Chemistry Reference
In-Depth Information
the probe beam is scanned across the surface (each footprint is on a fresh
section of surface.)
The above characteristics of the reflected SHG and SFG probes were
exploited by our group in developing these processes as
in situ
probes for
natural ocean surfactants. The instantaneous nature of the signal (time
scale for the duration of the incident laser pulse of either 5 to 3 nano-
seconds), the extremely narrow spectral spread of the signal (essential the
same spectral bandwidth as the pump laser which is less than 0.01 nm), the
squared dependence of signal intensity on pump laser intensity, and the
nondestructive nature of the optical processes were all invaluable in en-
abling us to use these probes in the field successfully. Tuning the laser
wavelength to where we expect some form of resonance or preresonance
(vibronic) for almost any organic surfactant, we were able to enhance the
surfactant signal well above the background water signal. The temporal re-
sponse of the signal, the pump laser intensity dependence, and its narrow
spectral width made it easily identified and isolated from the ambient
background light. In addition, the collinear nature of the signal and reflect-
ed pump laser allowed us to correct or scale the signal for fraction of sig-
nal collected. This is especially important, when surface waves create an-
gular spreading in the surface reflection (Frysinger et al. 1993).
Figure 1 is an example of a time series SFG signal (532 nm and 355 nm
pump beams and SFG at 213 nm) generated from surfactant gradients on
the ocean surface (Korenowski et al. 1993, Korenowski 1997). The ex-
periment was carried out during the SLIX 89 experiment just off the coast
of California in the vicinity of Santa Rosa Island. The increases in the SFG
signal correspond with visible records of crossing through a series of
banded surface slicks. The increasing signal while crossing the banded
slicks indicates an increase in surfactant concentration in the first molecu-
lar layer of the surface (The SFG and SHG signals increase quadratically
with surfactant concentration). The largest signal occurs at the upwind
edge of the banded slick and sharply decreases upon exiting the slick in the
upwind direction. The variation of surfactant concentration is a maximum
at the upwind edge decreasing to a minimum at the leeward edge. Such a
concentration distribution would be consistent with the wind assisting the
natural spreading tendencies of a surfactant monolayer downwind and op-
posing spreading on the upwind edge leading to compaction of the slick
and a more abrupt change in concentration. Unfortunately, no information
was available about any subsurface flow and its potential role in creating
this concentration distribution. Another interesting feature occurs between
22 and 30 minutes in the data. A visible slick was observed but only a
modest increase in the SFG signal occurred (slightly larger than back-
ground). This indicates only a small surface concentration of surfactant in
