Broadband Sum Frequency Generation Studies of Surface Intermediates Involved in Fuel Cell Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 374

The focused spot sizes (1/e 2 beam diameter) are 200 mm, and as the IR wavelength is
varied in the 2.5 - 16 mm range, the IR spot size can be maintained by refocusing the
ZnSe lens or by switching to a different ZnSe focal length. The SFG output pulses are
filtered using a short wavelength pass filter and detected using a 0.3 m spectrograph
and CCD (Andor Corp.).
In BB-SFG, all vibrations lying within the spectral region defined by the BBIR
pulse spectrum are probed simultaneously. With our laser, this region is about
250 - 300 cm 21 in width. The spectral resolution is set by the spectral width of the
NBVIS pulses, which in turn is set by the spectral bandpass of the etalon. We have
several etalons available, providing a range of spectral widths. It is generally desirable
to use the largest bandpass possible, because the intensity of the SFG signal is, all
things being equal, proportional to the square of the etalon bandpass. Doubling the
bandpass doubles the NBVIS pulse energy and reduces the NBVIS pulse duration
by a factor of two. Since the vibrational transitions observed in our electrochemical
measurements are typically 15 cm 21 or more in width, we ordinarily use an etalon
that transmits a portion of the femtosecond visible pulse that is 11 cm 21 wide.
When higher resolution is needed, we can add a second etalon in series to improve
the resolution at the expense of signal.
The arrangement used to phase-match the BB-SFG signal is depicted in Fig. 12.3c.
With a series of polarization rotators and analyzers, we can obtain spectra in all eight
possible polarization conditions, but on metal electrodes we use only the ppp-
polarization condition, where the two incident beams and the output beam are
polarized in the plane of incidence of the NBVIS pulses. The BBIR pulses are incident
upon the electrode from a few millimeters out of this plane. The spectrograph slit is per-
pendicular to this plane. Because there is a range of IR frequencies, the BB-SFG output
has a range of output wavevectors. Since the CCD detector has a height of several milli-
meters, all these different wavevectors fall onto the CCD, albeit at slightly different
heights h, and output spectra are obtained by integrating the signals along the h-axis.
To align the system, the NBVIS beam is directed onto the slit, falling near the edge
of the CCD detector. The short wavelength pass filter is then inserted. With this
arrangement, tuning the BBIR pulse wavelength simply moves the SFG signal parallel
to the spectrograph slit onto a slightly different region of the CCD detector, so the IR
wavelength can be easily changed with minimal height realignment.
The TLE SFG electrochemical cell depicted in Fig. 12.2 was made of Kel-F and glass,
with an optical window (CaF 2 or MgF 2 ) as the input window [Lu et al., 2005; Lagutchev
et al., 2006; Vidal et al., 2002; Biggin and Gewirth, 2001]. A 25 mm thick Teflon spacer
was placed between the electrode and the optical window. Originally [Lu et al., 2005] the
Teflon holder was used as a plunger to compress the spacer between the working elec-
trode and the optical window to create a uniform electrolyte layer of known thickness.
In more recent work, single-crystal electrodes were used that were attached to the plunger
without protecting the walls against the electrolyte contact. Voltammetry associated with
spectral acquisition for the single crystals is obtained in the spectroelectrochemical cell in
a meniscus configuration [Lagutchev et al., 2006].
The CCD detector and the potentiostat were synchronized in an open-loop con-
figuration by starting the experiment with a common trigger. Careful measurements