Polarization and angle-resolved NEXAFS of benzene adsorbed on oriented single-domain Si(100)-2x1 surface,
N. Witkowski(France), F. Hennies, A. Pietzsch, S. Mattsson, A. Fohlisch, and W. Wurth (Germany)
M. Nagasono(Kyoto U., Japan), M. N. Piancastelli(Italy),
Phys. Rev. B 68, 115408 (2003).
Craig (1993)–tilted, calculation
Taguchi (1991)–butterfly or tilted, ~1 benzene/ 2 dimers (TDS, AES, HREELS)
Jeong (1995)–Pedestal, Semiempirical calculation
Birkenheuer (1998)–butterfly, First princlple, density functional calculation
Gokhale (1998)–butterfly, TDS, UPS
Lopinski (JVST,1998)–butterfly -> pedestal, STM, FTIR Borovsky (PRB,1998)–butterfly -> tilt, STM,
Kong (SS,1998) – butterfly -> tilt, (NEXAFS,TDS,IR)
Molecular benzene
Chemisorbed benzene
Physisorbed benzene
No peak at 288.8 eV -> Breaking of conjugated benzene ring
At RT, benzene is chemisorbed like (a) then, is transformed into (c) after a long time.
Experiment
At MAX-Lab BL I511 (providing linearly polarized light)
Endstation - Prep. + Analysis chamber(XPS, NEXAFS, XES)
Sample - 5o-off vicinal Si(100) -8.5 dimers per terrace - two SD-samples 90o-rotated each other
Sample cleaning & preparation - Xe-sputtering at RT(800K) & annealing (1250K) cycles - 10L of benzene exposure at RT ~ saturation coverage
NEXAFS measurements - scanning photon energies while monitoring CFS (KLL carbon Auger peak)
- Emission angle = 55o,
• 10L benzene/Si(100) at RT
• 284.8 eV – C 1s -> π*
• 286.9 eV – C 1s -> σC-H*
• Still, C=C double bond is preserved at RT.
• ---- > Pedestal can be excluded !!!!
• Overall decrease of the peak at 248.8 eV compared with Fig. 2 -> molecule is close to in-plane (but, not parallel to the surface)
• Apparent difference between Fig. 3 and Fig. 4 -> π* resonance has an in-plane component normal to the dimers
• No asymmetric shape (contrary to the previous NEXAFS results (Tong, SS 1998)) -> Not a metastable species ! (Also, no change after further annealing)
• σC-H* resonance appears all in Fig. 2-4. -> C-H is tilted away from in-plane or out-of plane
• No 289 eV peak (due to conjugated benzene ring) -> aromatic ring is broken.
• ---- > Butterfly rather than tilted geometry !!!!
• From Fig. 5, adsorbed benzene has a mirror plane perpendicular to the dimer.
• From Fig. 6, adsorbed benzene has a mirror plane parallel to the dimer.
• Small difference in the CH* resonance -> a slight distortion on each side -> step edge effect(?)
• Major conclusions that can be drawn from the above results are,
Summary
• Still, π-bond is preserved. (OK) • No other bonding structure other than “butterfly” is observe (?) - in conflict with previous STM, NEXAFS results
• From Fig. 3-4, butterfly rather than tilted ----- ?
• butterfly still comply with the results of Fig. 5-6.
