Polarization properties of light scattered off solutions of chiral molecules in non-forward direction
Measuring the optical activity from an ensemble of chiral molecules is a common tool to know their stereo-structure. These measurements are done in the same propagation direction of the probe beam of light, because that is the direction where most signal is emitted. We provide experimental and theoretical evidence that, even though other interesting information may be gathered when collecting light emitted in other directions, for most molecules, the phenomenon of optical activity is only present in the forward scattering direction. The fundamental reason behind this is that forward scattered light preserves the circular polarization states due to the cylindrical symmetry of the system, an essential requirement for optical activity. An important exemption happens in dual molecules, i.e., molecules which present the same response to electric and magnetic fields. We present a series of experiments measuring the optical activity and the scattering of chiral solutions in the forward and perpendicular directions. We experimentally show that these molecules present optical activity and preservation of circular polarization in the forward direction, while the polarization pattern in non-forward directions is much more complex and, in particular, does not preserve the circular polarization. Finally, we show that when probing the particle with different wavelengths, the scattering in non-forward directions presents some interesting structural features which are hidden in the forward measurements.