Chemistry Reference
In-Depth Information
to resolve the fine structure within the concentration gradients. In particu-
lar, it is especially important to obtain millimetre spatial resolution with
these surfactant measurements. Millimetre spatial resolution would pro-
vide detailed information of the surfactant gradients on capillary-gravity
(CG) and capillary waves where surfactants have their largest impact on
wave damping and wind-wave coupling. With this goal in mind, we ex-
plored the possibility of making single laser pulse SHG and SFG measure-
ments using a CCD camera to record an image of the surfactant based sig-
nal.
3 Experimental
Figure 2 is a schematic of the imaging experiment. An unfocussed 6 milli-
metre diameter 532 nm second harmonic beam from a high repetition rate
(100 Hz) Coherent Infinity Nd:YAG laser was made incident at 60 degrees
from normal on a water surface in a channel. An insoluble monolayer of a
hemicyanine surfactant, 4-[4-(dimethylamino)-styryl]-1-docosyl-pyridini-
um bromide, was spread on the distilled water surface. The hemicyanine
surfactant is a model surfactant system that we have found useful in preli-
minary laboratory studies (Hirsa et al. 1997). (Hemicyanine is a model sur-
factant for which we have synthesised derivatives with varying degrees of
solubility ranging from derivatives that form insoluble monolayers to solu-
ble surfactant versions). The water in the channel was subjected to a lami-
nar flow of nominal velocity 20 cm s
-1
. Details of the fluid measurements
were presented elsewhere (Hirsa, et al., 2001). At the end of the channel a
surface barrier was placed across the channel perpendicular to the flow di-
rection. The SHG signal and the collinear reflected pump laser beam were
collect in the direction of reflection. The collinear beams were passed
through a dichroic image splitter to separate the green pump beam (532
nm) and the 266 nm second harmonic signal beam. The green pump laser
beam was passed through a series of neutral density (ND) filters to limit
the detected intensity. The UV SHG signal was passed through a 7-54 col-
our glass filter to eliminate any residual green pump laser light. The two
beams are recombined side by side and sent through a Nikon quartz lens to
image the two beams on the photocathode of the pulse gated microchannel
plate image intensifier. This image is amplified and optically transferred
via a fiber optic coupler to the chip of a CCD camera (Princeton Instru-
ments now named Roper Scientific). The KG-1 filter was used in the pump
beam path to eliminate any spurious 266 nm light from the pump beam.
