Auger Stimulated Ion Desorption Mechanism Research Articles

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.

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.

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.

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-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 +.

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).