Sum frequency generation (SFG) is a second order nonlinear
optical process in which a tunable infrared beam is mixed
with a visible beam to general an output at the sum
frequency, it has surface specificity because the process
is electric-dipole forbidden in centrosymmetric bulk, but
allowed at an interface where the inversion symmetry is
necessarily broken. This newly developed vibrational
spectroscopy is a very powerful tool that has great impact
on electrochemistry where interface is the centerpiece. It
provides absolute spectra (unlike EC-IRAS, which needs
modulation or difference techniques to extract surface
signal) for various kinds of interfaces (unlike EC-Raman,
generally limited to roughened coinage metal surfaces)
The SFG in the MRL Laser Facility consists of a flashlamp
pumped Nd:YAG laser mode-locked with a non-linear mirror.
This pump laser is adequate for synchronously pumping a
picosecond LiNbO3 or an AgGaS2 optical parametric
oscillator (OPO) to provide infrared pulses tunable from
2.5 to 9 µm in one single conversion step. By using a
synchronously pumped KTP OPO followed by a different
frequency generation (DFG) in CdSe, the tunability of the
infrared beam can be extended to the range of 9 to 20 µm.
SFG is opening up new research fields in our research
group. Currently we use in-situ SFG to study bi-metallic
catalysts, focusing on platinum surface decorated by other
metals. Typical example is Pt(111) decorated by
spontaneously deposited Ru, as Pt and Ru are supposed to be
the best combination for methanol oxidation. We have used
CO as a molecule probe to study this surface and
preliminary results have been obtained. CO adsorbed on both
Pt domains and Ru domains from the same surface are clearly
identified.