Broadband Sum Frequency Generation Studies of Surface Intermediates Involved in Fuel Cell Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 371

Figure 12.3 (a) Block diagram of the third-generation SFG spectrometer. A beam splitter
(BS) directs 4 of the output through a Fabry - Perot etalon (FPE) to produce the narrowband vis-
ible (NBVIS) pulses. The remaining output pumps an optical parametric amplifier to generate
the broadband IR pulses (BBIR). (b) Photograph of the SFG apparatus on a 2 m 0.8 m optical
table. The laser, spectrograph, and sample chamber are shown in the foreground. (c) Schematic
of the arrangement for SFG phase-matching. Different BBIR frequencies result in slightly
different SFG output angles, but all SFG signals pass through the spectrograph slit and illumi-
nate slightly different height (h) regions of the CCD. Output spectra are obtained by integrating
along the CCD h-direction. (See color insert.)
spectroscopy [Shen, 1989]. SFG helps solve the two biggest problems of surface
molecular spectroscopy: sensitivity and selectivity. Sensitivity results from the non-
linear coherent nature. The signal is proportional to the product of IR and visible inten-
sities at the surface, which is huge with focused femtosecond pulses [Patterson et al.,
2005]. The signal is emitted coherently, so almost all SFG photons are collected and
detected by high sensitivity detectors. Selectivity for surfaces or interfaces results from
the symmetry properties of the x (2) tensor, which in the dipole approximation
vanishes in centrosymmetric media [Shen, 1989; Bonn et al., 2000]. Despite the
low number density of interfacial molecules, the combination of intense femtosecond
pulses (typically 10 GW/cm 2 ), coherence and the high sensitivity of CCD detectors to the
visible sum-frequency photons generate excellent signals from many interfacial species.
Figures 12.1 and 12.2 show that the spectroelectrochemical cell is basically a thin-
layer electrochemistry cell (TLE) with a solution gap of 25 mm [Hubbard, 1973]. The
metal working electrode may be polycrystalline or a single crystal. Emptying the gap
out of the adsorbate molecules due to molecules' oxidation, and refilling via molecular