Publikationsserver der RWTH Aachen University
Aufbau eines 500 Hz-Lasersystems zur Summenfrequenzschwingungsspektroskopie
Abstract
dc:descriptionThis work reports on the design and performance of a 500 Hz picosecond laser system specifically developed for sum-frequency spectroscopy of molecular vibrations at interfaces, and on its applications to the study of potential-dependent CO adsorption at the Pt(111)/aqueous electrolyte interface. The experimental setup consists of a picosecond Nd:YAG laser system operating with a pulse frequency of 500 Hz, and on optical parametric generator/amplifier system for the generation of tunable picosecond pulses in the medium infrared. Laser pulses at 1.064 µm with 0.9 mJ pulse energy and about 25 ps pulse duration are produced by regenerative amplification of picosecond pulses from a laser diode-pumped, mode-locked Nd:YAG oscillator in a laser diode-pumped Nd:YAG amplifier at a repetition rate of 500 Hz. Since much larger pulse energies are required for efficient frequency conversion in the parametric generator/amplifier system, the avialable laser pulse energy at 1.064 µm is increased to a level of about 9 mJ by one-pass amplification in two flashlamp-pumped Nd:YAG laser amplifiers. Spectrum and autocorrelation traces indicate considerable self-phase modulation of the amplified pulses which have a pulse duration as short as 12 ps. Tunable pulses in the medium infrared for vibrational spectroscopy are obtained by optical parametric amplification of near-infrared seed pulses in two angle-tuned AgGaS2 crystals pumped with a fraction of the energy of the amplified Nd:YAG laser pulses. The seed pulses which are tunable between 1.2 and 1.7 µm are produced by optical parametric generation and two-pass amplification in two angle-tuned barium borate (BBO) crystals pumped by the second harmonic (0.532 µm) of the remaining fraction of the amplified Nd:YAG laser pulses. The spectrally broad-band parametric emission of the BBO crystals is narrowed down with a diffraction grating between the two passes of amplification. The optical parametric system produces at a repetition rate of 500 Hz high-power infrared pulses tunable from 1.2 to 8.3 µm with pulse durations of approximately 9 ps, bandwidths of typically 4 cm^(-1) in the medium infrared, and pulse energies of 70 µJ at 5 µm and 160 µJ at 3.33 µm. The potential of this new laser system is demonstrated in infrared-visible sum-frequency generation (SFG) experiments to study the potentail dependence of CO adsorption and CO adlayer phase transition at the Pt(111)/sulfuric acid interface. This Pt(111)/CO/electrolyte system has been selected as a well-defined and well-characterized model system for molecular chemisorption. Moreover, CO adsorption at Pt electrocatalysts is an important intermediate step in the electrooxidation of organic fuels such as methanol. In the SFG experiments of the present work the resonant excitation of the intramolecular stretching vibration of adsorbed CO molecules with tunable infrared pulses is up-converted to the sum-frequencv using 0.532 µm laser pulses. Experiments where performed on flame-tempered Pt(111) single crystal electrodes in a CO-saturated aqueous solution of 0.5 M H2SO4. When CO is adsorbed at a potential of 80 mV versus the reversible hydrogen electrode (RHE), resonances at approximately 1786 cm^(-1) and 2066 cm^(-1) are observed in the SFG spectra corresponding to CO adsorption at three-fold hollow sites and on-top sites, respectively. These bands are indicative of a (2x2) adsorption structure of CO as revealed in previous scanning tunneling microscopy studies of this system. A fast increase of the potential to 500 mV/RHE causes the appearance of new bands at approximately 1847 cm^(-1) and 2062 cm^(-1) which are characteristic of CO adsorption at (near-)bridge and (near-)on-top sites, respectively, of a (sqrt(19) x sqrt(19))R23.4° adsorption structure, and the disappearance of the bands of the (2x2) structure. When CO is adsorbed at 500 mV/RHE the SFG spectra show that the phase transition from the (2x2) to the (sqrt(19) x sqrt(19))R23.4° structure is completed within few minutes, while for adsorption at 600 mV/RHE the (2x2) structure is rather stable.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2003
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Kuß, Michael
- Contributors dc:contributor
-
- Daum, Winfried
Subjects
dc:subject × 12Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- ger
Identifiers
dc:identifier.*- OAI identifier oai:identifier
- oai:publications.rwth-aachen.de:60238