Ion Desorption Mechanism Research Articles

Study of ion desorption induced by carbon core excitation for poly-methylmethacrylate thin film using electron–ion coincidence spectroscopy

We have developed a new electron–ion coincidence apparatus combined with synchrotron radiation in order to examine the various ion desorption mechanisms related to the Auger process induced by core excitation. Photon stimulated ion desorption (PSID) of a poly-methylmethacrylate (PMMA) thin film has been investigated by this apparatus. The PSID of PMMA induced by carbon core excitation has been examined using Auger electron yield, total ion yield, resonant Auger electron, and Auger electron–photoion coincidence (AEPICO) spectra. The spectrum of the total ion yield divided by the Auger electron yield shows that the desorption efficiency is largely increased at the resonant excitation of carbon 1s electron in the O–CH3 side chain to σ*(O–CH3) orbital. In AEPICO measurement, H+ and CHn+ (n=1–3) ions are observed at various resonant excitations. The AEPICO signal intensity depends on the Auger electron energy. Particularly, the CH3+ ion desorption in coincidence with Auger electron at 270 eV shows strong enhancement with σ*(O–CH3) resonant excitation. The results of the resonant Auger spectra and AEPICO yield spectra demonstrate the relation of the ion desorption mechanism to the bonding/antibonding character and localized character of the excited σ*(O–CH3) orbital and the Auger final state.

Study of photon stimulated desorption on aluminum alloy with synchrotron light

Photon stimulated desorption (PSD) from the aluminum surface was investigated by synchrotron light at 50–150 eV. The photoelectron spectroscopy (PES) and the secondary ion mass spectroscopy (SIMS) were used to analyze the variation of surface compositions. Target currents of the aluminum alloy irradiated by synchrotron light were measured to interpret the mechanism of ion desorption. The depth profile analysis with ion beam sputtering was used for comprehensive understandings of the correlations between desorption behaviors and surface oxides.

Positron induced ion-desorption from the Ni surface

We performed the measurement of positron-induced hydrogen ion-desorption from a Ni surface, and estimated the effective cross-section of hydrogen ion-desorption with reemitted slow positrons experimentally. The positron-induced ion-desorption mechanism will be discussed.

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.

Control of chemical bond scission by using site-specific photochemical surface reactions

Using continuous variable synchrotron radiation, site selective core excitation can be achieved. The site-specificity of the consequent chemical reactions has been studied for adsorbates, thin films and molecules condensed onto a substrate. During the present research, a site-specific reaction was found in the case of photon stimulated ion desorption (PSID) of PMMA (polymethylmethacrylate) thin films. The PSID of condensed acetonitrile was also examined in order to elucidate the Auger process that follows resonant core excitation, and the ion desorption mechanism related to this Auger process. We discuss from the experimental results obtained so far, the possibility of controlling chemical bond scission through the use of site-specific photochemical surface reactions.

Ultra fast H+ Desorption from an Isolated NH3 Monolayer Adsorbed on a Xe Film Induced by a Resonant Core Electron Transition Studied by Auger Electron-Photoion Coincidence Spectroscopy

Mechanism of ion desorption from an isolated NH3 monolayer adsorbed on a Xe film (NH3/Xe) induced by a resonant core electron excitation is studied using Auger electron-photoion coincidence (AEPICO) spectroscopy. The total ion yield spectrum of NH3/Xe exhibits a threshold peak at the resonant excitation from N 1s to the 4a1 N-H antibonding orbital. The Auger electron spectrum of the isolated NH3 at the 4a1←N 1s resonant transition is found to be mainly due to spectator-Auger transitions. A series of AEPICO spectra at the 4a1←N 1s resonance is measured for the electron kinetic energies corresponding to the spectator Auger transitions. The AEPICO spectra show that H+ is the only desorbed ion species. The electron kinetic energy dependence of the H+ AEPICO yield displays a structure similar to that of the spectator-Auger electron spectrum. This result indicates that the H+ desorption probabilities are independent of the final states of spectator-Auger transitions. Based on these results, we suggest that the repulsive potential surface of the (N 1s)-1(4a1)1 state is responsible for the H+ desorption, that is, ultrafast H+ desorption mechanism is favorable in this system.

Auger electron photoion coincidence technique combined with synchrotron radiation for the study of the ion desorption mechanism in the region of resonant transitions of condensed H2O

The Auger electron photoion coincidence (AEPICO) technique has been applied for the study of H+ desorption induced by resonant excitations of O 1s of condensed H2O. The peak positions of the AEPICO yield spectrum at the 4a1←O 1s resonance (hν=533.4 eV) are found to correspond to spectator-Auger transitions leaving (O 2s)−2(4a1)1, (O 2s)−1(O 2p)−1(4a1)1, and (O 2p)−2(4a1)1 states. The H+ AEPICO yield is greatly enhanced at 4a1←O 1s while it is suppressed at 3p←O 1s (hν=537 eV) as compared with that at the O 1s ionization (hν=560 eV). On the basis of these results, the ultrafast ion desorption mechanism is suggested to be favorable for the H+ desorption at 4a1←O 1s, that is, the repulsive potential energy surface of the (O 1s)−1(4a1)1 state is responsible for the H+ desorption. For H+ desorption at 3p←O 1s, a spectator-Auger stimulated ion desorption mechanism is concluded to be probable. The suppression of the H+ AEPICO yield is ascribed to the reduction of the hole–hole repulsion due to the shield effect of the 3p electron. These results demonstrate the power of the AEPICO technique to clarify the mechanism of ion desorption induced by core–electron excitations.

Photon stimulated ion desorption of condensed acetonitrile molecules induced by core excitation

Photon stimulated ion desorption (PSID) of condensed acetonitrile molecules induced by carbon and nitrogen core excitation has been examined to elucidate the Auger process following resonant core excitation and the ion desorption mechanism related to this Auger process using total electron yield, total ion yield, resonant Auger electron spectra. Resonant Auger electron spectra show a strong dependence on the character of excitation. In particular, these Auger spectra are drastically changed between the carbon 1s to theπ * CN excitation and that of the C H * excitation. The spectrum of the total ion yield divided by the total electron yield shows that the desorption efficiency is largely increased at the resonant excitation of carbon 1s electron in the CH 3 group to C H * orbital. The results of the resonant Auger spectra and the Auger electron-photoion coincidence (AEPICO) measurements at various resonant excitations are discussed in relation to the bonding/anti-bonding character and localized character of the excited state and the Auger final state.

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.

Study of ion desorption induced by a resonant core-level excitation of condensed H 2O using Auger electron photo-ion coincidence (AEPICO) spectroscopy combined with synchrotron radiation

Ion desorption induced by a resonant excitation of O 1s of condensed amorphous H 2O has been studied by total ion and total electron yield spectroscopy, nonderivative Auger electron spectroscopy (AES) and Auger electron photo-ion coincidence (AEPICO) spectroscopy. The spectrum of total ion yield divided by total electron yield exhibits a characteristic threshold peak at hν = 533.4 eV, which is assigned to the 4a 1 ← O 1s resonant transition. The AES at the 4a 1 ← O 1s resonance is interpreted as being composed of the spectator-AES of the surface H 2O, and the normal-AES of the bulk H 2O, where the 4a 1 electron is delocalized before Auger transitions. H + is found to be the only ion species in AEPICO spectra measured at the 4a 1 ← O 1s resonance and at the O 1s ionization ( hν = 560 eV). The electron kinetic energy dependence of the AEPICO yield (AEPICO yield spectrum) at the 4a 1 ← O 1s resonance is found to be greatly different from that at the O 1s ionization. The peak positions of the AEPICO yield spectrum at the 4a 1 ← O 1s resonance are found to correspond to those of the spectator-AES of the surface H 2O, which is extracted from the AES at the 4a 1 ← O 1s resonance. Furthermore, the AEPICO yield is greatly enhanced at the 4a 1 ← O 1s resonance as compared with that at the O 1s ionization. On the basis of these results, a spectator-Auger-stimulated ion desorption mechanism and/or ultra-fast ion desorption mechanism are concluded to be responsible for the H + desorption at the 4a 1 ← O 1s resonance. The enhancement of the H + yield is ascribed to the OH anti-bonding character of the 4a 1 orbital.

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.

Auger electron-ion coincidence study for H 2O adsorbed on [formula omitted] at 80 K

Electron stimulated ion desorption induced by core level excitations has been studied for H 2O condensed on a SiO 2 Si(111) surface at 80 K [H 2O/SiO 2/Si(111)] by using Auger electron-ion coincidence spectroscopy. The coincidence H + yield from the H 2O/SiO 2/Si(111) was found to be enhanced at the electron kinetic energies corresponding to O(KVV) Auger transitions. This result presents direct and clear evidence of an Auger stimulated ion desorption mechanism accompanied by the cleavage of a covalent bond. A KV NB V NB Auger process is concluded to lead to ion desorption less efficiently than KV BV NB and KV BV B processes (V NB and V B represent the non-bonding and bonding valence orbitals, respectively).

Development of Electron-Ion Coincidence Spectroscopy for the Study of Surface Dynamics

Abstract Electron-ion coincidence (EICO) spectroscopy for studying surface dynamics has been developed. The apparatus consists of an electron gun, a cylindrical mirror analyzer (CMA), and a time-of-flight ion mass spectrometer (TOF-MS). A sample surface is excited with an electron beam, and the energy of the emitted electrons is analyzed by the CMA. The TOF spectra of desorbed ions are measured with the TOF-MS and a multichannel scaler, taking the energy-analyzed electron signal as the starting trigger. Those ions coincidently desorbed with electrons give a characteristic peak in the TOF spectrum. The apparatus has been evaluated on the basis of electron-ion coincidence measurements of an F/SrTiO3 (100) surface. The F+ coincidence signal was observed to be enhanced at electron kinetic energies of 650—655 eV, corresponding to the F(KL23L23) Auger transition. The result provides direct evidence of the Auger-stimulated ion-desorption mechanism for an ionically bonded surface (the Knotek–Feibelman (KF) mechanism).

Photon-stimulated ion desorption and photoelectron spectroscopy of a chlorine-adsorbed GaAs(111)B surface

Synchrotron radiation has been employed to study photon-stimulated ion desorption and X-ray photoelectron spectroscopy of chlorine-adsorbed GaAs(111)B surfaces. Upon chlorine adsorption, the Ga 3d and As 3d core levels are shifted by 0.55 eV and 0.83 eV, respectively, to higher binding energy. The photon-stimulated desorption products observed for the chlorine-adsorbed surface are dominantly Cl + ions. The photon energy dependence of Cl + ion yield suggests that photo-ionization of Cl core levels is much more effective for the ion desorption than that of As core levels. The PSD results obtained are discussed from the viewpoint of the Auger process-related ion desorption mechanism.

The Mechanism of Ion Desorption from H2O/Si(100) by O 1s Electron Excitation

The photon stimulated ion desorption (PSID) of H2O on the Si(100) surface has been studied in the photon energy region of 500-750 eV. Not only H+ but also O+ ion was observed. PSID yield curves of these ions indicate characteristic behavior near and above the O K edge; H+ ion exhibits sharp rises at ca. 530 eV and two broad peaks below (ca. 533 eV) and above (ca. 555 eV) the O K-edge, O+ exhibits a delayed threshold at ca. 570 eV and gradual increase up to 700 eV. The O KVV auger electron yield curve, the photoion-photoion coincidence spectrum and the gas-phase photo fragment ion spectrum have also been obtained and compared with the PSID curves. The results are consistent with a mechanism of formation of multiple charged OHm+ (m≥3) ions followed by reneutralization of the excess of charge with strong interactions with the substrate and finally desorption as single charged H+ and O+ ions.

Mechanism of ion desorption by electronic transitions: A density-functional study.

Etude du mecanisme de la desorption ionique et a l'etat neutre via les transitions electroniques grâce a la theorie de la fonctionnelle de densite. On utilise F sur Al comme prototype

Resonant electron and ion emission and desorption mechanism in rare earth oxides

Resonant electron and ion emission and desorption mechanism in rare earth oxides

Resonant electron and ion emission and desorption mechanism in rare earth oxides

The resonant enhancement in photoelectron spectra at the 4d edges of rare earth atoms and metals is also found in yield spectra of desorbed ions from the surfaces of the oxides of Sm, Eu and Yb following the photon excitation. The analysis of the 4d → 4f resonance leads to a picture of an indirect mechanism of ion desorption which is mainly caused by the flux of energetic 4f photoelectrons from the bulk. In this case the dominant desorption through secondary processes limits the use of the photon-stimulated desorption (PSD) to determine to which type of atom the desorbing species was attached.

Resonant photoemission and the mechanism of photon-stimulated ion desorption in a transition-metal oxide.

Resonant photoemission and the mechanism of photon-stimulated ion desorption in a transition-metal oxide.