Chemistry Reference
In-Depth Information
on the ocean surface (Frysinger et al. 1992, Korenowski et al. 1993, Kore-
nowski 1997).
2 Background
Over the last twenty years, reflected SHG and reflected SFG have emerged
as important new laboratory probes for studying the one or two molecular
layers that comprise an interface. These techniques have been reviewed
elsewhere and the reader is directed to several of these reviews for more
detailed information (Shen 1989, Korenowski 1997) and a text on the sub-
ject of nonlinear optics is referenced (Shen 1984). It will suffice to say
here that SHG and SFG are laser based second order nonlinear optical in-
teractions (nonlinear dependence on the incident optical field). In general,
SHG and SFG occur when the electric field from the pump laser light
beam (SHG) or beams (SFG) drive the electrons in a condensed medium to
oscillate in response to the sinusoidal varying electric field, but with an
anharmonic component because of the strong electric field or fields. These
electrons radiate light not only at the pump laser or laser frequencies but
also at the harmonic and sum frequencies. For an isotropic medium, SHG
and SFG are forbidden optical processes (electric dipole approximation).
For a liquid medium such as water, the random molecular ordering in the
bulk is sufficient to qualify the medium as isotropic. Only at the interface,
where the isotropy is necessarily broken because of the interface, is there
enough molecular ordering to permit SHG and SFG. Consequently, any
SHG or SFG signal generated in transmission or reflection is the result of
the optical interaction in the interfacial layer of molecules. When reflection
occurs into an essentially non-dispersive (optical) medium like atmo-
spheric pressure air, the conservation of momentum requires that the re-
flected pump laser beam at optical frequency Z (wavelength O) be colli-
near with the reflected SHG signal at optical frequency 2Z (wavelength
O/2). The magnitude of this reflected nonlinear optical signal is dependent
upon the vibronic (vibrational and electronic) structure of the interfacial
molecules. When either the pump optical frequency or that of the nonlinear
signal matches an allowed vibronic transition in the interfacial molecules,
the resonance occurs and the signal increases. For strong resonances, the
signal increase may be many orders of magnitude. Tuning the optical
probe to a vibronic resonance of a surfactant molecule, enables one to
monitor surfactant concentrations along with the much smaller signal from
the interfacial water molecules (not in optical resonance). Any small po-
tential for heating or photochemical degradation of the surfactant is not a
problem when only a single laser pulse is used to record the signal or when
