203
"b 15.0
~
HELIX SCATIER A = 700 run
*
10.0
5.0
rE 0.0 -./ -) .....................•....
\
\
I
-5.0
\
/
\
I
-10.0
\ /
I
-15.0
0
30
60
90
120
150
180
Scattering Angle (deg)
Figure 12. Coupled dipole approximation for the S34 scattering matrix element for a helix with different values
of radius (R), pitch (P) and thickness (T) at a wavelength of 700 nm. S34 is sensitive to helix pitch and
radius. (Reproduced from Salzman et al. (1990))
elements is phase differential scattering (PDS) (Johnston et al., 1986; Johnston et aI., 1987;
Johnston et al., 1988). The method uses a laser that produces two, collinear lines at nearly
the same frequency (633 nm) with orthogonal linear polarizations. The lines differ in
frequency by only 250 kHz. The laser beam is incident on a cuvette suspension of particles.
A detector is placed at a specified scattering angle with a polarizer oriented at 45 degrees
from the horizontal scattering plane. A scattering object retards the incident horizontal and
vertical polarizations of the laser beam by different amounts. This produces a phase shift in
the 250 kHz beat frequency at the detector. In terms of the scattering matrix elements, this
phase shift, a, is given by