Nonlinear vibrational spectroscopy of surfactants at liquid interfaces Page: 48 of 229
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spatial overlap of the two beams due to variations in the IR beam direction as its
frequency is tuned, as in the case where difference-frequency generation in a nonlinear
crystal is used to generate the IR pulses. It is also good to have the spot size of one of the
beams at the sample a few times larger than the other beam. This minimizes the effect of
overlap changes on the output signal.
A major component in an SFG setup is a broadly tunable source of high power
infrared pulses. They can be generated by frequency conversion in nonlinear crystals, as
done in our laboratory, by stimulated Raman scattering of a tunable visible laser in a
high-pressure gas cell27 or by a free-electron laser.28,29 For the work reported in this
Thesis, two SFG setups have been used. Most data were obtained in an existing setup
(hereafter called setup A) but the spectra in the CO stretch range in Chapter 4 and all
spectra in Chapter 5 were taken in a newly constructed setup (hereafter called setup B). In
setup A the main laser source is a homebuilt high power Nd:YAG laser with wavelength
1.06 pm, 10 mJ pulse energy and ~ 40 ps pulsewidth, operating at a repetition rate of 10
Hz. The infrared generation is done in an optical parametric oscillator and amplifier
(OPG/OPA) based on LiNbO3 crystals. It generates pulses of energy ~ 300 pJ and
pulsewidth ~ 20 ps, tunable from 1.45 to 4 pm (6900 to 2500 cm-) and it has been
thoroughly describe elsewhere.30 The IR bandwidth varies throughout the tuning range
from ~ 8 cm1 at 2800 cm1 to - 45 cm1 at 3700 cm 1. The visible pulses at 532 nm are
generated by SHG in a KD*P crystal and have energies about 700 pJ and pulsewidth ~ 28
ps. The visible and IR pulses are incident on the sample at angles 42* and 510 and have
spot sizes of approximately 300 pm and 500 jm, respectively. The reflected SFG output
was selected with a set of pinholes defining a narrow detection solid angle, spectrally34
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Miranda, P.B. Nonlinear vibrational spectroscopy of surfactants at liquid interfaces, thesis or dissertation, December 14, 1998; California. (https://digital.library.unt.edu/ark:/67531/metadc702752/m1/48/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.