S. Nagaoka, K. Mase, M. Nagasono, S. Tanaka, T. Urisu, J. Ohshita, and U. Nagashima
[Chem. Phys., 249, 15-27 (1999)]
We used photoelectron spectroscopy and the energy-selected-photoelectron photoion coincidence ESPEPICO method to study site-specific phenomena in the Si:2p photoionization of X3Si(CH2)n Si(CH3)3(X=F or Cl, n=0-2) condensed on a Si(111) surface. The site-specific excitation and the occurrence of different chemical shifts at two Si sites were revealed in the total electron-yield spectra and the photoelectron spectra of F3Si(CH2)nSi(CH3)3 (n= 1, 2) , although they were not clearly revealed in those of Cl3SiSi(CH3)3 . We conclude that these site-specific phenomena are easily observed in molecules in which the two Si sites are located far apart and in which electron migration between the two Si-containing groups does not occur. This was supported by our ab initio calculation. Site-specific fragmentation was revealed in the ESPEPICO spectrum of F3SiCH2Si(CH3)3 , although it was negligible for Cl3SiSi(CH3)3 and was less remarkable in F3SiCH2Si(CH3)3 than in F3SiCH2CH2Si(CH3)3 . Site-specific fragmentation also occurred when the two Si sites were located far apart.
K. Mase, M. Nagasono, and S. Tanaka
[J. Electron Spectrosc. Relat. Phnom., 101-103, 13-19 (1999)]
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 H2O (hν=564 eV), H3 desorption is attributed to the normal Auger stimulated ion desorption (ASID) mechanism, that is normal Auger final states are concluded to be responsible for H3 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 4a1<--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(4a1)1 state is responsible for the H+ desorption. For H+ desorption at 3p<--O:1s (hν=5537 eV), the spectator Auger stimulated ion desorption mechanism is concluded to be probable. In the case of H2O 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.
T. Sekitani, E. Ikenaga, K. Fujii, K. Mase, N. Ueno, and K. Tanaka
[J. Electron Spectrosc. Relat. Phenom., 101-103, 135-140(1999)]
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.
Mitsuru Nagasono, Kazuhiko Mase, Shin-ichiro Tanaka, and Tsuneo Urisu
[Jpn. J. Appl. Phys. Suppl., 38-1, 325-327 (1999)]
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 4a(1) N-H antibonding orbital. The Auger electron spectrum of the isolated NH3 at the 4a(1) <-- N 1s resonant transition is found to be mainly due to spectator-Auger transitions. A series of AEPICO spectra at the 4a(1) <-- 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)(4a(1))(1) state is responsible for the H+ desorption, that is, ultrafast H+ desorption mechanism is favorable in this system.
Kazuhiko Mase and Shin-ichiro Tanaka
[Jpn. J. Appl. Phys. Suppl., 38-1, 233-238 (1999)]
Ion desorption induced by core-electron transitions is studied for various surfaces by electron-ion coincidence spectroscopy combined with synchrotron radiation. In a study of F+ desorption from Si(100) terminated by fluorine using photoelectron photoion coincidence spectroscopy, the site-selective F+ desorption is directly verified; that is, F+ desorption is induced by Si 2p photoionizations at the ≡SiF, =SiF2, and -SiF3 sites. An Auger electron photoion coincidence study of a CaF2(111) film epitaxially grown on a Si(111) surface also presents direct evidence of F+ desorption, in this case induced by F 1s surface core exciton. These investigations demonstrate that electron-ion coincidence spectroscopy combined with synchrotron radiation is a powerful tool for studying ion desorption induced by core-electron excitations.
間瀬一彦、永園充、田中慎一郎
[真空、42、84 (1999)]
表面に吸着した水の電子励起に由来する脱離過程の研究は、 ガス放出の少ない素材の探索や真空槽内壁処理技術 の開発において極めて重要であるとともに、表面 基礎科学としても非常に魅力的である。我々は電 子−イオン・コインシデンス分光法という新しい研究手法 を開発し、表面に吸着した分子の内殻電子励起に 由来するイオン脱離研究を行ない、脱離機構につ いて様々な知見を得た。本稿では、水の内殻電子 励起に由来する水素イオン脱離に関する我々の最 近の成果を簡単に紹介する。