Shin-ichi Nagaoka, Kazuhiko Mase, and Inosuke Koyano
[Trends in Chemical Physics, 6, 1-30 (1997)]
This article reviews our recent work on site-specific fragmentation followong core-level photoexcitation of organosilicon molecules in the vapor phase and on the solid surface. The ionic fragmentation following core-level photoexcitation occurs selectively around the atom where the photoexcitation has taken place. Synchrotron radiation can indeed play the part of an optical knife for molecules.
間瀬一彦、永園充、田中慎一郎、長岡伸一
[放射光、 10、375-391 (1997)]
Study of ion desorption induced by core-electron excitations of molecules on surface by using energy-selected electron ion coincidence (EICO) spectroscopy combined with synchrotron radiation is reviewed. The significance and the historical backgrounds of desorption induced by electronic transitions are introduced. The photoelectron photoion coincidence (PEPICO) and the Auger-electron photoion coincidence (AEPICO) spectroscopies for the investigations of the ion desorption induced by core-electron excitations of molecules on surface are described. The PEPICO results for condensed Si(CH3)4 and Si(CH3)3CH2CH2SiF3 provided direct evidences of site- specific ion fragmentation induced by core-electron excitations. AEPICO results for condensed H2O and NH3, on the other hand, show that the character of the orbitals where holes are created, as well as the effective hole-hole Coulomb repulsion are important factors in the ion desorption induced by Auger processes of covalent molecules. AEPICO study of H2O chemisorbed on a Si(100) surface demonstrated the validity of the EICO spectroscopy for the quantitative investigations of photostimulated ion desorption from well-defined surfaces. Finally, prospects of ion desorption study by using EICO spectroscopy are described.
Shin-ichi Nagaoka, Kazuhiko Mase, Mitsuru Nagasono, Shin-ichiro
Tanaka, Tsuneo Urisu, and Joji Ohshita
[J. Chem. Phys., 107,
10751-10755 (1997)]
We used photoelectron spectroscopy and the energy-selected-photoelectron photoion coincidence method to study site-specific fragmentation following Si:2p photoionization of 1-trifluorosilyl-2-trimethylsilylethane [F3SiCH2CH2Si(CH3)3, FSMSE] condensed on a Au surface. The photoelectron spectrum of FSMSE has two peaks for 2p-electron emission: one for the Si atom bonded to three methyl groups (Si[Me]) and one for the Si atom bonded to three F atoms (Si[F]). H+ and F+ ions are predominantly desorbed coincidentally with the Si[Me]:2p and Si[F]:2p electrons.
Mitsuru Nagasono, Kazuhiko Mase, Shin-ichiro Tanaka, and Tsuneo
Urisu
[Surface Sci., 390, 102-106 (1997)]
Photostimulated ion desorption at the 4a1 - N 1s resonant transition of condensed NH3 was studied using electron emission spectroscopy and Auger-electron photoion coincidence (AEPICO) spectroscopy. The total ion yield divided by Auger-electron yield exhibited a threshold peak at hu = 399 eV, which was ascribed to the resonant transition from the N 1s to the N-H antibonding 4a1 orbital. The electron emission spectrum at the 4a1 - N 1s resonance was decomposed into three components, i. e., a valence photoelectron emission spectrum, normal- and resonant-Auger-electron emission spectra. We ascribed the resonant-Auger-electron emission spectrum mainly to the spectator-Auger transitions on the basis of the peak assignment. A series of AEPICO spectra at the 4a1 - N 1s resonance was also measured as a function of the Auger-electron kinetic energy. The electron kinetic energy dependence of the H+ AEPICO yield displayed a shape approximately similar to that of the mixed spectrum of normal- and spectator-Auger-electron emission spectra. Based on this result the H+ desorption at the 4a1 - N 1s resonance was concluded to originate from the spectator-Auger transitions and from the normal-Auger transitions following the delocalization of the excited electron.
Kazuhiko Mase, Mitsuru Nagasono, Shin-ichiro
Tanaka, Tsuneo Urisu, Eiji Ikenaga, Tetsuji Sekitani, and Ken-ichiro
Tanaka
[Surface Sci., 390, 97-101 (1997)]
Ion desorption induced by a resonant excitation of O 1s of condensed amorphous H2O has been studied by total ion and total electron yield spectroscopy, non-derivative Auger-electron spectroscopy (AES), and Auger-electron photoion coincidence (AEPICO) spectroscopy. The spectrum of total ion yield divided by total electron yield exhibits a characteristic threshold peak at hu = 533.4 eV, which is assigned to the 4a1 - O 1s resonant transition. The AES at the 4a1 - O 1s resonance is interpreted as composed of the spectator-AES of the surface H2O, and the normal-AES of the bulk H2O where the 4a1 electron is delocalized before Auger-transitions. H2 is found to be the only ion species in AEPICO spectra measured at the 4a1 - O 1s resonance and at the O 1s ionization (hu = 560 eV). The electron kinetic energy dependence of the AEPICO yield (AEPICO yield spectrum) at the 4a1 - 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 4a1 - O 1s resonance are found to correspond to those of the spectator-AES of the surface H2O which was extracted from the AES at the 4a1 - O 1s resonance. Furthermore, the AEPICO yield is greatly enhanced at the 4a1 - O 1s resonance as compared with that at the O 1s ionization. On the basis of these results, spectator-Auger-stimulated ion desorption mechanism and/or ultrafast ion desorption mechanism are concluded to be responsible for the H2 desorption at the 4a1 - O 1s resonance. The enhancement of the H2 yield is ascribed to the O-H anti-bonding character of the 4a1 orbital.
Shin-ichiro Tanaka, Kazuhiko Mase, Mitsuru
Nagasono, and Masao Kamada
[Surface Sci., 390, 204-208 (1997)]
Tetsuji Sekitani, Eiji Ikenaga, Kenichiro
Tanaka, Kazuhiko Mase, Mitsuru Nagasono, Shin-ichiro Tanaka, and Tsuneo
Urisu
[Surface Sci., 390, 107-111 (1997)]
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 245~250eV electron energy. It seems to be related to the spectator resonant Auger stimulated ion desorption. That is, H+ desorption is enhanced due to a 2 hole 1 electron state at which the electron is in a anti-bonding C-H* orbital and the two holes can be in a 1π bonding orbital localized on the -CH3 group. We have also measured similar spectra for other resonant excitation (π*CN, σ*CC, σ*CN). The results are discussed in connection to the bonding/anti-bonding character and localized character of the excited state.
Kazuhiko Mase, Mitsuru Nagasono, Shin-ichiro
Tanaka, Tsuneo Urisu, and Shin-ichi Nagaoka
[Surface Sci., 377-379, 376-379 (1997)]
Photoelectron-photoion coincidence (PEPICO) spectra of condensed Si(CH3)4 were measured with the C 1s and the Si 2p photoelectron by using synchrotron radiation. H+ was found to be the only species desorbed in both the 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.
Mitsuru Nagasono, Kazuhiko Mase, Shin-ichiro
Tanaka, and Tsuneo Urisu
[Surface Sci., 377-379, 380-383 (1997)]
Photostimulated ion desorption induced by core-level excitations was studied for condensed NH3 and ND3 by using Auger electron-photoion coincidence (AEPICO) spectroscopy with synchrotron radiation. Series of AEPICO spectra for condensed NH3 and ND3 were measured as a function of the electron kinetic energy around the N(KVV) Auger electron energy range. Both the AEPICO yields of H+ and D+ were enhanced at the electron kinetic energies corresponding to the N(KVV) Auger transitions, which provide a clear evidence of 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 the ion desorption. The AEPICO yield of D+ was found to be one-third to one-half as small as that 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+.
Kazuhiko Mase, Mitsuru Nagasono, Shin-ichiro
Tanaka, Masao Kamada, Tsuneo Urisu, and Yoshitada Murata
[Rev. Sci. Instrum., 68, 1703 (1997)]
Energy-selected electron-ion coincidence (EICO) spectroscopy for the study of surface dynamics combined with synchrotron radiation (SR) has been developed. The equipment consists of an electron gun, a cylindrical mirror analyzer (CMA), and a time-of-flight ion mass spectrometer (TOF-MS). A sample surface was excited by synchrotron radiation, and energy of the emitted electron was analyzed by the CMA. The TOF spectrum of the desorbed ions was measured taking the energy-analyzed electron signal as the starting trigger. The Ions coincidently desorbed with the electron gave a characteristic peak in the TOF spectrum. The apparatus was evaluated on the basis of photoelectron-photoion coincidence (PEPICO) and Auger electron-photoion coincidence (AEPICO) measurements of H2O condensed on a gold foil. The results demonstrate that PEPICO and AEPICO combined with SR are powerful methods for investigating the ion desorption induced by core-level excitations.