.

Electron and Photon Stimulated Ion Desorption from Poly(thiophene)

Poli(tiofeno) (PT) foi estudado atraves de tecnicas de dessorcao estimulada por eletrons (ESID) e fotons (PSID) acopladas a espectrometria de massas por tempo-de-voo (TOF-MS), a fim de investigar a resposta desse polimero, quando submetido a feixes de eletrons e fotons. Ions atomicos de enxofre e suas contribuicoes isotopicas foram claramente identificados. A intensidade maxima das curvas de rendimento ionico de ESID foi medida para todos os ions em cerca de 900 eV, estao relacionadas com o processo de ionizacao do carbono 1s e explicadas em termos do processo Auger. O estudo da influencia de eletrons secundarios no processo de desorcao foi tambem realizado, o que e altamente importante, uma vez que envolvem niveis mais baixos de energia, afetando as propriedades de interesse de materiais fotovoltaicos. A analise de TOF-PSID de filmes de PT com diferentes espessuras mostrou um aumento na eficiencia da dessorcao de especies mais pesadas, como C3HS + . Este resultado foi atribuido aos eletrons secundarios originados nas camadas internas do material. Poly(thiophene) (PT) was studied by the electron (ESID) and photon (PSID) stimulated ion desorption techniques coupled with time-of-flight mass spectrometry (TOF-MS) in order to investigate the response from this semiconducting polymer when submitted to electron and photon beams. Atomic sulfur ions and its isotopic contributions could be clearly identified. Maximum intensity in the ESID ion yield curves was measured for all ions around 900 eV, which may be related to carbon 1s ionization and explained in terms of the Auger stimulated ion desorption process. The study of the influence of secondary electrons for the desorption process was also investigated, which is highly important, since they involve lower energy levels, affecting the properties of interest of photovoltaic materials. TOF-PSID analysis on PT films with different thicknesses showed an increase in the efficiency to desorb higher mass species, like C 3HS + . This result was attributed to the role of secondary electrons originated in the inner layers of the material.

Study on selective adsorption of deuterium on boron nitride using photon-stimulated ion-desorption

Adsorption behavior of atomic deuterium on a hexagonal boron nitride (h-BN) thin film is studied by photon-stimulated ion desorption (PSID) of D+ and near edge X-ray absorption fine structure (NEXAFS) at the B and N K-edges. After the adsorption of atomic deuterium, D+ desorption yield η(hν) shows clear enhancement at the B K-edge and almost no enhancement at the N K-edge. NEXAFS spectra show a large change in the B K-edge and a small change in the N K-edge after the adsorption. We propose selective adsorption of atomic deuterium on the h-BN thin film based on the experimental results, and mention the effectiveness of applying the PSID method with X-ray to study hydrogen storage materials.

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.

Polymer Surface Functionalization Using Plasma, Ultraviolet and Synchrotron Radiation

Polyurethane (PU) and polystyrene (PS) films were functionalized by ultraviolet (UV) or selective synchrotron radiations (SR) in the presence of reactive gases. The UV-PU results were compared with lowpressure plasma treatments of the same films. Oxygen or acrylic acid vapours (AA) were used as reactive gases. X-ray photoelectron spectroscopy measurements of UV modified films in the presence of oxygen or AA matched the RF-plasma treatments results. It is shown that COO and C=O functional groups were incorporated at the polymer surface efficiently with both methodologies. In addition, near-edge X-ray absorption fine structure showed that a thin film of poly(acrylic acid) is formed over the PU and PS films during the UV irradiation in the presence of AA vapours. These results resemble previous AA low-power plasma treatments. PU and PS films were also selectively functionalized by SR using oxygen as reactive gas. Surface concentrations of COO and C=O functional groups were enhanced by C1s → σ* C–C excitation after irradiation and oxygen introduction. This efficient surface functionalization was clearly observed in PS films which do not have CO and COO groups in their molecular structure. Excitations involving transitions to π* orbital (π*C=C, π*C=O) led to much lower functionalization efficiency. The SR results can be explained by taking into account previous photon stimulated ion desorption studies of polymers. SR results may open new ways to functionalize polymer surfaces selectively and efficiently.

Photoresists comparative analysis using soft X-ray synchrotron radiation and time-of-flight mass spectrometry

Positive photoresists are widely used in lithographic process for the fabrication of relief components. When exposed to UV radiation they suffer chemical reactions modifying their chemical and physical properties. Aiming to follow molecular modifications among two different photoresists unexposed and previously exposed to ultraviolet light we have employed spectroscopic techniques coupled with mass spectrometry in the study of the AZ-1518 and AZ-4620 photoresists. The photon stimulated ion desorption (PSID) technique following the S K-edge NEXAFS spectrum was employed at the brazilian synchrotron light source (LNLS), during single-bunch operation and using time-of-flight mass spectrometry (TOF-MS) for ion analysis. NEXAFS and PSID mass spectra on both AZ-1518 and AZ-4620 photoresists (unexposed and exposed) were obtained and relative desorption ion yield curves determined for the main fragments as a function of the photon energy. They present marked different PSID spectra. Fragments related to the photochemical decomposition of the AZ-1518 photoresist could be clearly identified differently from the AZ-4620. Studies on the hardness of both photoresists were performed using O 2 plasma reactive ion etching (RIE) technique, analyzed by scanning electron microscopy (SEM) and used to explain different desorption yields in the PSID spectra. These results show that the PSID technique is adequate to investigate structural changes in molecular level in different unexposed and exposed photoresists, which is crucial for improving our knowledge about the breakup process.

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.

Photoabsorption and desorption studies on poly-3-hexylthiophene/multi-walled carbon nanotube composite films

Photon stimulated ion desorption (PSID) and Near-edge X-ray absorption fine structure (NEXAFS) studies have been performed on poly-3-hexylthiophene and nanocomposites thin films made of poly-3-hexylthiophene/multi-walled carbon nanotubes (MWNT) filled with iron/iron-oxide. The experiments were performed 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. Both PSID mass spectra show great similarity and exhibit desorption of the polymer fragments only. This result seems to be in accordance with previous morphological studies on these materials, which suggested that the nanotubes are highly dispersed and involved by the polymer. Although similar, the spectra present shifts in the direction of greater time-of-flights in the case of the poly-3-hexylthiophene/multi-walled carbon nanotube composite. This behavior may be related to the donation of electronic charge between the polymer matrix and the carbon nanotubes. In both cases, S + desorption seems to be suppressed, which may be due to the hexyl side-chains. Relative desorption ion yield curves have been determined as a function of the photon energy, which reproduced the photoabsorption spectrum. These results are discussed in terms of the indirect XESD (X-ray induced electron stimulated desorption) process.

Enhancement effect in photon-stimulated ion desorption for benzene adsorbed on silicon surfaces observed using angle-dependent technique

We have investigated photon-stimulated ion desorption from deuterated benzene (C 6D 6) adsorbed on Si(1 0 0) and Si(1 1 1) surfaces following C 1s core excitation. Using time-of-flight mass spectrometry combined with angle-dependent technique, we measured the dependences of mass-spectra of desorption ions on photon energies and on incident angle ( θ) of synchrotron beam. We have found the ion yields for adsorbate-derived fragments of CD + and CD 2 + are enhanced in very small angles of incident X-rays. Moreover, molecular orientation effect appeared in excitation energy dependences of D + ions from the Si(1 0 0) and Si(1 1 1) surfaces; that is, ion yield spectra measured at θ = 10° are different from that at θ = 65°. Furthermore, it was found that desorption ion yields increase greatly with decreasing incident angles. The angular dependences are consistently similar for all ion species, excitation energies, and indexes of substrates. Possible desorption processes are described on the basis of the observations.

Configuration dependence of photon stimulated ion desorption from methyl ester compounds induced by core excitation

Photon stimulated ion desorption (PSID) from methyl ester terminated self-assembled monolayer (MHDA-SAM, HS(CH 2) 15COOCH 3) and methyl mercaptoacetate (MA, HSCH 2COOCH 3) on Ag has been investigated using soft X-ray in the C and O K-edge regions. In MHDA-SAM on Ag, site-selective ion desorption has been clearly observed at resonant core excitations of C 1s, O 1s(OCH 3) → σ ∗(O CH 3) and O 1s(OCH 3) → σ ∗(C OCH 3). Ion intensity in MA on Ag is obviously reduced for CH n + ( n = 1–3) at C 1s, O 1s(OCH 3) → σ ∗(O CH 3) excitations, and no site-selective reaction at O 1s(OCH 3) → σ ∗(C OCH 3) excitations has been observed. These reactions may be influenced by configurational difference of reactive sites. It is suggested that surface effects on the selective reaction due to positioning methyl ester group near the surface plays an important role.

AZ-1518 Photoresist analysis with synchrotron radiation using high-resolution time-of-flight mass spectrometry

With the aim of identifying molecular modifications among photoresists unexposed and previously exposed to the ultraviolet light the photon stimulated ion desorption (PSID) technique was employed in the study of the AZ-1518 photoresist. Data acquisition was performed at the Brazilian Synchrotron Light Source (LNLS), during a single-bunch operation mode of the storage ring and using high-resolution time-of-flight mass spectrometry (TOF-MS) for ion analysis. PSID mass spectra on both photoresists (unexposed and exposed) were obtained following the S K-shell photoexcitation and desorption ion yield curves have been determined for the main fragments as a function of the photon energy. The AZ-1518 photoresists presented different PSID spectra, showing characteristic fragments. Most of the analyzed ions showed larger relative yields for the exposed photoresist. Fragments related to the photochemical decomposition of the photoresist could be clearly identified. These results showed that the PSID technique is adequate to investigate structural changes in molecular level in unexposed and exposed photoresists.

Photon stimulated ion desorption from condensed thiolane photoexcited around the S 1s-edge

Photon stimulated ion desorption (PSID) and Auger decay spectra were measured for condensed thiolane around the S 1s-edge. For thiolane a much richer fragment ions were observed in PSID mass spectrum than that for thiophene. Partial ion yield (PIY) curves have been determined for the fragment ions as a function of the photon energy. The first resonance in thiolane, attributed to the resonance from S 1s into an antiboning σ *(C–S) state, seems to induce highly specific S + desorption as compared to thiophene. Furthermore, it was found that the indirect X-ray induced electron stimulated desorption (XESD) mechanism plays an important role in ionic desorption from thiolane with exception for S + ions.

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 from condensed thiophene photoexcited around the S1s-edge

Photon stimulated ion desorption and Auger decay spectra were measured for condensed thiophene around the S1s-edge. Three kinds of ions, H+, S+, and S2+, were observed as main desorbed species. Partial ion yields (PIY) have been determined for these three fragment ions as a function of the photon energy. It was found that the indirect x-ray induced electron stimulated desorption mechanism plays an important role in H+ ion desorption. For S+ ion desorption, on the other hand, a pronounced enhancement at ∼3eV away from the first core-to-valence resonance was observed in the PIY curve. On the basis of the results for the Auger decay spectra, the enhancement of S+ desorption at ∼3eV away from the first resonance is interpreted in terms of the effective screening of positive holes due to the existence of Rydberg electrons.

Photon stimulated ion desorption of condensed CO2 at ~ 85 K studied by synchrotron radiation

Photon stimulated ion desorption (PSID) from condensed carbon dioxide has been studied for photon excitation energies ranging from 93 to 193 eV. PSID studies have been performed at the Brazilian synchrotron light source (LNLS), Campinas, during a multi-bunch operation mode of the storage ring. The results showed that after photon excitation several ions desorbed from the CO2 films: C+ , O+ , CO+ and O2+. PSID experiments showed that ion desorption was enhanced only at the Si resonance excitations. When the thickness of the CO2 was ~ 500 L or higher, almost no desorption yield was observed. The study of the dependence of the relative partial ion yield on the photon excitation showed that the X-ray induced Electron Stimulated Desorption (XESD) mechanism has to be invoked to explain the origin of the desorbed ions in the energy region studied.

Photon stimulated desorption of hydrogen from diamond surfaces via core-level excitations:fundamental processes and applications to surface studies

In this paper we review a number of works on photon stimulated ion desorption (PSID) ofhydrogen from various diamond surfaces. In particular, we elaborate on the bondingconfiguration of adsorbed hydrogen (deuterium) and its neighbouring atoms, and itsthermal stability in surface and sub-surface regions. Usage of hydrogen PSID as acharacterization method involves detailed description of the mechanism and dynamics ofhydrogen ion desorption initiated by a synchrotron generated photon beam. Severalcomplimentary analytical techniques (near edge x-ray absorption fine structurespectroscopy, x-ray photoelectron spectroscopy, secondary electron emission measurements)were used to clarify the results of the PSID measurements. The studied systems includehydrogenated/deuterated single-crystal and polycrystalline diamonds, diamond filmshydrogenated ex situ using microwave hydrogen plasma versus in situ hydrogenation bypassing hydrogen gas through a hot tungsten filament, hydrogenated diamond filmsexposed to atomic and molecular oxygen gas, and ion beam damaged diamond films.

Ionic desorption in valence- and core- excited polymers: poly(vinyl chloride) and poly(vinylidene chloride)

Photon stimulated ion desorption (PSID) studies have been performed in poly(vinyl chloride) (PVC) and poly(vinylidene chloride) (PVDC) using synchrotron radiation, covering from valence to core electron (Cl 2p and C 1s) energy ranges. Data acquisition was performed at the TGM beam line from the Brazilian Synchrotron Light Source (LNLS), operating in a multi-bunch mode and using a time-of-flight mass spectrometer (TOF-MS). A new pulsed system developed uses as a trigger for the TOF-MS experiments the pulsed extraction high voltage applied to the sample. Ionic desorption from PVC and PVDC shows strong selectivity in the formation of chlorine ions around the Cl 2p-edge while very similar fragmentation patterns are observed for the other energies studied.

Ionic fragmentation of C 1s excited and ionized formic acid

Time of flight mass spectrometry with multi-ion coincidence detection has been used to investigate ionic photofragmentation of four isotopes of gaseous formic acid [HCOOH, HCOOD, DCOOH and DCOOD] following C 1s excitation and ionization. C 1s spectra, branching ratios and quantitative yields of ions and ion pairs are reported. For equivalent ionic decay channels in the four isotopes the partial ion and ion pair yield spectra are very similar. These gas phase results are compared to results from photon stimulated ion desorption (PSID) of deuterated formate adsorbed on Si(1 0 0) [H. Ikeura-Sekiguchi, T. Sekiguchi, K. Tanaka, Phys. Rev. B 53 (1996) 12655] where there is a very strong enhancement of the D + yield at C 1 s → σ C - D ∗ resonance and the COD + yield at the C 1 s → σ C – O ∗ resonance. Such enhancements were not observed in the fragmentation of gas phase formic acid. From this we conclude that these very specific bond breaking processes in surface adsorbed formate are associated with a minority channel such as a (2 h, 1 e) spectator Auger process, which becomes prominent through elimination of the dominant molecular fragmentation mechanisms by electronic recombination.

X-ray initiated molecular photochemistry of Cl-containing adsorbates on a Si(1 0 0) surface using synchrotron radiation

X-ray initiated molecular photochemistry for SiCl 4 and CCl 4 adsorbed on Si(1 0 0) at ∼90 K following Cl 2p core-level excitation is investigated by photon stimulated ion desorption and ion kinetic energy distribution measurements. The Cl 2 p → 8 a 1 ∗ excitation of solid SiCl 4 induces the significant enhancement (∼900%) of the Cl + yield, while the Cl 2 p → 7 a 1 ∗ excitation of condensed CCl 4 leads to a moderate enhancement (∼500%) of the Cl + yield. The enhancement of Cl + yield at the specific core-excited states is strongly correlated to the ion escaped energy. Upon X-ray exposure for CCl 4 adsorbed on Si(1 0 0) (20-L exposure), the Cl + yields at 7 a 1 ∗ resonances decrease and new structures at higher photon energies are observed. Cl + yields at these new resonances are significantly enhanced compared to those at other resonances. These changes are the results of desorption and surface reaction of the CCl 4–Si surface complex due to X-ray irradiation. We have demonstrated that state-specific enhancement of ion desorption can be successfully applied to characterize the reaction dynamics of adsorbates adsorbed on surfaces by X-ray irradiation.