Optische Dreiwellenspektroskopie an GaAs(001), Si(111) und Pt-Elektrolyt-Grenzflächen

Nonlinear optical spectroscopy was used to investigate GaAs(001), Si(111) and Pt-electrolyte interfaces. GaAs(001): The spectral dispersion of the magnitude of the second-order nonlinear susceptibility |æxyz| of GaAs has been determined by second-harmonic generation (SHG) for two-photon energies between 2 eV and 5 eV. In this range the frequency dependence of |æxyz| is shown to be dominated by the E_1, E_1+Delta_1and E_0' and E_2 critical-point-transitions of GaAs. At the E_1 transition ( 2.91 eV) the value of |æxyz| is 870 nm/V . An interference effect is observed between the E_0' and E_2 contributions to |æxyz|, in agreement with recent calculations. A strong and spectrally narrow resonance at 3.35 eV in the SHG spectrum of GaAs is assigned to a surface transition related to the gallium oxide surface layer of the GaAs samples. Si(111): The ideal Si(111)(1x1)H surface, this surface during oxidation on air and various oxidized Si(111)-SiO2 interfaces are studied with SHG for two-photon energies between 2.5 and 5 eV. Native oxidation of the Si(111)(1x1)H surface causes the appearance and disappearance, respectively, of SHG resonances which are identified with specific Si-Si bonding configurations at the interface. Specifically, resonances at 3.52 eV two-photon energy observed in p-polarized SHG spectra are related to a Si suboxide configuration present in a partially oxidized Si surface bilayer. The same resonances are observed in spectra of thermally oxidized Si(111) and identified with a Si2+ suboxide state at the buried interface. Pt-electrolyte Vibration bands of CO and H2O adsorbed to Pt at the Pt/aqueous electrolyte interface are studied by spectroscopic sum-frequency generation (SFG) in the spectral range between 1700 and 4000 cm-1. For the Pt(111)-electrolyte interface, dipol-dipol coupling of two coexisting domains with different CO-adsorbate structures is observed. It is found an increasing SFG intensity of the stretching mode of "on-top" adsorbed CO during the phase transition from the (2x2)-3CO to the (root 19 x root 19) -13CO structure. To investigate surface reactions with SFG with enhanced reaction rates a new kind of electrochemical cell is developed. This cell works with thin Pt films evaporated on IR-transparent substrates. The new electrochemical cell and the Pt films, evaporated on sapphire(0001) substrates, are also presented in detail. The vibrations of the electrolyte close to the interface of thick Pt(111) single crystals (using the common cell) and thin Pt films (using the new cell) are investigated under varying electrode potentials. While SFG spectra obtained in the common cell have a gap in the spectral range of OH-vibrations, the new cell allows investigating vibrations with SFG in the whole spectral range between 1700 and 4000 cm-1. In previous experiments, using the common cell, CO is only observed at the Pt/aqueous electrolyte interface at potentials below 500 mV/RHE because of a limited thickness of the electrolyte. Experiments with the new cell do not have a limited electrolyte thickness and show clearly CO adsorbed on Pt until the potential of maximum CO oxidation rate at 850 mV/RHE.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2002

Author dc:creator

• Bergfeld, Stefan

Contributors dc:contributor

• Daum, Winfried

• info:eu-repo/classification/ddc/530
• Galliumarsenid
• Silicium
• Kristallfläche
• Suszeptibilität
• Frequenzverdopplung
• Platin
• Elektrolyt
• Grenzfläche
• Summenfrequenzerzeugung
• Physik
• Grenzflächen
• SHG
• SFG
• GaAs
• Si
• Pt

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-120984%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:59174