Do polar molecules diffract at nanomechanical gratings?

In our quest for coherent optical elements for biomolecules we have explored the compatibility of polar biomolecules with nanomechanical diffraction gratings. We have studied the naturally occurring antibiotic hypericin and compared it also with derivatives of tetraphenylporphyrin.  We observe coherent diffraction but the diffraction peaks are largely diffusively broadened. Like most biomolecules, hypericin has a permanent electric dipole moment due to the functional groups it needs to fulfill its biological function. Hence, the question arises whether this dipole moment is responsible for the blurring.

This hypothesis was tested by comparing the diffraction patterns of two organic molecules, which are nearly identical except for their dipole moment. While the non-polar molecule tetraphenylporphyrin TPP shows high contrast both at a grating machined into 20 nm thin amorphous carbon and also in silicon dioxide, this is not the case for the polar derivative MeOTPP. At the carbon grating we observe again diffuse beam broadening as already seen for hypericin. However, behind the insulating silicon dioxide mask, coherence seems to be entirely lost.

This can be traced back to charges implanted into the diffracting membrane during the milling process using fast gallium ions. The electric field of the random charge distribution deflects the molecules depending on the orientation of their dipole moment and the local field, either to the left or to the right.

While in a conducting carbon membrane the charges can be largely neutralized the insulating silicon dioxide maintains local charge patches over long periods of time. The stocahstic deflection suffices to eliminate the interference contrast for fast molecules and slow molecules are even deflected to be beyond the detector area.

References

• C. Knobloch, B. A. Stickler, C. Brand, M. Sclafani, Y. Lilach, T. Juffmann, O. Cheshnovsky, K. Hornberger, M. Arndt, On the role of the electric dipole moment in the diffraction of biomolecules at nanomechanical gratings, Fortschr. Phys., doi:10.1002/prop.201600025