The experiment is performed using the same setup as in Fig. 2.6 with a quarter-wave plate placed in the beam path after the recombination of pump 2 and 3 to make both circularly polarized. Then pump 2 and 3 recombine with pump 1 on a BS that has different transmis- sion efficiency for, and adds different phase to the s and p polarization components, making the circularly polarized light elliptically polarized but with major axes tilted away from Z axis. A quartz plate is placed in the path after the recombination of all three pumps to rotate the major axes back to the LF Z axis. Polarizations of all pump and probe pulses are carefully characterized by measuring Stokes parameters. The target DFIB molecules are seeded in high pressure helium gas through the supersonic gas jet and rotationally cooled
to about 1 K. We use VMI to measure the momentum distribution of both I+ and F+ frag-
ments. The former indicates the alignment of the C-I axis and the latter the confinement of the benzene plane.
Figure 3.5: End and side views of both iodine and fluorine ion fragments at different moments, all images have been Gaussian smoothed and four fold symmetrized (Since the polarization of the elliptically polarized pulses have four fold symmetry in the polarization plane, the resulting momentum distributions have four fold symmetry.). “Isotropic” gives the momentum distribution for both ions before any aligning pumps interact with the molecules. “After 1 pump” shows the distributions at the peak of the FF1DA of C-I axis, which is about 3.8 ps. “After 2 pumps” and “After 3 pumps” show the distributions at the peak of the FF3DA after two kicks and three kicks, respectively.
Images for both ions at different times are shown in Fig. 3.5. For both I+ and F+,
images collected with the C-I axis aligned perpendicular to the detector are referred to as “end view” images, and those with the C-I axis parallel to the detector are referred to as “side view” images. The polarization of the probe pulse is chosen to be perpendicular to the detector plane for both I+end and side views. For F+, the probe is perpendicular to the
detector for end views and parallel to the detector for side views. By doing so, the effect of the probe on the measured angular distributions is minimized. Before any aligning pulses,
both end and side views of I+ give uniform distributions, while for F+, because the probe
preferentially dissociates molecules that have the C-I axis along the probe polarization axis, the resulting F+ distribution has a donut shape for both side and end views. The first
linearly polarized pulse aligns the C-I axis but leaves the benzene ring spinning. At the alignment peak, the I+ side view clearly shows more counts along pump 1’s polarization
axis, and because of the alignment of the C-I axis and the distribution of the I+ in the
end view becomes tighter. For F+, because the C-I axis is fixed, more F+ will be driven
away from the center, so in the end view, the number of counts in the center of the donut decrease significantly. Since the benzene ring is still spinning, the projection of the spinning F+ rings produces four spots in the side view images. Pump 2 aligns both the C-I axis
and the benzene ring at the same time. The side view distribution of I+ gets stretched and
further confined to the major axis and the end view distribution becomes even tighter. The minor axis of pump 2 starts to align the benzene ring, in the end view of F+ the distribution
starts going towards the direction of the minor axis and since the rotation of the benzene ring is more confined now, the four spots on the side view start to merge into two spots. After pump 3 arrives, it further increases FF3DA, especially for the F+ distribution, where both the side and end views of F+ become more confined. We can define an angle θ2D in
the I+ side views as the angle between vector of an individual hit with respect to the image center and laser major axes. We can define the same angle for the F+end views as the angle with respect to the laser minor axes. Then we can plot the evolution of the cosines of both angles after each pump pulse, as shown in Fig. 3.6. A projected 2D value of 0.8 is reached for the C-I axis and 0.65 for the benzene plane at the peak of the FF3DA which, to the best of our knowledge, is the highest degree of FF3DA achieved thus far. It is also comparable to the peak values measured in [18], where the peak alignment occurs in the presence of the aligning laser field. Furthermore, this value can be further enhanced by adding more elliptically polarized pulses as shown theoretically in Fig. 3.4.
Figure 3.6: Top panel shows the time evolution of the hcos2θ
2D measured in I+ side views
for one pump, two pumps and all three pumps. Lower panel shows a similar figure measured in F+ end views. Laser parameters are the same as in Fig. 3.4.
priately timed, is also effective for other asymmetric tops with a prolate-like polarizability, however, the same pulse sequence is not effective for those with oblate-like polarizability. In the latter case, changing the linear first pulse to an elliptical pulse is predicted to induce and enhance FF3DA.